XPost: sci.electronics.design   
      
   In article ,   
   Cursitor Doom   wrote:   
   >Gentlemen,   
   >   
   >Last week I got an old (1968) pulse generator out of mothballs and managed   
   >to get it fully functional again. However, before replacing the case, I   
   >(true to form) dropped it on the bench and something on the PCB must have   
   >shorted out against the metal tools it fell on, because it no longer works   
   >properly.   
   >I've found an issue with the principal oscillator. It's generating   
   >distorted sine waves. It's a wien bridge type using BJTs as the gain   
   >element and fine tungsten filaments as thermistors, so should produce near   
   >perfect sine waves before they're chopped and shaped by subsequent   
   >circuitry, but since the fall, it's not.   
   >   
   >Here's the oscillator output:   
   >https://disk.yandex.com/i/eKAe95xMsiIvNA   
   >   
   >I found some weird periodic spikes on the power supply rails in the   
   >oscillator stage. They are actually present on the rail, not just picked   
   >up by the ground lead of the scope out of the ether, as I used a short   
   >ground clip in this instance. I'm not sure if these could cause the   
   >distortion or not.   
   >https://disk.yandex.com/i/eKAe95xMsiIvNA   
   >   
   >I'm out of ideas. What could cause such distortion if the PS rail isn't   
   >responsible?   
      
   Not entirely unrelated to this subject.   
   Image a Wien bridge oscillator, stabilized with a lightbulb.   
   This could be described with differential equations, including   
   the heating and cooling of the tungsten wire.   
      
   I suspect a relation between the cooling time constant and the   
   periods of an oscillator, and the distortion.   
   A rule could be   
   if you need N periods to have the distance to the   
   the stable wave halved, you have circa 1/N distortion.   
      
   Is this known territory?   
   >   
   >Your pal,   
   >   
   >CD.   
      
   Groetjes Albert   
   >   
   >   
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