[time-nuts] GPSD-Rb

Azelio Boriani azelio.boriani at screen.it
Wed Aug 22 12:41:09 EDT 2012


Exactly. Every GPSRb works by adjusting the C-field: sort of let me (the
algorithm/GPS pair) calibrate your Rb for the long term (aging) variation
that every Rb has. The Rb is not good in the long term (100000 seconds),
not as good as a working and receiving GPS receiver. The Rb (as a whole)
*is* an expensive OCXO that has good short term stability, paired with a
disciplining algorithm and a GPS it becomes like a Cs (and maybe even
better in the short term).

On Wed, Aug 22, 2012 at 6:03 PM, Bob Camp <lists at rtty.us> wrote:

> 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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