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

Bob Camp kb8tq at n1k.org
Sun Sep 14 14:03:30 EDT 2014


Hi

If you are seeing modulo 2 ns jumps, then there is something in your setup causing them. The Res-T can only put out something about every 40 ns. 

That edge is locked to nothing at all, so it does drift. If your particular 12.504 MHz clock is off by 1 ppm, then it’s edge will drift 1 us per second. That will get quantized modulo 40 ns and you will get a new edge. If your clock is within 1 ppb it will drift 1 ns  per second and you will see very little change due to drift. Unless you have an OCXO on your unit, the 1 ppm is a better guess than the 1 ppb.

Bob

On Sep 14, 2014, at 12:38 PM, Peter Reilley <peter at reilley.com> wrote:

> Thank you for that link.   I read the section of quantization error.
> I am confused with what I am seeing on my scope verses what they are saying
> 
> They say that the system clock is 12.504 MHz and that they use both the
> rising and falling edges.   That is about 40 nS between quantization time
> slots.   The PPS can only appear on a 40 nS edge.   I should be seeing 
> 40 nS jumps in the waveforms.   I do see ~40 nS jumps but they are less 
> common.
> 
> The waveform timing is clearly quantized but I am seeing ~2 nS jumps.
> My scope is a Rigol DS2202 which samples at 1 GSP in 2 channel mode.
> Could this 2 nS quantization be a result of the scope?   Perhaps I should
> get my 400 MHz analog scope out (Tek 2465B) and repeat the measurements?
> 
> Pete.
> 
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob Camp
> Sent: Sunday, September 14, 2014 11:30 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
> signalfromaGPSreceiver.
> 
> Hi
> 
> If you look at:
> 
> ftp://ftp.trimble.com/pub/sct/embedded/bin/Manuals/Old%20Manuals/Resolution%
> 20T%20Final%20Final.pdf
> 
> (first hit on Google for Resolution T manual, no other reason for picking
> it).
> 
> In the index, "PPS Quantization Error" is on page 46.
> 
> On page 46 they talk about the +/- 20 ns error from sawtooth and how to
> correct it out.
> 
> Page 47 shows the plot with the sawtooth still in the signal. Page 48 shows
> the plot with it removed.
> 
> Bob
> 
> 
> 
> On Sep 14, 2014, at 9:19 AM, 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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