[time-nuts] Fury Interface Board: 5MHz needed?

[Tom Van Baak]

> I believe the problem is that the EFC closed loop has a significant time
> constant, so any current variation that is faster than the EFC loop time
> constant will induce an uncorrectable error (at least as far as the EFC loop
> is concerned). With a separate ground pin, or a circuit designed to
> compensate for the effects of ground pin current, there will be no such
> error.
> 
> Didier KO4BB

Thanks for that explanation.

Note thermal effects have a time constant as well. Depending
on the OCXO, or the enclosure used, it may be shorter; more
likely it's actually longer than the GPSDO time constant. This
is especially true for diurnal temperature changes where the
thermal TC is 100x to 1000x slower than the GPSDO TC!

I understand what you're saying, though. However, in your
scenario of a more rapidly changing temperature, I suspect the
frequency changes due to the OCXO tempco itself (*internal*
resonator temperature, thermal lag(s), gradients, etc.) will far
exceed any changes in *external* EFC due to changes in oven
heater current.

You might check this yourself by shorting the EFC pin(s) and
then run some rate dependent temperature tests.

Another check someone could run is to deliberately place a
1R or 10R or 100R carbon resistor in series with the EFC and 
see what it takes to make this effect rise *above* the noise.
With these measured points in hand, you could then extrapolate
down to 0.01R or 0.001R (PCB trace) to see how far *below*
the noise you are in real life.

/tvb