[time-nuts] frs-c rubidium osc.

george george_margetts at nss.net
Wed Mar 16 20:32:56 EST 2005

Oops I have I little more info Yes external power is pure and stable.
I've monitored my efc with analogue vtvm at 8 volts the manual states
normal efc range to be 4 to 8 volts I'm wondering if the quartz osc has
aged near the limit of normal efc range. However upon power up I see the
efc slew from 12 down to 1 volt then back to 12 and lock on the second
sweep at 8 volts every time which almost seems premature. Unit always
locks at about 2 minutes.
My external frequency adjust wiper is at .675 volts.
I think my unit has minor problems other than ambient temp allthough at
around or above 91.9 f it becomes very stable +/- 50 nanoseconds over
several hours..

Brian Kirby wrote:
> I know Rex and myself have them.  I also have a FRK-L and a mutate FEI5680A.
> When I got my FRS-C, back in December, I put it together in a permanent
> package and I let it run for a month before I ran any test on it.
> Efratom did not warrant any specs until the unit is burned in.  Looking
> at yesterdays raw data and my unit was tweaked on frequency 4 days ago,
> it had a drift of  311 nanoseconds for 24 hours.
> My unit is frequency controlled by an external 1 kilo-ohm 10 turn pot.
> When an external pot is hooked up to the unit, it overrides the internal
> frequency control.  The top of the pot is connected to pin 8, the bottom
> of the pot to pin 3 (ground) and the wiper to pin 2 of the FRS-C
> connector.  In this method I see 6.9138 volts on the top of the pot from
> the rubidium and its solid, no variation (using a Fluke 45 voltmeter).
> A month ago I did a check and at 6.9 volts the rubidiums frequency was
> +2.709x10-9, and at zero volts is was -1.052x10-9 in respect to UTC.
> Currently its on frequency at 2.1048 volts.
> Is your meter stable enough to monitor the EFC ?  Is your power tightly
> regulated to the unit ?  Is your ambient temperature stable ?  Does the
> unit have an adequate heat sink ?
> -----
> I am also working on using this unit with Brooks Shera's GPS controller
> - I had to build a stable amplifier and voltage reference to interface
> with his controller.   My circuit uses a LM399 voltage reference and
> OP07 op amps and this regulator is more stable than the circuitry in the
> FRS-C.  It may be overkill.  The FRS-C unit (mine) has a 1 volt
> sensitivity of 5x10-10, so 100 mV would be 5x10-11, 10 mV 5x10-12, and
> maybe 1 mV 5x10-13.
> Also, I have read that the FRS-C is not as stable as the older FRK-L and
> it has to do with the way the rubidium cells are manufactured - the new
> process is considerably more cheaper.
> Brian - N4FMN
> george wrote:
> >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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> >
> >

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