Hi > On Dec 11, 2014, at 8:20 PM, d...@irtelemetrics.com wrote: > > 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.
or 1x10^-8 per volt. If it’s a 10 MHz OCXO. That would be 1x10^-14 per uV 4.7 x 10^-13 for 47 uV > The temp swing is +/- 2 degF with ~45 minute period. So, in the ballpark of > your +/- 1 Deg C guess. That’s pretty normal for a modern HVAC system. > +/-4.7uHz / 10e6 Hz oscillator = +/-4.7e-13, or near a 1e-12 full swing over > 2.2 Deg C. 1x10^-12 for full swing is about right. > (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. That’s a very common (and legit) thing to do. > 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. What makes you believe that the OCXO’s temperature performance it not the issue? 1x10^-12 per 2 C -> 1x10^-10 over 100 C (say -30 to +70C). That’s a very good spec on an OCXO. Also consider that gradients could easily amplify the impact by 2X or more. > 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… I’d guess that the analog stuff is much better than it needs to be. Bob > 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 > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
