[time-nuts] frs-c rubidium osc.

george george_margetts at nss.net
Wed Mar 16 14:13:11 EST 2005

Anybody have experience with ball efratom frs-c oscillators?

Are these any good?  I purchased one on ebay and have been waiting for
about a week to see good stability.. AT 10k samples against gps 1pps
indicate 5-11th  accuracy.. which is exactly at spec.. I was hoping for
something close to 3-12th.

To be more specific with my line of questions;
This standard is adjusted with precision 20 turn pot supplied from in
internal 17 volt requlator which I think isn't that stable of a voltage
source because the manual shows and alternate customer provides variable
5 volt source for the adjustment (these adjustments vary the C-field).
The internal adjustment pot has voltage divider resisters that bring the
semi-stable 17 volt source down to around 5 volts to the adjustment pot.

I've been monitoring the wiper voltage day by day I see .05 volt
difference and think that this variation is effecting my stability.
Its only changing day by day not hour to hour.

This wouldn't be a big deal to provide my own stable 5 volt supply to an
external adjustment pot.. but further investigation of the schematics
indicate internal temperature compensation circuitry is varying the
C-field as well.. along with another 17 volt source to the C-field via
Select In Test Resistor (the manual calls fixed C-field).

Is it customary for Atomic standards to vary the C-field to obtain
compensate for internal temperature variation?

Cant the closed loop servo handle these temperature/frequency errors
alone.  Without the need of additional circuitry trying to compensate
via the C-field ?

Or is the Closed loop servo a low resolution adjustment and maybe the
C-field a better resolution adjustment?
Or maybe efratoms just have low resolution servo loops and require
messing with C-field.. How can I set the C-field manually if the
internals are playing with the c-field to..

What would you do?
A. Disable internal c-field temperature compensation.

B. Run external stable 5 volt sourced C-field trim pot.

C. Wire spare pin on interface to provide external 17 volt supply that
feeds C-field trim pot and fixed C-field supply?
D. All the above..

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