[time-nuts] 1PPS for the beginner

[jimlux]

On 8/14/18 12:29 AM, Mike Cook wrote:

>> Le 14 août 2018 à 04:29, Chris Caudle <chris at 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)