[time-nuts] frs-c rubidium osc.

Brian Kirby kirbybq at bellsouth.net
Wed Mar 16 16:32:12 EST 2005

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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