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

[Bruce Griffiths]

Tom Van Baak wrote:
> Bruce,
>
> Thanks. Here's my math. An average 10 MHz OCXO has an
> EFC range of, say, 1e-7 over 10 V. That's 1e-8/V; 1e-11/mV.
>
> Now look at the short-term frequency error of a Fury:
> http://www.leapsecond.com/pages/fury/log31837v.gif
> ( from http://www.leapsecond.com/pages/fury/ )
>
> You see Fury frequency varies by at least 5e-11 over minutes,
> as is normal for this class of OCXO. To make that much noise
> with EFC effects alone you'd need on the order of 5000 uV
> variation on EFC line. Another reason why I think the single
> ground pin question is a non-issue.
>
> The steady-state oven current variations are quite small. See
> this plot, for example:
> http://www.leapsecond.com/pages/fury/fury-oven.gif
>
> Said, have you measured the ground return loop resistance on
> a Fury? We can do the rest of the calculations based on that.
>
> /tvb
>
>   
Tom

For a single (or other) oven OCXO with finite thermal gain the change in
oven temperature due to changes in heater power are, for small power
fluctuations at least, proportional to OCXO heater current fluctuations.
Thus it is, in principle at least, possible to use the fluctuations in
OCXO power supply current to correct for both the effects of finite
thermal gain and ground voltage variations.

However its probably difficult to impossible to separate the two effects
from ambient temperature fluctuations and the associated OCXO current
and frequency fluctuations.

The real question is off course how large a temperature fluctuation is
required before such effects become significant?

If one makes a few crude approximations:
The MTI 260 series frequency varies by around 1E-9 over a -30 to 70C
temperature range corresponding to a tempco of around 1E-11/C.
Typically the ADEV at 1s is about 2E-11 or so, so such correction is
perhaps only useful for temperature fluctuations > 1 C or so.
Even then such correction is only useful if the temperature fluctuations
are slow enough for the OCXO to track them, whilst the temperature
fluctuations are too fast for the disciplining loop to track them.

Bruce