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

[Didier Juges]

An illustration of what I meant would be what happens when someone opens the door to the lab where the GPSDO is located. The room temperature may change by a few degrees over a few minutes time, causing the oven to kick in (or out). That kind of change is too fast for the GPS to compensate for, and the increased (or decreased) oven current will drive a frequency change.

Slow diurnal changes would be compensated for, but as Said commented, the loop can only compensate for an error it sees. If there is no error to begin with, there is no error to compensate for.

Didier

---- Tom Van Baak <tvb at LeapSecond.com> wrote: 
> > 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
> 
> 
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