[time-nuts] Linear voltage regulator hints...

[dan at irtelemetrics.com]

     Hi Bob,
  Some numbers. Maybe you can double check my math, just to be sure I'm 
not getting something completely wrong. That is very possible, since 
I'm new here... ;) 
  The DAC is moving up and down about 7.5 counts with room temp 
swings. 20 bit resolution at 6.6volts full scale output. 
  6.6 volts * 7.5counts / (2^20) =  +/- 47uV.  (This is verified as 
reasonable with a 24bit data logger, as it's seeing about +/-50 uV temp 
swings on EFC. Resolution of about 1uV.)
  Tuning sensitivity of the oscillator is 1Hz/10Volts. Or 47uV * 
1Hz/10V = +/- 4.7uHz. 
  The temp swing is +/- 2 degF with ~45 minute period. So, in the 
ballpark of your +/- 1 Deg C guess. 
  +/-4.7uHz / 10e6 Hz oscillator = +/-4.7e-13, or near a 1e-12 full 
swing over 2.2 Deg C. (Or, am I completely out to lunch here???) 
  I should qualify, there is aging/retrace here. It's in the range of 
3e-11 per day right now, and I took the +/- 7.5counts off of what was 
left after removing the slope of the aging drift. The aging looks huge 
over a day compared to the thermal cycling. 
  Currently the system has ~2ppm/C reference, and .04nV/C opamps. So, 
Yeah a little overkill. But these things are getting cheap nowadays, so 
why not? Before the 'good' reference and opamps, there was about 10 
times as much swing in the PWM DAC over temp cycles. As you have 
suggested there is probably some room to 'relax the spec', and still be 
fine... 
   
  Thanks,
  Dan
   
   
   
   
   
   
  >
  > Hi
  >
  > If your OCXO has a stability of +/-3x10^-10 over 0 to 60C (numbers 
picked to make math easy):
  >
  > 6x10^-10 / 60 = 1x10^-11 per C
  >
  > If the OCXO is 10X better than that (unlike) you are at 1x10^-12/C
  >
  > If the room temperature swings 1 C, you get a 1x10^-11 swing in the OCXO. 
  >
  > Keep in mind that the OCXO also responds to things like gradients 
as much as it does to absolute temperature. 
  >
  > If your EFC range is 1x10^-7 for 5V (pretty common):
  >
  > Each 1 mv change is 2 x10^-11. 
  >
  > A 78L05 will hold 1 mv over 1C. Roughly 90% of them will hold that 
over 20C. That’s a cheap regulator for 2x10^-12. 
  >
  > A 10 ppm / C reference will get you to 1x10^-13 / C
  >
  > You don’t *need* an EFC at 1x10^-7. Something 1/10 that size is 
probably good enough. Knocking it down to that level is just a couple 
of resistors. Way less money than fancy references. 
  >
  > Bob
  >