[time-nuts] Anybody want a Thunderbolt power supply?

Attila Kinali attila at kinali.ch
Fri Oct 21 11:05:10 EDT 2016

On Fri, 21 Oct 2016 06:14:03 -0700
Nick Sayer via time-nuts <time-nuts at febo.com> wrote:

> Well, because it's easily an order of magnitude more expensive than a 7912. $5 instead of 50¢ (Q:1).
> If it *matters*, then fine, but I am sensitive to cost efficiency in addition to efficacy.

Sorry, I've been hunting ps level delay uncertainties in the last weeks
and kind of forgot that not everyone does things to that extreme.

If you want to go for cheap LDOs, I would recommend going for the
modern variants available from Ti and Linear. Those have much less
noise and drift than the 79xx and 78xx families and are also zero
load impedance stable (ie can be used with ceramic capacitors).
But these are already all in the 1-3USD range if you want to supply 800mA. 
Ie they are at least a factor 2 more expensive than an 78xx.

> If you put the board in a box in a stable temperature environment
> (which I'd kind of assume you'd do if you cared about temperature stability
> generally), then how far do you really have to go?

Let's put a few numbers here:
The stability of a cheap OCXO (SCOCXOHS by Microcrystal) is specced
as <5*10^-7/V. An 7812 (LM7812 by Fairchild) is specced to 0.8mV/°C @ 5mA.
Assuming that using 500mA output makes the Voltage drifft worse by a factor
of 10 (i hope this conservative enough) we are at 8mV/°C. Ie. we get a
total, temperature to supply voltage induced frequency dependence of 4*10^-9/°C.
This is quite noticable. Compare this to the 75ppb over 0°C to 60° of
the temperatur spec of the SXOCXOHS. I.e. we are suddenly adding as much
temperature dependence to the OCXO trough the power supply as it has itself.
Even if you discard the factor 10 for the 7812, that's still 4*10^-10/°C.

Using a more expensive OCXO (AOCJY4 by Abracon) gets you to 3*10^-11/°C
to 3*10^-12/°C, still quite noticable for a GPSDO and again in the order
of its intrinsic temperature coefficient.

And these calculations are only for the temperature of the LDO,
ie they assume an otherwise stable environment. If you start accounting
for drafts of air and changes in humidity, then things become even worse.

Also keep in mind, that unless you start putting cardboard boxes around
everything, then changes of 2-3°C within 10s is pretty normal when you
have people opening/closing doors/windows.

So.. does all this matter? Maybe, maybe not. :-) It depends on what you
actually do with the LDOs. If you put them in a metal box, maybe
with an cardboard box around it, then you dampen the temperature variations
quite a bit and give the GPSDO and its control loop  a chance to compensate.
If you leave them in the open, possibly with an heatsink attached, then
you definitely want to have a lower temperature coefficient.

			Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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