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

Peter Reilley peter at reilley.com
Sun Sep 14 16:48:01 UTC 2014


I don't want to repeat an old discussion.   You certainly can
use unterminated coax as long as you are aware of the ringing
issues and the possibility of false triggering.

I am using a terminator on the scope end of the cable.   I can
see ringing similar to what is shown on the leapsecond web site
when I don't terminate.   With termination the waveforms are
also similar so I am comfortable that my cables and terminators
are working as they should.   My scope does not have a built-in
terminator.

Is there a good way of searching the archives? 

My scope is a Rigol DS2202 200 MHz scope that samples at 1 GSP
in two channel mode.  I understand the problems that you can run
into as you approach the sampling limits of digital scopes.

Pete.
 

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Said
Jackson via time-nuts
Sent: Sunday, September 14, 2014 11:36 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
signalfromaGPSreceiver.

Peter,

You don't need nor do you want a 50 ohms end-termination on a
series-terminated 50 ohms coax cable.

This has been discussed here extensively before, please check the archives.
Your last sentence is not correct.

Also, you are running into your scope's BW limit if you are measuring a 4ns
risetime with a 200MHz scope..

Bye,
Said

Sent From iPhone

> On Sep 14, 2014, at 6:19, "Peter Reilley" <peter at reilley.com> wrote:
> 
> 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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