[time-nuts] GPSD-Rb

Bob Camp lists at rtty.us
Wed Aug 22 12:03:21 EDT 2012


Hi

The problem you run into with this approach is that it is relatively high
jitter. You output "jumps" by what ever your microcontroller step time step
size is every so often. Since that's going to be 10's or possibly 100's of
ns, it's a major hit.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Michael Tharp
Sent: Wednesday, August 22, 2012 10:47 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPSD-Rb

On 08/22/2012 01:22 AM, Edgardo Molina wrote:
> b. Is it possible to build a GPSDRb? I would like to know if it is
reasonable to pursue the goal to discipline the 5065a with a TB which I also
got recently.

Some Rbs have a "C-field" input that can technically be used to 
discipline it, but this is not the approach I am going to take with 
mine. A Rb oscillator is internally an OCXO that is disciplined to the 
Rb physics package, with the goal of holding a frequency over a long 
period of time with the only variance coming from deficiencies in the 
measurement circuitry. Using the C-field to discipline it is kind of 
throwing out the long-term stability characteristics of the Rb and using 
it as an expensive OCXO. It's more interesting to me as a holdover 
source for a conventional GPSDO.

The design I'm thinking of is to have a separate microcontroller clocked 
by the Rb that will generate a third pulse-per-second (the first two 
being the raw one from GPS and the divided-down local oscillator). As 
long as the GPS is locked the divider will not output pulses but will 
monitor the pulses from the local oscillator and use it to count the 
frequency of the Rb. Once lock is lost or holdover is manually engaged, 
then it stops counting and starts outputting pulse-per-second based on 
the last known average frequency it counted. The GPSDO would then 
continue normal disciplining based on the Rb pulses until GPS lock 
returns. Of course while locked the measured frequency would also be 
reported so that the Rb could be calibrated in situ.

-- m. tharp

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