[time-nuts] Correcting jitter on the 1 PPS signalfrom a GPS receiver.

Fuqua, Bill L wlfuqu00 at uky.edu
Mon Sep 15 02:04:15 EDT 2014


A lot of devices have a low output impedance so that the signal can be split using a TEE adapter with little loss or need for a distribution amplifier.
However, the cables must be impedance matched at far end, scope input, to prevent reflections which are the source of the ringing. 
You can match the impedance at the source and you will get a reflection which will then be absorbed by the source resistance. One way to do this
is to get a small 15 turn pot about 100 Ohms put it, in series with the input source and adjust it until the ringing is gone or you can put it at the far end
,input of the scope, to ground and do the same.  But the best solution is to get a good feed thru 50 Ohm terminator and put it on the input of the scope.
Bill  

----------------------------------------------------------------------

Message: 1
Date: Sun, 14 Sep 2014 09:58:54 -0700
From: Said Jackson <saidjack at aol.com>


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.


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Today's Topics:

   1. Re: Correcting jitter on the 1 PPS        signalfromaGPSreceiver.
      (Said Jackson)
   2. Re: Correcting jitter on the 1 PPS        signalfromaGPSreceiver.
      (Peter Reilley)
   3. Re: Correcting jitter on the 1 PPS        signalfromaGPSreceiver.
      (Said Jackson)
   4. Fwd: Correcting jitter on the 1 PPS       signalfromaGPSreceiver.
      (Said Jackson)


----------------------------------------------------------------------

Message: 1
Date: Sun, 14 Sep 2014 09:58:54 -0700
From: Said Jackson <saidjack at aol.com>
To: Peter Reilley <peter at reilley.com>
Cc: Discussion of precise time and frequency measurement
        <time-nuts at febo.com>
Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
        signalfromaGPSreceiver.
Message-ID: <A77B8502-096E-4D03-864E-4A63AF3A9C9E at aol.com>
Content-Type: text/plain;       charset=us-ascii

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
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>


------------------------------

Message: 2
Date: Sun, 14 Sep 2014 12:04:57 -0400
From: "Peter Reilley" <peter at reilley.com>
To: "'Said Jackson'" <saidjack at aol.com>, "'Discussion of precise time
        and frequency measurement'" <time-nuts at febo.com>
Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
        signalfromaGPSreceiver.
Message-ID: <4BE3954409F947CDB0EA6BEEBC7BCFF6 at Win7>
Content-Type: text/plain;       charset="US-ASCII"

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
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.



------------------------------

Message: 3
Date: Sun, 14 Sep 2014 10:11:25 -0700
From: Said Jackson <saidjack at aol.com>
To: Peter Reilley <peter at reilley.com>
Cc: Discussion of precise time and frequency measurement
        <time-nuts at febo.com>
Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
        signalfromaGPSreceiver.
Message-ID: <27E4AB2F-82A2-4EE3-8B97-B01C67FBA03E at aol.com>
Content-Type: text/plain;       charset=us-ascii

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
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>


------------------------------

Message: 4
Date: Sun, 14 Sep 2014 10:12:49 -0700
From: Said Jackson <saidjack at aol.com>
To: Discussion of precise time and frequency measurement
        <time-nuts at febo.com>
Subject: [time-nuts] Fwd: Correcting jitter on the 1 PPS
        signalfromaGPSreceiver.
Message-ID: <E3DB52E0-A0B4-4F5B-B478-3E9141B4BB49 at aol.com>
Content-Type: text/plain;       charset=us-ascii


> 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
>>>>
>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>


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