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

[Peter Reilley]

I tried removing the termination and got a little better than 4 nS
risetime.

Isn't the ringing frequency simply a function of the length of
the coax?   Isn't it the price you pay for mismatched impedances?

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 11:43 AM
To: Said Jackson; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
signalfromaGPSreceiver.

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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