I have been checking the performance of a variable-reactance type of >frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase >modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the >2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
I have been checking the performance of a variable-reactance type of frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the 2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
I have been checking the performance of a variable-reactance type of frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the 2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Correction. <https://www.sigidwiki.com/wiki/Database> includes one
PM zombie satellite: <https://www.sigidwiki.com/wiki/Transit_5B-5>
Liz Tuddenham wrote:
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was
barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a
good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the
2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Good question. It's a shame the All Identified Signals database <https://www.sigidwiki.com/wiki/Database> lacks a filter for PM mode. Otherwise it might indicate countries where PM is popular.
Danke,
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >frequency modulator which 'pulls' a crystal oscillator. After >multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter) >This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase >modulation.
The handbook for the Icom refers throughout to frequency modulation and >does not mention phase modulation. Most references to modulation in the >2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and >the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was
barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a
good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the
2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at: >http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >tapping the signal off the cathode of the oscillator through a 90-degree >phase-shift network consisting of a choke and the cathode resistor of
the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its
grid, a variable amount of 90-degree phase-shifted signal is fed into
the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I
have tried removing most of the top-cut capacitors between the audio
clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the
audio response figures of the Icom were taken with a Marconi TF 2016A
signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source
was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >>> >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >>> >good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so >>> >the limitation was in the detector/A.F. stages, not in the I.F. filter) >>> >This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and >>> >does not mention phase modulation. Most references to modulation in the >>> >2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and >>> >the use of phase modulation would cause 'splash' into adjacent channels >>> >at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at: >>http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >>tapping the signal off the cathode of the oscillator through a 90-degree >>phase-shift network consisting of a choke and the cathode resistor of
the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its >>grid, a variable amount of 90-degree phase-shifted signal is fed into
the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I
have tried removing most of the top-cut capacitors between the audio >>clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the
audio response figures of the Icom were taken with a Marconi TF 2016A >>signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source
was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
I have been checking the performance of a variable-reactance type of >frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase >modulation.
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >> >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >> >good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so >> >the limitation was in the detector/A.F. stages, not in the I.F. filter) >> >This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and >> >does not mention phase modulation. Most references to modulation in the >> >2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and >> >the use of phase modulation would cause 'splash' into adjacent channels >> >at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at: >http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >tapping the signal off the cathode of the oscillator through a 90-degree >phase-shift network consisting of a choke and the cathode resistor of
the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its >grid, a variable amount of 90-degree phase-shifted signal is fed into
the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I
have tried removing most of the top-cut capacitors between the audio >clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the
audio response figures of the Icom were taken with a Marconi TF 2016A >signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source
was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
In article <1rf3e3v.uj0bkl16o5mqqN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
I have been checking the performance of a variable-reactance type of >frequency modulator which 'pulls' a crystal oscillator. After >multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter) >This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase >modulation.
It's the former, and "it's a feature, not a bug".
As I understand it: the normal convention on the ham bands is to apply
a 6 dB/octave high-pass equalization to the transmitted voice signal
prior to frequency modulation. The time constant puts the "knee" of
the curve above the voice band.
During reception, the signal from the discriminator is fed through
a corresponding low-pass filter ("knee" below the voice band) before
being fed to the audio amplifier and outputs.
It's a process similar to what's done in commercial FM broadcasting,
but with different time constants in the filters.
As I understand it, this was done for two reasons: to reduce the
incursion of high-frequency noise into the audio signal, and to
allow compatibility with PM transmitters (which don't require or
use the high-pass filter).
So, what you are observing is probably this: the signal you're
transmitting (from your own modulator or from your test oscillator)
is frequency-modulated, but the necessary pre-emphasis of the
upper frequencies is not being applied. When the signal is
demodulated by the Icom, it's passed through the de-emphasis low-
pass filter, and what you're observing in your frequency chart is
the response of that filter.
So, you could try putting your Icom into "9600-baud" mode, and look at
the discriminator-output signal at the "to TNC" output jack. I'm
going to guess that you find that you see a pretty flat frequency
response from the receiver when you feed it a signal from your
experimental modulator.
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com>
wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >> >>> >barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a
good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so >> >>> >the limitation was in the detector/A.F. stages, not in the I.F. filter) >> >>> >This looks as though EITHER a 6dB per octave response is being imposed >> >>> >on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and >> >>> >does not mention phase modulation. Most references to modulation in the
2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels >> >>> >at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve
tapping the signal off the cathode of the oscillator through a 90-degree >> >>phase-shift network consisting of a choke and the cathode resistor of
the reactance valve (which also carries the oscillator cathode current). >> >>
As the gain of the reactance valve is varied by the audio signal on its
grid, a variable amount of 90-degree phase-shifted signal is fed into
the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I
have tried removing most of the top-cut capacitors between the audio
clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the
audio response figures of the Icom were taken with a Marconi TF 2016A
signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source
was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason
to suppose it was causing such a bad frequency response.
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com>
wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >> >>> >frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found
it was barely intelligible, with severe high frequency cut. At
first I suspected my modulator but I checked the audio output of
the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in
the I.F. filter) This looks as though EITHER a 6dB per octave
response is being imposed on the output of the FM detector OR the
detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references
to modulation in the 2-metre band (144-146 Mc/s in the U.K.)
mention frequency modulation and the use of phase modulation would
cause 'splash' into adjacent channels at higher audio frequencies
because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is
the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >> >>tapping the signal off the cathode of the oscillator through a 90-degree >> >>phase-shift network consisting of a choke and the cathode resistor of
the reactance valve (which also carries the oscillator cathode current). >> >>
As the gain of the reactance valve is varied by the audio signal on its >> >>grid, a variable amount of 90-degree phase-shifted signal is fed into
the crystal oscillator frequency trimming inductor in the anode circuit >> >>of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I
have tried removing most of the top-cut capacitors between the audio
clipper and the input to the modulator but this made little difference >> >>as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >> >>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the
audio response figures of the Icom were taken with a Marconi TF 2016A
signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source >> >>was a Solartron CO 546 Wein-bridge oscillator which is stable to + or - >> >>0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason
to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I wonder if the crystal's Q limits modulation bandwidth somehow.
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com>
wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >> >> >>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >> >> >>> >frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found
it was barely intelligible, with severe high frequency cut. At
first I suspected my modulator but I checked the audio output of
the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in >> >> >>> >the I.F. filter) This looks as though EITHER a 6dB per octave
response is being imposed on the output of the FM detector OR the
detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >> >> >>> >to modulation in the 2-metre band (144-146 Mc/s in the U.K.)
mention frequency modulation and the use of phase modulation would >> >> >>> >cause 'splash' into adjacent channels at higher audio frequencies
because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is
the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >> >> >>tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of >> >> >>the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its >> >> >>grid, a variable amount of 90-degree phase-shifted signal is fed into >> >> >>the crystal oscillator frequency trimming inductor in the anode circuit >> >> >>of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I
have tried removing most of the top-cut capacitors between the audio
clipper and the input to the modulator but this made little difference >> >> >>as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >> >> >>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >> >> >>audio response figures of the Icom were taken with a Marconi TF 2016A >> >> >>signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source >> >> >>was a Solartron CO 546 Wein-bridge oscillator which is stable to + or - >> >> >>0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason
to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
On Mon, 7 Jul 2025 19:06:07 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >> >> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >> >> >>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance
type of frequency modulator which 'pulls' a crystal oscillator.
After multiplication and mixing, the signal appears at 145
Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found >> >> >>> >it was barely intelligible, with severe high frequency cut. At
first I suspected my modulator but I checked the audio output of >> >> >>> >the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in >> >> >>> >the I.F. filter) This looks as though EITHER a 6dB per octave
response is being imposed on the output of the FM detector OR the >> >> >>> >detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >> >> >>> >to modulation in the 2-metre band (144-146 Mc/s in the U.K.)
mention frequency modulation and the use of phase modulation would >> >> >>> >cause 'splash' into adjacent channels at higher audio frequencies >> >> >>> >because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >> >> >>> >the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass the frequency control input, the varactor
thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance
valve tapping the signal off the cathode of the oscillator through
a 90-degree phase-shift network consisting of a choke and the
cathode resistor of the reactance valve (which also carries the
oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal
on its grid, a variable amount of 90-degree phase-shifted signal
is fed into the crystal oscillator frequency trimming inductor in
the anode circuit of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >> >> >>have tried removing most of the top-cut capacitors between the audio >> >> >>clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but
the audio response figures of the Icom were taken with a Marconi
TF 2016A signal generator. This has an internal meter which
allows the modulation level to be accurately set and monitored.
The audio source was a Solartron CO 546 Wein-bridge oscillator
which is stable to + or - 0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >> >to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
Use a receiver in CW mode. That will heterodyne the FM down and zoom
up the deviation. Then look at the result on a scope to see the period variation.
A decent digital scope should display FM deviation directly.
Crystals modulate much faster than f0/Q, interestingly.
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com>
wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >>>>>>>> frequency modulator which 'pulls' a crystal oscillator. After >>>>>>>> multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found >>>>>>>> it was barely intelligible, with severe high frequency cut. At >>>>>>>> first I suspected my modulator but I checked the audio output of >>>>>>>> the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in >>>>>>>> the I.F. filter) This looks as though EITHER a 6dB per octave
response is being imposed on the output of the FM detector OR the >>>>>>>> detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >>>>>>>> to modulation in the 2-metre band (144-146 Mc/s in the U.K.)
mention frequency modulation and the use of phase modulation would >>>>>>>> cause 'splash' into adjacent channels at higher audio frequencies >>>>>>>> because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >>>>>>>> the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >>>>>> tapping the signal off the cathode of the oscillator through a 90-degree >>>>>> phase-shift network consisting of a choke and the cathode resistor of >>>>>> the reactance valve (which also carries the oscillator cathode current). >>>>>>
As the gain of the reactance valve is varied by the audio signal on its >>>>>> grid, a variable amount of 90-degree phase-shifted signal is fed into >>>>>> the crystal oscillator frequency trimming inductor in the anode circuit >>>>>> of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >>>>>> have tried removing most of the top-cut capacitors between the audio >>>>>> clipper and the input to the modulator but this made little difference >>>>>> as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >>>>>>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >>>>>> audio response figures of the Icom were taken with a Marconi TF 2016A >>>>>> signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source >>>>>> was a Solartron CO 546 Wein-bridge oscillator which is stable to + or - >>>>>> 0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >>> to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
I wonder if the crystal's Q limits modulation bandwidth somehow.
I had wondered about that but the measuements I made with the signal generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver. The reply
by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my usual sources.
When I have added the appropriate gross pre-emphasis to the transmitter
I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >>>>> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:47K and 1 nF (plus some strays) has a corner frequency of around 3 >>>>>> KHz.
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >>>>>>>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >>>>>>>>> frequency modulator which 'pulls' a crystal oscillator. After >>>>>>>>> multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found >>>>>>>>> it was barely intelligible, with severe high frequency cut. At >>>>>>>>> first I suspected my modulator but I checked the audio output of >>>>>>>>> the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in >>>>>>>>> the I.F. filter) This looks as though EITHER a 6dB per octave >>>>>>>>> response is being imposed on the output of the FM detector OR the >>>>>>>>> detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >>>>>>>>> to modulation in the 2-metre band (144-146 Mc/s in the U.K.) >>>>>>>>> mention frequency modulation and the use of phase modulation would >>>>>>>>> cause 'splash' into adjacent channels at higher audio frequencies >>>>>>>>> because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >>>>>>>>> the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >>>>>>> tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of >>>>>>> the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its >>>>>>> grid, a variable amount of 90-degree phase-shifted signal is fed into >>>>>>> the crystal oscillator frequency trimming inductor in the anode circuit >>>>>>> of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >>>>>>> have tried removing most of the top-cut capacitors between the audio >>>>>>> clipper and the input to the modulator but this made little difference >>>>>>> as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >>>>>>>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >>>>>>> audio response figures of the Icom were taken with a Marconi TF 2016A >>>>>>> signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source >>>>>>> was a Solartron CO 546 Wein-bridge oscillator which is stable to + or - >>>>>>> 0.1 dB.
The sig-gen tests confirmed what my ears were already telling me. >>>>>>
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >>>> to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
I wonder if the crystal's Q limits modulation bandwidth somehow.
I had wondered about that but the measuements I made with the signal
generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver. The reply
by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my
usual sources.
When I have added the appropriate gross pre-emphasis to the transmitter
I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
Crystals modulate much faster than f0/Q, interestingly. The mechanical movement and the piezoelectric coupling are a single degree of freedom, governed by an ordinary differential equation.
ODEs have no internal state, so the oscillation responds instantly to
changes in the capacitive load.
Something like a SAW resonator has lots of internal state, and so takes
much longer to respond.
Cheers
Phil Hobbs
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >>>>>> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:47K and 1 nF (plus some strays) has a corner frequency of around 3 >>>>>>> KHz.
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >>>>>>>>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >>>>>>>>>> frequency modulator which 'pulls' a crystal oscillator. After >>>>>>>>>> multiplication and mixing, the signal appears at 145 Mc/s. >>>>>>>>>>
Listening to this signal on an Icom 706 MkII transceiver I found >>>>>>>>>> it was barely intelligible, with severe high frequency cut. At >>>>>>>>>> first I suspected my modulator but I checked the audio output of >>>>>>>>>> the Icom with a good-quality signal generator and found the >>>>>>>>>> response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar >>>>>>>>>> results, so the limitation was in the detector/A.F. stages, not in >>>>>>>>>> the I.F. filter) This looks as though EITHER a 6dB per octave >>>>>>>>>> response is being imposed on the output of the FM detector OR the >>>>>>>>>> detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >>>>>>>>>> to modulation in the 2-metre band (144-146 Mc/s in the U.K.) >>>>>>>>>> mention frequency modulation and the use of phase modulation would >>>>>>>>>> cause 'splash' into adjacent channels at higher audio frequencies >>>>>>>>>> because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >>>>>>>>>> the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpassThe circuits are at:
the frequency control input, the varactor thing, pretty hard. >>>>>>>>
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm >>>>>>>>
The crystal oscillator is a modified Colpitts with the reactance valve >>>>>>>> tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of >>>>>>>> the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its
grid, a variable amount of 90-degree phase-shifted signal is fed into >>>>>>>> the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >>>>>>>> have tried removing most of the top-cut capacitors between the audio >>>>>>>> clipper and the input to the modulator but this made little difference >>>>>>>> as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >>>>>>>>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >>>>>>>> audio response figures of the Icom were taken with a Marconi TF 2016A >>>>>>>> signal generator. This has an internal meter which allows the >>>>>>>> modulation level to be accurately set and monitored. The audio source >>>>>>>> was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me. >>>>>>>
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >>>>> to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
I wonder if the crystal's Q limits modulation bandwidth somehow.
I had wondered about that but the measuements I made with the signal
generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver. The reply
by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my >>> usual sources.
When I have added the appropriate gross pre-emphasis to the transmitter
I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
Crystals modulate much faster than f0/Q, interestingly. The mechanical
movement and the piezoelectric coupling are a single degree of freedom,
governed by an ordinary differential equation.
ODEs have no internal state, so the oscillation responds instantly to
changes in the capacitive load.
Something like a SAW resonator has lots of internal state, and so takes
much longer to respond.
Cheers
Phil Hobbs
Interesting!
What are ODEs?
... The crystal mode is a mass-spring
oscillation, so an instantaneous change in the spring constant causes an instantaneous change of frequency.
There’s no need for the resonant energy to die out, unlike the case of external forcing, where a change in the forcing frequency takes on the
order of Q cycles to change the response.
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 19:06:07 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >> >> >> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid >> >> >> >(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance
type of frequency modulator which 'pulls' a crystal oscillator. >> >> >> >>> >After multiplication and mixing, the signal appears at 145
Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found >> >> >> >>> >it was barely intelligible, with severe high frequency cut. At >> >> >> >>> >first I suspected my modulator but I checked the audio output of >> >> >> >>> >the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in
the I.F. filter) This looks as though EITHER a 6dB per octave
response is being imposed on the output of the FM detector OR the >> >> >> >>> >detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references
to modulation in the 2-metre band (144-146 Mc/s in the U.K.)
mention frequency modulation and the use of phase modulation would
cause 'splash' into adjacent channels at higher audio frequencies >> >> >> >>> >because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >> >> >> >>> >the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass the frequency control input, the varactor
thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance
valve tapping the signal off the cathode of the oscillator through >> >> >> >>a 90-degree phase-shift network consisting of a choke and the
cathode resistor of the reactance valve (which also carries the
oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal
on its grid, a variable amount of 90-degree phase-shifted signal
is fed into the crystal oscillator frequency trimming inductor in
the anode circuit of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >> >> >> >>have tried removing most of the top-cut capacitors between the audio >> >> >> >>clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is
in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but
the audio response figures of the Icom were taken with a Marconi
TF 2016A signal generator. This has an internal meter which
allows the modulation level to be accurately set and monitored.
The audio source was a Solartron CO 546 Wein-bridge oscillator
which is stable to + or - 0.1 dB.
The sig-gen tests confirmed what my ears were already telling me.
47K and 1 nF (plus some strays) has a corner frequency of around 3
KHz.
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >> >> >to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
Use a receiver in CW mode. That will heterodyne the FM down and zoom
up the deviation. Then look at the result on a scope to see the period
variation.
A decent digital scope should display FM deviation directly.
I only have analogue equipment and the test with a signal generator has >already told me what is going on. When I have the modulator working, I
can easily check for unwanted harmonics by listening on the two adjacent >channels.
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
... The crystal mode is a mass-spring
oscillation, so an instantaneous change in the spring constant causes an
instantaneous change of frequency.
Thereâs no need for the resonant energy to die out, unlike the case of >> external forcing, where a change in the forcing frequency takes on the
order of Q cycles to change the response.
Could I ask you to explain this a bit more please. Where is the
'spring' that is variable?
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >>>>> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:47K and 1 nF (plus some strays) has a corner frequency of around 3 >>>>>> KHz.
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >>>>>>>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >>>>>>>>> frequency modulator which 'pulls' a crystal oscillator. After >>>>>>>>> multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found >>>>>>>>> it was barely intelligible, with severe high frequency cut. At >>>>>>>>> first I suspected my modulator but I checked the audio output of >>>>>>>>> the Icom with a good-quality signal generator and found the
response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar
results, so the limitation was in the detector/A.F. stages, not in >>>>>>>>> the I.F. filter) This looks as though EITHER a 6dB per octave >>>>>>>>> response is being imposed on the output of the FM detector OR the >>>>>>>>> detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >>>>>>>>> to modulation in the 2-metre band (144-146 Mc/s in the U.K.) >>>>>>>>> mention frequency modulation and the use of phase modulation would >>>>>>>>> cause 'splash' into adjacent channels at higher audio frequencies >>>>>>>>> because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >>>>>>>>> the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpass
the frequency control input, the varactor thing, pretty hard.
The circuits are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm
The crystal oscillator is a modified Colpitts with the reactance valve >>>>>>> tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of >>>>>>> the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its >>>>>>> grid, a variable amount of 90-degree phase-shifted signal is fed into >>>>>>> the crystal oscillator frequency trimming inductor in the anode circuit >>>>>>> of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >>>>>>> have tried removing most of the top-cut capacitors between the audio >>>>>>> clipper and the input to the modulator but this made little difference >>>>>>> as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >>>>>>>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >>>>>>> audio response figures of the Icom were taken with a Marconi TF 2016A >>>>>>> signal generator. This has an internal meter which allows the
modulation level to be accurately set and monitored. The audio source >>>>>>> was a Solartron CO 546 Wein-bridge oscillator which is stable to + or - >>>>>>> 0.1 dB.
The sig-gen tests confirmed what my ears were already telling me. >>>>>>
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >>>> to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
I wonder if the crystal's Q limits modulation bandwidth somehow.
I had wondered about that but the measuements I made with the signal
generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver. The reply
by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my
usual sources.
When I have added the appropriate gross pre-emphasis to the transmitter
I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
Crystals modulate much faster than f0/Q, interestingly. The mechanical >movement and the piezoelectric coupling are a single degree of freedom, >governed by an ordinary differential equation.
ODEs have no internal state, so the oscillation responds instantly to
changes in the capacitive load.
Something like a SAW resonator has lots of internal state, and so takes
much longer to respond.
Cheers
Phil Hobbs
piglet <erichpwagner@hotmail.com> wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >>>>>>> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid >>>>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:47K and 1 nF (plus some strays) has a corner frequency of around 3 >>>>>>>> KHz.
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >>>>>>>>>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After >>>>>>>>>>> multiplication and mixing, the signal appears at 145 Mc/s. >>>>>>>>>>>
Listening to this signal on an Icom 706 MkII transceiver I found >>>>>>>>>>> it was barely intelligible, with severe high frequency cut. At >>>>>>>>>>> first I suspected my modulator but I checked the audio output of >>>>>>>>>>> the Icom with a good-quality signal generator and found the >>>>>>>>>>> response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar >>>>>>>>>>> results, so the limitation was in the detector/A.F. stages, not in >>>>>>>>>>> the I.F. filter) This looks as though EITHER a 6dB per octave >>>>>>>>>>> response is being imposed on the output of the FM detector OR the >>>>>>>>>>> detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency >>>>>>>>>>> modulation and does not mention phase modulation. Most references >>>>>>>>>>> to modulation in the 2-metre band (144-146 Mc/s in the U.K.) >>>>>>>>>>> mention frequency modulation and the use of phase modulation would >>>>>>>>>>> cause 'splash' into adjacent channels at higher audio frequencies >>>>>>>>>>> because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >>>>>>>>>>> the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpassThe circuits are at:
the frequency control input, the varactor thing, pretty hard. >>>>>>>>>
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm >>>>>>>>>
The crystal oscillator is a modified Colpitts with the reactance valve
tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of >>>>>>>>> the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its
grid, a variable amount of 90-degree phase-shifted signal is fed into >>>>>>>>> the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >>>>>>>>> have tried removing most of the top-cut capacitors between the audio >>>>>>>>> clipper and the input to the modulator but this made little difference
as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >>>>>>>>>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >>>>>>>>> audio response figures of the Icom were taken with a Marconi TF 2016A >>>>>>>>> signal generator. This has an internal meter which allows the >>>>>>>>> modulation level to be accurately set and monitored. The audio source
was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me. >>>>>>>>
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >>>>>> to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
I wonder if the crystal's Q limits modulation bandwidth somehow.
I had wondered about that but the measuements I made with the signal
generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver. The reply >>>> by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my >>>> usual sources.
When I have added the appropriate gross pre-emphasis to the transmitter >>>> I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
Crystals modulate much faster than f0/Q, interestingly. The mechanical
movement and the piezoelectric coupling are a single degree of freedom,
governed by an ordinary differential equation.
ODEs have no internal state, so the oscillation responds instantly to
changes in the capacitive load.
Something like a SAW resonator has lots of internal state, and so takes
much longer to respond.
Cheers
Phil Hobbs
Interesting!
What are ODEs?
Ordinary differential equations. The crystal mode is a mass-spring oscillation, so an instantaneous change in the spring constant causes an instantaneous change of frequency.
There’s no need for the resonant energy to die out, unlike the case of external forcing, where a change in the forcing frequency takes on the
order of Q cycles to change the response.
Cheers
Phil Hobbs
On Tue, 8 Jul 2025 11:19:35 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 7 Jul 2025 18:28:20 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:
On Mon, 07 Jul 2025 08:07:32 -0700, john larkin <jl@glen--canyon.com> >>>>>> wrote:
On Mon, 7 Jul 2025 15:04:46 +0100, liz@poppyrecords.invalid.invalid >>>>>>> (Liz Tuddenham) wrote:
john larkin <jl@glen--canyon.com> wrote:47K and 1 nF (plus some strays) has a corner frequency of around 3 >>>>>>> KHz.
On Mon, 7 Jul 2025 08:44:41 +0100, liz@poppyrecords.invalid.invalid >>>>>>>>> (Liz Tuddenham) wrote:
I have been checking the performance of a variable-reactance type of >>>>>>>>>> frequency modulator which 'pulls' a crystal oscillator. After >>>>>>>>>> multiplication and mixing, the signal appears at 145 Mc/s. >>>>>>>>>>
Listening to this signal on an Icom 706 MkII transceiver I found >>>>>>>>>> it was barely intelligible, with severe high frequency cut. At >>>>>>>>>> first I suspected my modulator but I checked the audio output of >>>>>>>>>> the Icom with a good-quality signal generator and found the >>>>>>>>>> response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar >>>>>>>>>> results, so the limitation was in the detector/A.F. stages, not in >>>>>>>>>> the I.F. filter) This looks as though EITHER a 6dB per octave >>>>>>>>>> response is being imposed on the output of the FM detector OR the >>>>>>>>>> detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency
modulation and does not mention phase modulation. Most references >>>>>>>>>> to modulation in the 2-metre band (144-146 Mc/s in the U.K.) >>>>>>>>>> mention frequency modulation and the use of phase modulation would >>>>>>>>>> cause 'splash' into adjacent channels at higher audio frequencies >>>>>>>>>> because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is >>>>>>>>>> the norm or is there another explantion?
Is your "crystal oscillator" a packaged VCXO?
No
They generally lowpassThe circuits are at:
the frequency control input, the varactor thing, pretty hard. >>>>>>>>
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm >>>>>>>>
The crystal oscillator is a modified Colpitts with the reactance valve >>>>>>>> tapping the signal off the cathode of the oscillator through a 90-degree
phase-shift network consisting of a choke and the cathode resistor of >>>>>>>> the reactance valve (which also carries the oscillator cathode current).
As the gain of the reactance valve is varied by the audio signal on its
grid, a variable amount of 90-degree phase-shifted signal is fed into >>>>>>>> the crystal oscillator frequency trimming inductor in the anode circuit
of the reactance valve.
The RC time constant in the grid circuit is 3dB down at 3.4 Kc/s. I >>>>>>>> have tried removing most of the top-cut capacitors between the audio >>>>>>>> clipper and the input to the modulator but this made little difference >>>>>>>> as all those time constants took effect above 3 Kc/s.
If your rig is all tubes, probably not. But I suspect the rolloff is >>>>>>>>> in the transmitter, not the receiver.
The initial tests were done with the experimental transmitter but the >>>>>>>> audio response figures of the Icom were taken with a Marconi TF 2016A >>>>>>>> signal generator. This has an internal meter which allows the >>>>>>>> modulation level to be accurately set and monitored. The audio source >>>>>>>> was a Solartron CO 546 Wein-bridge oscillator which is stable to + or -
0.1 dB.
The sig-gen tests confirmed what my ears were already telling me. >>>>>>>
Why not measure the FM and see who the bad guy is?
The FM was coming from a high quality signal generator, I have no reason >>>>> to suppose it was causing such a bad frequency response.
I meant measure the actual FM from your crystal oscillator.
I have no way of making those measurement other than with a VHF
receiver.
I wonder if the crystal's Q limits modulation bandwidth somehow.
I had wondered about that but the measuements I made with the signal
generator indicate quite clearly that the large drop in audio HF
response is mainly (if not entirely) caused by the receiver. The reply
by Dave Platt ofers a completely plausible explanation for this and
confirms something I had suspected but couldn't find stated in any of my >>> usual sources.
When I have added the appropriate gross pre-emphasis to the transmitter
I shall be able to hear what I am doing and can then set about
correcting any remaining nuances of the modulator's performance.
Crystals modulate much faster than f0/Q, interestingly. The mechanical
movement and the piezoelectric coupling are a single degree of freedom,
governed by an ordinary differential equation.
ODEs have no internal state, so the oscillation responds instantly to
changes in the capacitive load.
Something like a SAW resonator has lots of internal state, and so takes
much longer to respond.
Cheers
Phil Hobbs
I suppose that's still true for a shear-mode overtone crystal. Just
harder to think about.
The question now is where to put the pre-emphasis in the feed to the >modulator? If I put it in front of the clipper it will be rendered >ineffective by clipping - but if I put it after the clipper it is liable
to over-deviate the signal.
I have been checking the performance of a variable-reactance type of frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the 2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and
the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
I meant measure the actual FM from your crystal oscillator.
I wonder if the crystal's Q limits modulation bandwidth somehow.
In article <pu1o6kp8b27fbq470dh8lrkjiu15cldu76@4ax.com>, jl@glen--
canyon.com says...
I meant measure the actual FM from your crystal oscillator.
I wonder if the crystal's Q limits modulation bandwidth somehow.
Sure it does. You can only move the frequency a small amount. FM and
PM is usually generated at 8 to 12 MHz and multiplied up to 50 to 450
MHz.
This is for the old crystal controled ham and pulic servce radios.
In article <1rf3e3v.uj0bkl16o5mqqN%liz@poppyrecords.invalid.invalid>, liz@poppyrecords.invalid.invalid says...
I have been checking the performance of a variable-reactance type of frequency modulator which 'pulls' a crystal oscillator. After multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter) This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase modulation.
The handbook for the Icom refers throughout to frequency modulation and does not mention phase modulation. Most references to modulation in the 2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Frequency and phase modulation should come out of a transmitter almost identical. As they start off different in the first stage of the
transmitter the audio frequency going into the modulastor stage must be modified so the end results is the same for FM and PM.
In article <1rf4563.inglu7k0vp30N%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
The question now is where to put the pre-emphasis in the feed to the >modulator? If I put it in front of the clipper it will be rendered >ineffective by clipping - but if I put it after the clipper it is liable
to over-deviate the signal.
If you push the signal into clipping, the very act of clipping is
going to create a whole bunch of nasty high-frequency harmonics and IM
that sound terrible. In that situation, applying further emphasis to
this grunge is probably the opposite of what you want to do. Rather,
let the THD/IM be rolled off by the receiver's deemphasis filter, and consider this an act of corporal mercy to the listener's ears :-)
If you put the emphasis after the clipper, then you end up having to
set your deviation limit based on a worst-case prediction of high
frequency content in the signal, and this may force you to use
a lower deviation level than you'd otherwise prefer.
So, my vote is "emphasize, then clip".
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
... The crystal mode is a mass-spring
oscillation, so an instantaneous change in the spring constant causes an >> instantaneous change of frequency.
Thereâs no need for the resonant energy to die out, unlike the case of
external forcing, where a change in the forcing frequency takes on the
order of Q cycles to change the response.
Could I ask you to explain this a bit more please. Where is the
'spring' that is variable?
The spring is the mechanical stiffness of the quartz, plus the
piezoelectric force caused by the interaction with the external circuit.
The mass is the mass. ;)
Dorking the load capacitance slows down or speeds up the piezo restoring force.
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
... The crystal mode is a mass-spring
oscillation, so an instantaneous change in the spring constant causes an >>>> instantaneous change of frequency.
ThereâÂÂs no need for the resonant energy to die out, unlike the case of
external forcing, where a change in the forcing frequency takes on the >>>> order of Q cycles to change the response.
Could I ask you to explain this a bit more please. Where is the
'spring' that is variable?
The spring is the mechanical stiffness of the quartz, plus the
piezoelectric force caused by the interaction with the external circuit.
The mass is the mass. ;)
Dorking the load capacitance slows down or speeds up the piezo restoring
force.
Interesting, I hadn't come across that before. Presumably the effect
would be the same if the controlling reactance were an inductor or a
tuned circuit? In my case, the 'variable reactance' is an amplitude-modulated 90-degree phase-shifted current injected into the controlling inductor.
Ralph Mowery wrote:
liz@poppyrecords.invalid.invalid says...
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was
barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >> > good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so
the limitation was in the detector/A.F. stages, not in the I.F. filter)
This looks as though EITHER a 6dB per octave response is being imposed
on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and
does not mention phase modulation. Most references to modulation in the >> > 2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and >> > the use of phase modulation would cause 'splash' into adjacent channels
at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Frequency and phase modulation should come out of a transmitter almost
identical. As they start off different in the first stage of the
transmitter the audio frequency going into the modulastor stage must be
modified so the end results is the same for FM and PM.
Yes - but it would appear that PM is the de-facto standard for commercially-built 'amateur' 2-metre transceivers despite all the
literature calling it FM.
Liz Tuddenham wrote:
Ralph Mowery wrote:
liz@poppyrecords.invalid.invalid says...
I have been checking the performance of a variable-reactance type of
frequency modulator which 'pulls' a crystal oscillator. After
multiplication and mixing, the signal appears at 145 Mc/s.
Listening to this signal on an Icom 706 MkII transceiver I found it was >> > barely intelligible, with severe high frequency cut. At first I
suspected my modulator but I checked the audio output of the Icom with a >> > good-quality signal generator and found the response was:
200c/s : -3dB
400c/s : 0dB
750c/s : -3dB
1 Kc/s : -6dB
1k5 : -10dB
2k0 : -13dB
2k5 : -16dB
3k0 : -18dB
(Using the wideband FM setting of the Icom produced similar results, so >> > the limitation was in the detector/A.F. stages, not in the I.F. filter) >> > This looks as though EITHER a 6dB per octave response is being imposed >> > on the output of the FM detector OR the detector is expecting phase
modulation.
The handbook for the Icom refers throughout to frequency modulation and >> > does not mention phase modulation. Most references to modulation in the >> > 2-metre band (144-146 Mc/s in the U.K.) mention frequency modulation and >> > the use of phase modulation would cause 'splash' into adjacent channels >> > at higher audio frequencies because of the rising characteristic.
Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Frequency and phase modulation should come out of a transmitter almost
identical. As they start off different in the first stage of the
transmitter the audio frequency going into the modulastor stage must be
modified so the end results is the same for FM and PM.
Yes - but it would appear that PM is the de-facto standard for commercially-built 'amateur' 2-metre transceivers despite all the literature calling it FM.
Yes, your sentiment seems "on the money."
This thread apparently advanced from an IC-706 to a DIY receiver. If
your schematic indeed pertains to your DIY receiver - excellent work showcased in an excellent thread! (It taught me a few things, thank
you.)
For what it's worth, the IC-706 probably uses the 75 micro-second pre- emphasis standard prevalent in the Americas and Japan. Meanwhile,
Europe favors a 50 micro-second standard.
Although Phase Modulation is popular in Asia:
<https://pmc.ncbi.nlm.nih.gov/articles/PMC11501851/>
it's doubtful the IC-706 was developed for an Asian market.
Don wrote:
Liz Tuddenham wrote:
Ralph Mowery wrote:
liz@poppyrecords.invalid.invalid says...Has my Icom been designed for a market where phase modulation is the
norm or is there another explantion?
Frequency and phase modulation should come out of a transmitter almost
identical. As they start off different in the first stage of the
transmitter the audio frequency going into the modulastor stage must be >> >> modified so the end results is the same for FM and PM.
Yes - but it would appear that PM is the de-facto standard for
commercially-built 'amateur' 2-metre transceivers despite all the
literature calling it FM.
Yes, your sentiment seems "on the money."
This thread apparently advanced from an IC-706 to a DIY receiver. If
your schematic indeed pertains to your DIY receiver - excellent work
showcased in an excellent thread! (It taught me a few things, thank
you.)
It is based on an idea that has been in the back of my mind for many
years. The change from an exchange-powered to a mains-dependent
telephone service in the UK was the final factor that made me decide
that I neded an alternative communications system. I thought that,
while I was making one, it might be a good idea if it was EMP-proof, so
I decided it would have no semiconductors. It has turned into an
interesting project.
For what it's worth, the IC-706 probably uses the 75 micro-second pre-
emphasis standard prevalent in the Americas and Japan. Meanwhile,
Europe favors a 50 micro-second standard.
Those are the standards for wide-band broadcast FM, but they are outside
the range of permitted modulating frequencies for narrow-band amateur
work. The time constant for the Icom appears to be around 2000
microseconds.
Although Phase Modulation is popular in Asia:
<https://pmc.ncbi.nlm.nih.gov/articles/PMC11501851/>
it's doubtful the IC-706 was developed for an Asian market.
Apparently it was. There is a Japanese version with a slightly
different specification from the European one. You may have hit upon
the real explanation, I haven't checked to see if mine is a Japanese one which has been sold in the UK by accident.
Yes - but it would appear that PM is the de-facto standard for >commercially-built 'amateur' 2-metre transceivers despite all the
literature calling it FM.
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