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

[Said Jackson]

Peter,

I should mention that all you need to avoid amplitude-modulating your 10MHz signal by the 1PPS DC return current and pumping massive current through your coax and termination resistor is to add a 41 to 47 Ohms series resistor to your 1PPS source as close to the driver as possible... It is really that simple.

Bye,
Said

Sent from my iPad

On Sep 14, 2014, at 9:48, "Peter Reilley" <peter at reilley.com> wrote:

> 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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