On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave
oscillators.
The problem is that if you want a get stable output from a sine wave
oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't'
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>oscillators.
The problem is that if you want a get stable output from a sine wave >>oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't.
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>oscillators.
The problem is that if you want a get stable output from a sine wave >>oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't.
On Thu, 6 Feb 2025 16:47:10 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 6/02/2025 4:08 pm, JM wrote:
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>>> oscillators.
The problem is that if you want a get stable output from a sine wave
oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't'
Counter-example?
Clipping is a non-linear process. The most linear op amp becomes
non-linear as soon as its output hits the supply rails.
<snip>
There have been quite a few postings about 1kHz low distortion sine wave oscillators.
The problem is that if you want a get stable output from a sine wave oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
In the original example - the Hewlett Packard sine wave oscillator which
got the company going - the non-linear element was the filament in an incandescent lamp whose resistance increased as it got hotter when the circuit put more current through it. It had enough thermal mass that the resistance didn't change much over a single cycle of the sine wave.
The popular option today is a FET where you can modulate the channel resistance by changing the gate-to-channel voltage. The channel
resistance isn't completely independent of the current through the
channel - it tends to increase a bit with current, independent of the polarity of the current. There's also some ripple on the control voltage applied to the FET gate.
It can still work very well.
I like precision four quadrant multipliers. You can set one up to add a controlled amplitude copy of the output to vary the gain around the oscillating loop - which is handy at start-up - or subtract it from the output. This means that you can trim the oscillating loop so that the multiplier normally only contributes the minimal correction required to compensate for component drift and temperature excursions.
I've set up an LTSpice simulation which illustrates the point, but it
used an AD734 as it's analog multiplier, which was horribly expensive at
the time and is $A72.99 now.
"JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:isg8qj15nkgl5cg41lgt4h4oav3bbgej2n@4ax.com...
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>> oscillators.
The problem is that if you want a get stable output from a sine wave
oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't.
There are plenty of examples out there claiming to be a stable output sinewave oscillator, with no obvious non-linear element.
https://www.google.ca/search?q=sinewave+oscillator&udm=2
But a quick simulation of one of them (a 2kHz oscillator) shows that it's not even 40dB down at 4kHz.
Maybe follow that with a Chebychev low pass filter with a zero in the stop band at 4kHz.
On Thu, 06 Feb 2025 05:08:16 +0000, JM
<sunaecoNoChoppedPork@gmail.com> wrote:
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>> oscillators.
The problem is that if you want a get stable output from a sine wave
oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't.
What limits the amplitude?
On 2025-02-06 00:44, john larkin wrote:
On Thu, 06 Feb 2025 05:08:16 +0000, JMWe had a long discussion of this in one of the myriad other 1-kHz
<sunaecoNoChoppedPork@gmail.com> wrote:
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>>> oscillators.
The problem is that if you want a get stable output from a sine wave
oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't.
What limits the amplitude?
oscillator threads. One approach is to use a comparator+integrator to >control the tail current source (suitably cascoded).
The key is for the gain-setting mechanism to be outside the oscillator
loop, so that it doesn't get run through its range on each cycle. The
bias of the active device does change some, of course, but that's harder
to avoid.
Cheers
Phil Hobbs
On Fri, 7 Feb 2025 12:43:17 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2025-02-06 00:44, john larkin wrote:
On Thu, 06 Feb 2025 05:08:16 +0000, JMWe had a long discussion of this in one of the myriad other 1-kHz
<sunaecoNoChoppedPork@gmail.com> wrote:
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>>>> oscillators.
The problem is that if you want a get stable output from a sine wave >>>>> oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
You don't.
What limits the amplitude?
oscillator threads. One approach is to use a comparator+integrator to
control the tail current source (suitably cascoded).
The key is for the gain-setting mechanism to be outside the oscillator
loop, so that it doesn't get run through its range on each cycle. The
bias of the active device does change some, of course, but that's harder
to avoid.
Cheers
Phil Hobbs
But where can I buy those linear diodes?
The idea of using a s/h to pick off the sine amplitude, for level
feedback, is interesting. Properly done, it should result in a
zero-ripple amplitude signal.
Or use an active full-wave rectifier to get the average, and filter
the heck out of that.
I suspect that nobody needs a way-sub-PPM THD sine wave, so it's
pretty much a game.
One might Spice using an ohmic mosfet or two as a low distortion
variable resistor. The i/v curves look awfully straight around zero.
john larkin <JL@gct.com> wrote:
On Fri, 7 Feb 2025 12:43:17 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2025-02-06 00:44, john larkin wrote:
On Thu, 06 Feb 2025 05:08:16 +0000, JMWe had a long discussion of this in one of the myriad other 1-kHz
<sunaecoNoChoppedPork@gmail.com> wrote:
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org> >>>>> wrote:
There have been quite a few postings about 1kHz low distortion sine wave >>>>>> oscillators.
The problem is that if you want a get stable output from a sine wave >>>>>> oscillator you have to add a non-linear element to control the gain >>>>>> around the oscillating circuit.
You don't.
What limits the amplitude?
oscillator threads. One approach is to use a comparator+integrator to
control the tail current source (suitably cascoded).
The key is for the gain-setting mechanism to be outside the oscillator
loop, so that it doesn't get run through its range on each cycle. The
bias of the active device does change some, of course, but that's harder >>> to avoid.
Cheers
Phil Hobbs
But where can I buy those linear diodes?
The idea of using a s/h to pick off the sine amplitude, for level
feedback, is interesting. Properly done, it should result in a
zero-ripple amplitude signal.
Or use an active full-wave rectifier to get the average, and filter
the heck out of that.
I suspect that nobody needs a way-sub-PPM THD sine wave, so it's
pretty much a game.
One might Spice using an ohmic mosfet or two as a low distortion
variable resistor. The i/v curves look awfully straight around zero.
I spent some quality time with that complementary Class AB car stereo amp
of JT’s last summer, and the more time I spent, the more impressed I was.
Its bias loop used an LM311 comparator sensing the minimum collector
current at the zero crossing, and charged up a biggish cap that set the >voltage between the PNP and NPN bases. Every time it got too low, the >comparator dumped a bit of charge into the cap, and a bleed resistor took
it out again.
Lots of us have done similar things, e.g. the class-H TEC driver in our
LC120 laser controller. The really nifty thing about Jim’s circuit was that >it measured what you actually care about, namely the minimum class-A
current right at the crossover point, rather than some DC average that >depends on the waveform, power supply droop, and other stuff with bupkis to >do with the crossover distortion. It worked brilliantly, according to the >spherical cows.
Something like that, measuring the instantaneous peak voltage of our >oscillator, would do an excellent job of regulating the tail current to
keep the amplitude constant.
Cheers
Phil Hobbs
On Sat, 8 Feb 2025 01:46:37 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
john larkin <JL@gct.com> wrote:
On Fri, 7 Feb 2025 12:43:17 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2025-02-06 00:44, john larkin wrote:
On Thu, 06 Feb 2025 05:08:16 +0000, JMWe had a long discussion of this in one of the myriad other 1-kHz
<sunaecoNoChoppedPork@gmail.com> wrote:
On Thu, 6 Feb 2025 03:58:59 +1100, Bill Sloman <bill.sloman@ieee.org> >>>>>> wrote:
There have been quite a few postings about 1kHz low distortion sine wave
oscillators.
The problem is that if you want a get stable output from a sine wave >>>>>>> oscillator you have to add a non-linear element to control the gain >>>>>>> around the oscillating circuit.
You don't.
What limits the amplitude?
oscillator threads. One approach is to use a comparator+integrator to >>>> control the tail current source (suitably cascoded).
The key is for the gain-setting mechanism to be outside the oscillator >>>> loop, so that it doesn't get run through its range on each cycle. The >>>> bias of the active device does change some, of course, but that's harder >>>> to avoid.
Cheers
Phil Hobbs
But where can I buy those linear diodes?
The idea of using a s/h to pick off the sine amplitude, for level
feedback, is interesting. Properly done, it should result in a
zero-ripple amplitude signal.
Or use an active full-wave rectifier to get the average, and filter
the heck out of that.
I suspect that nobody needs a way-sub-PPM THD sine wave, so it's
pretty much a game.
One might Spice using an ohmic mosfet or two as a low distortion
variable resistor. The i/v curves look awfully straight around zero.
I spent some quality time with that complementary Class AB car stereo amp
of JTÂ’s last summer, and the more time I spent, the more impressed I was. >>
Its bias loop used an LM311 comparator sensing the minimum collector
current at the zero crossing, and charged up a biggish cap that set the
voltage between the PNP and NPN bases. Every time it got too low, the
comparator dumped a bit of charge into the cap, and a bleed resistor took
it out again.
Lots of us have done similar things, e.g. the class-H TEC driver in our
LC120 laser controller. The really nifty thing about JimÂ’s circuit was that >> it measured what you actually care about, namely the minimum class-A
current right at the crossover point, rather than some DC average that
depends on the waveform, power supply droop, and other stuff with bupkis to >> do with the crossover distortion. It worked brilliantly, according to the
spherical cows.
Something like that, measuring the instantaneous peak voltage of our
oscillator, would do an excellent job of regulating the tail current to
keep the amplitude constant.
Cheers
Phil Hobbs
My NMR gradient coil drivers had PPM current accuracy and microsecond settling. I used many parallel mosfets with an opamp per fet to turn
each one into an essentially ideal device, zero threshold voltage.
That's fairly easy to bias to zero-deadband class AB.
https://www.dropbox.com/scl/fi/yyxfzyn7ro8070lxoy78q/Amp.jpg?rlkey=acaf000itexnaaj4r3rex3yq2&raw=1
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