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

[Said Jackson]

Ok, did the math, a 4ns risetime should be ok on a 200MHz scope.

You likely won't see the oscillations and reflections visible in Toms 58503A plots for example, they are faster than the risetime.

Bye,
Said

Sent From iPhone

> On Sep 14, 2014, at 8:35, Said Jackson via time-nuts <time-nuts at febo.com> wrote:
> 
> 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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