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

Said Jackson saidjack at aol.com
Sun Sep 14 12:58:54 EDT 2014


Peter,

That depends. To use 1M Ohms input impedance, you need a 50 Ohms series impedance at the driver chip. Most sources such as the 58503A and Thunderbolt violate that requirement by having only a couple of Ohms output impedance, and are thus not suitable and do need the 50 Ohms termination at the scope least you get horrible ringing as shown in Tom's plots from yesterday.

However that means you are pumping up to 100mA through your coax, and scope termination. That makes your coax ground jump many 10's of millivolts (depending on cable length and quality). This IR induced ground jump now also shows up on your 10MHz coax and messes with that signal, as the 1PPS return current partially goes through the 10MHz coax shield and generates a voltage rise on the shield. It's a cluster....

You can take a multimeter and actually measure the voltage drop on your coax cable shield from one connector to the other. On units with longer 1PPS pulse you see the multimeter twitch once per second (Symmetricom XLI for example) even on a short 1m cable.

But if you look at Tom's plots you see that there is some high frequency ringing on the 58503A 1PPS when terminated into 1M, I am not sure thats coming from cable reflections. For those high frequency rings a 1G scope may be better to see what's really going on in the driver.

Think about it this way: why would you want to drive a 50 Ohms coax with a 5 Ohms output impedance? That's an absolutely horrible impedance mismatch. But that is what the Trimble Thunderbolt does, and likely also the Resolution-T.. Resulting in ringing up to 10V on your cable.

Bye,
Said


Sent from my iPad

On Sep 14, 2014, at 9:04, "Peter Reilley" <peter at reilley.com> wrote:

> 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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