[continued from previous message]   
      
   >> If I wanted to build a very low distortion 1k Hz oscillator then I'd build   
   the circuit below.   
   >> I'd be happy to see 90dB in reality if that could be measured.   
   >> If better than 90dB can be measured then that's fine but the expense of   
   doing so will likely   
   >> start to increase exponentially.   
   >>   
   >> Bill will be along soon to tell us who designed what and how he did this or   
   that but we   
   >> all know that anyway.   
   >>   
   >> I don't claim any credit for this circuit except for the Q1 Q2 circuit   
   which I arrived at   
   >> by doing online research into how it might be implemented. AS3944 might be   
   a good   
   >> choice for that circuit.   
   >> (This won't stop Bill, for the nth time, pointing out how further   
   refinements were added   
   >> but who cares.)   
   >>   
   >> I wanted to see whether it could be done without thermistors, opto devices,   
   lamps, FETs   
   >> or similar devices. And also without large capacitors. After all if you   
   wanted to implement   
   >> it all on a chip then you don't want those kinds of components in the   
   circuit if you can   
   >> avoid them. And none of the FET based circuits I ever saw could match the   
   LTSpice   
   >> performance of this circuit.   
   >>   
   >   
   > 16 to 20-bit DACS are available. Given a sine lookup table in ram, one   
   > could fine tune it for sub-PPM distortion. Or use a 16-bit DAC and sum   
   > in another 16-bit DAC that nulls the distortion, even the distortion   
   > of output amps or whatever.   
   >   
   > The problem is how to measure the distortion to close the loop.   
      
      
   About 25 years ago I used a 20-bit dac followed by a 3rd order low-pass   
   filter with a corner frequency of around 1.5kHz in a test oscillator   
   for evaluating a 16-bit codec.  The circuit was dead-bug hand wired on   
   a sheet of FR4.  I never discovered exactly how good the oscillator was,   
   but the combined response of the test oscillator and 16-bit adc gave   
   about 80dB SINAD.   
   The results are in fig 4-1 on page 36 of the following pdf.   
      
   https://www.ti.com/lit/ug/slau039/slau039.pdf   
      
   The op-amps in the signal path were TLC2272, chosen because TI wanted   
   to promote them at the time.   
      
   John   
      
      
   >   
   >   
   > John Larkin   
   > Highland Tech Glen Canyon Design Center   
   > Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)