XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   john larkin  wrote:   
   > 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.   
      
   Yup.   
      
   Vernier adjustments to a nearly perfect basic platform are Good Medicine,   
   very generally.   
      
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
   --- SoupGate-DOS v1.05   
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