Re: Oscillator Distortion

On Tue, 19 Nov 2024 18:49:33 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
 
[...]

Any oscillator with a nonlinear or bilinear gain control element that
has to respond during a cycle has to deal with the distortion caused by
that element.  OTAs, JFET variable resistors, PIN diode attenuators,
Vactrols, light bulbs, and so on, all have that problem.

 
Light bulbs and thermistors can have a controlling DC superimposed on a
miniscule signal current, so that the distortion caused by the latter is
negligible. 
 
Another alternative is an indirectly-heated thermistor with a very small
signal current in a large thermistor which is primarily heated by a
separate resistive element.  It would be slow to respond, but at 1 Kc/s
and -90 dB distortion, a long response time is essential to avoid
distortion from the amplitude-settling transient.
 
 

>
Depending on omega*tau_th, sure.  The HP 200 exhibits increasing
second-order distortion at lower frequencies.
>
Down at -90 dBc, depending on the signal level you might have to worry
about deviations from Ohm’s law in an oxide thermistor.  (Metals are pretty
linear, but the carrier density in an oxide is going to be much much
lower.)
>
Eventually it’s bound to be a tradeoff between distortion and noise.
>
Cheers
>
Phil Hobbs

Use the opamp dual-integrator sort of oscillator with a loop gain of
1.01, and give the (always nonlinear) variable-gain element 2%
influence.

In the old HP Wein bridge oscillators, the light bulb had a huge
influence on gain.