XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   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   
      
   --   
   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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