On 8/14/18 12:29 AM, Mike Cook wrote:
Le 14 août 2018 à 04:29, Chris Caudle <ch...@chriscaudle.org> a écrit :
On Mon, August 13, 2018 9:16 pm, Chris Burford wrote:
I have a (generic?) GPSDO which contains an Oscilloquartz STAR 4+ OCXO
that I am using to steer a PRS10 RFS. I'm a little confused on where the
1PPS is coming from with respect to the GPSDO.
As Chris points out the 1PPS from a GPSDO will « generally » be derived from
the primary frequency and can show better performance than directly from a GPS
receiver.
However this is becoming less and less true.
If you look at the Oscilloquarz blurb for the Star 4+ ( I found some here
<http://pdf.directindustry.com/pdf/oscilloquartz-sa/star3-4/62169-330779.html#search-en-oscilloquartz-star-4>
) , you will see that the phase stability (jitter) on the 1PPS output is +/- 30ns
when locked to GPS, an it has a timing grade GPS receiver. This is not as good as
other GPS modules now. 15ns is normal, with some less than half that.
The PRS10 has outstanding PLL control already. The SRS product doc gives +/-
10ns accuracy with +/-1ns resolution.
I don’t think that you are buying much with disciplining the PRS10 with a GPSDO
1PPS. Do you have any TIC measurements in this config to compare with a direct
GPS 1PPS feed?
I think the 1pps uncertainty spec for a receiver is more a function of
how they generate the 1pps - particularly if it's basically the period
of the internal clock. A tight spec might just mean they've got a high
frequency clock.
The spec is probably also for a test condition where the GPS signals
into the receiver are perfect. In real life, there's the ever changing
multipath, ionospheric scintillation, etc..
So I can imagine a receiver with very good performance on perfect
signals, but with poor "averaging", so that in a real signal
environment, the 1pps varies quite a lot on a pulse by pulse basis.
Likewise, I can imagine a receiver with a fairly large 1pps uncertainty
spec, but since the oscillator it is derived from is fairly stable and
high quality, over the long term, it tracks a 1pps more precisely and
predictably (e.g. the receiver puts out offset/sawtooth correction data)
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