[time-nuts] Making the most of SRS Rb source

David Forbes dforbes at dakotacom.net
Wed Jan 18 11:17:31 EST 2006

At 6:43 AM -0800 1/18/06, James Maynard wrote:
>Christopher Hoover wrote:
>>  If you've got the bucks and/or can find a deal, the Stanford Research PRS-10
>>  is a nice Rb oscillator that can be disciplined from an external 1 PPS
>>  signal.  This is what I am running, using a Motorola M12+ timing receiver to
>>  feed it.
>I can second that! In my previous post, I described what I did back in
>the seventies to calibrate a 10 MHz oscillator against the local Loran-C
>ch ain.  However, the PRS-10, disciplined with the M12+ is what I use today.

I'm interested in this feature.

The radiotelescope I work on has a Stanford Research FS725 rubidium 
source in its station clock rack. We also have an SR626 counter 
connected to a data logger, a Datum 9390-ish GPS receiver and an HP 
5087A distribution amp. Oh, and an old Rohde & Swartz crystal 
oscillator with matching LED clock.

Amusingly, the GPS receiver is NOT feeding the FS725 Rb source. 
Instead, the 10 MHz of the FS725 is divided down to 1PPS by the R&S 
clock and phase-compared against the GPS 1PPS by the SR620 counter. 
The result is about one microsecond P-P phase jumps, and some drift 
over time since the Rb source is free running.

Our need for accurate time/frequency is:
1) Local oscillator frequency +/-1kHz at several hundred GigaHertz;
2) Time correct to +/-10 milliseconds to keep the telescope pointed correctly.

I suppose it would make a lot more sense to run the GPS 1PPS into the 
Rb source so that it will discipline itself, and the phase error be 
monitored to make sure it stays at zero.

Am I barking up the right tree here?

--David Forbes, Tucson, AZ

More information about the time-nuts mailing list