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

[Said Jackson]

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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