Hi Separate from the analysis of the voltage on the OCXO, there is another part to this:
Ok, so why am I harping on the “need” for all this from a system standpoint ? 1) Adding stuff to a design that does not make it measurably better is simply a waste of money. That’s ok if it’s your money. 2) Others read these threads and decide “maybe I need to do that..”. Now it’s a waste of somebody else’s money. 3) Still others look at this and decide “If I need to do that, I’m not even going to start”. That’s not good either. 4) Analysis *is* part of the design process. It should very much be part of this. 5) Focusing on a design aspect “because I can” is a very common thing. I do it all the time :) Because I fall into the trap often, I recognize just how much time gets soaked up on dead ends this way. 6) There are very real issues when doing this. Sorting out what’s real and what isn’t is far from easy. The more “noise” in with the signal, the less likely others are to figure out a good approach. I’m quite sure this thread will keep going for quite a while on references. At some point we will be talking about 1 pV / C 1KV designs with 0.1 nV/Hz noise levels. It worries me that people may believe that in some way that applies to a time or frequency standard …. Bob > On Dec 11, 2014, at 8:47 PM, Bob Camp <kb...@n1k.org> wrote: > > 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.
