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

Bob Camp kb8tq at n1k.org
Thu Dec 11 20:47:59 EST 2014


> On Dec 11, 2014, at 8:20 PM, dan at 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.


> 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
> > 
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