XPost: sci.electronics.design   
   From: jl@glen--canyon.com   
      
   On Tue, 19 Nov 2024 18:49:33 -0000 (UTC), Phil Hobbs   
    wrote:   
      
   >Liz Tuddenham  wrote:   
   >> Phil Hobbs  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.   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)