[time-nuts] Correcting jitter on the 1 PPS signal fromaGPSreceiver.

[Peter Reilley]

My rise time is about 4 nS.   I am measuring that with my 200 MHz
scope.   I am only using 50 Ohm termination, anything else is not
valid when using 50 Ohm coax. 

Pete.
 

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Tom Van
Baak
Sent: Saturday, September 13, 2014 10:34 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Correcting jitter on the 1 PPS signal
fromaGPSreceiver.

> The cables are not exactly the same lengths.   Differences in length
> will result in a fixed offset.   I am not concerned about such fixed
> errors, only jitter.
> 
> I am comparing the rising edges which is what the spec defines as the 
> reference edge.
> 
> Pete.

Pete,

Correct, the survey position is determined only by the phase center of the
antenna, not by cable length. And cable length mismatches should make no
difference in your jitter measurements.

But one thing to check is how sharp the 1PPS rising edge is -- right at the
input to your TI counter. I use a BNC tee with one leg open allowing a
'scope check (set to 1M input). If your risetime is a couple of ns like mine
is, then all is well. Slow risetime can be a huge source of timing jitter.
Check both 50R and 1M at the counter input. Use DC, not AC coupling. Use
fixed trigger, never auto-trigger. Pick a trigger level that matches the
maximum slope.

Some examples of good/bad GPS 1PPS risetimes:
http://leapsecond.com/pages/gpsdo-rise/

/tvb


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