[time-nuts] Capturing a 1PPS signal with a Keysight 53230A

[Peter Membrey]

Hi guys,

I'm looking to measure and characterize the 1PPS output of a uBlox MAX M6Q using a Keysight 53230A. Specifically, I'm mostly interested in the period of the signal, how close it is to exactly 1Hz and how that signal changes over time.

The Keysight is using the 10Mhz output of a SRS FS-725 Rubidium Frequency Reference (which is being disciplined by a PPS from a microsemi S600 GPS) as an external reference.

I've currently got the channel configured as follows:

* DC
* 50Ohm
* Bandwidth filter enabled

The default gate time is 100ms, which if I leave it configured for that, it will never show a reading. I can get it to start giving a reading if I set the gate time to 3 seconds but for reliable readings, I need to set it to at least 5 seconds.

So, here's my questions:

1. I would like to capture the period of each pulse - is this doable with my set up?
2. What does gate time actually mean? I've read a number of descriptions, but I'm still not 100% clear on what it means in this context and what impact it has
3. How exactly does the gate time work in this case? I mean if it captures data over 5 seconds and gives one reading - what actually is that reading? An average? One of the samples?
4. Should I be doing anything differently?

Ideally, what I'd like to do is capture the period of each pulse individually so that I can do an Allan variance on the data over a period of time.

I've googled around and I've read numerous guides, but they mostly seem to cover faster signals (like 10Mhz sine wave) rather than the much slower PPS. I asked a very similar question on the EEVBlog forum, and the suggestion was to use time-stamp capture with the rise time, but unfortunately, it never seems to see the signal, even though the gate indicator flashes every second.

Thanks in advance for any advice or guidance!

Kind Regards,

Peter Membrey
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[Anders Wallin]

4.
With 1PPS you probably want to run it in time-interval mode and measure the
phase difference of two clocks: have it start on CH1 with (say) the 1PPS
from your microsemi, and stop on CH2 with your uBlox.
You should still have it on ext-REF with 10MHz from the Rb-clock.

gate time refers to measurements in frequency counter mode, usually not
used with 1PPS signals.

Post by Peter Membrey
Hi guys,
I'm looking to measure and characterize the 1PPS output of a uBlox MAX M6Q
using a Keysight 53230A. Specifically, I'm mostly interested in the period
of the signal, how close it is to exactly 1Hz and how that signal changes
over time.
The Keysight is using the 10Mhz output of a SRS FS-725 Rubidium Frequency
Reference (which is being disciplined by a PPS from a microsemi S600 GPS)
as an external reference.
* DC
* 50Ohm
* Bandwidth filter enabled
The default gate time is 100ms, which if I leave it configured for that,
it will never show a reading. I can get it to start giving a reading if I
set the gate time to 3 seconds but for reliable readings, I need to set it
to at least 5 seconds.
1. I would like to capture the period of each pulse - is this doable with my set up?
2. What does gate time actually mean? I've read a number of descriptions,
but I'm still not 100% clear on what it means in this context and what
impact it has
3. How exactly does the gate time work in this case? I mean if it captures
data over 5 seconds and gives one reading - what actually is that reading?
An average? One of the samples?
4. Should I be doing anything differently?
Ideally, what I'd like to do is capture the period of each pulse
individually so that I can do an Allan variance on the data over a period
of time.
I've googled around and I've read numerous guides, but they mostly seem to
cover faster signals (like 10Mhz sine wave) rather than the much slower
PPS. I asked a very similar question on the EEVBlog forum, and the
suggestion was to use time-stamp capture with the rise time, but
unfortunately, it never seems to see the signal, even though the gate
indicator flashes every second.
Thanks in advance for any advice or guidance!
Kind Regards,
Peter Membrey
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[Tom Van Baak]

Post by Peter Membrey
* DC
* 50Ohm
* Bandwidth filter enabled

DC and 50R is ok. I would not use the filter; you want the fastest edge possible.

You forgot to mention the most important setting of all - the trigger voltage. Check the signal at your inputs with a 'scope. I typically use 1.0 VDC, unless the 'scope suggests otherwise.

You should expect a risetime of a few ns. If it's a lot more something's wrong and it will affect your ublox vs. Rb conclusions.

/tvb
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[Magnus Danielson]

Hi,

Post by Tom Van Baak

Post by Peter Membrey
* DC
* 50Ohm
* Bandwidth filter enabled

DC and 50R is ok. I would not use the filter; you want the fastest edge possible.
You forgot to mention the most important setting of all - the trigger voltage. Check the signal at your inputs with a 'scope. I typically use 1.0 VDC, unless the 'scope suggests otherwise.

Oh, and turn of "auto" from trigger voltage, many counters tries to fix
it way quicker than the PPS occurs. The 53131/132 has this issue with PPS.

Cheers,
Magnus
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[Peter Membrey]

Hi guys,

Thanks for all the guidance!

I was ultimately able to make this work by applying various suggestions from the list. In the end I needed to offset the uBlox in order to get a good read and then fix this in post processing. I checked the pulse before and after making the offset, and the frequency seemed unaffected. Originally as part of the experiment I was considering the FS-725 as a perfect reference for simplicity (we all know that does not match reality) and if that assumption is made, it is possible where the pulses do overlap (that is they cross over and you start to see very large millisecond counts), to remove 1 second from the time and get the negative delta. Obviously this only works when you assume the reference is right even when it's wrong, which is not desirable in many cases.

However there was still a gotcha - it seems it is possible for it to "flip" within a single pulse so that you end up with some very strange results. I couldn't think of any good way to compensate for that so I went with the offset approach.

We did find some interesting things when looking at the performance of a relatively low cost GPS receiver (uBlox M8Q) when compared to the output of an SRS FS-725 which was being disciplined by the 1PPS of a Microsemi S650. We've got a paper being considered, but I'd love to share it here for critical feedback once it's published.

Thanks again to everyone!

Kind Regards,

Peter Membrey

----- Original Message -----
From: "Magnus Danielson" <***@rubidium.dyndns.org>
To: time-***@febo.com
Cc: ***@rubidium.se
Sent: Monday, 23 May, 2016 08:38:50
Subject: Re: [time-nuts] Capturing a 1PPS signal with a Keysight 53230A

Hi,

Post by Tom Van Baak

Post by Peter Membrey
* DC
* 50Ohm
* Bandwidth filter enabled

DC and 50R is ok. I would not use the filter; you want the fastest edge possible.
You forgot to mention the most important setting of all - the trigger voltage. Check the signal at your inputs with a 'scope. I typically use 1.0 VDC, unless the 'scope suggests otherwise.

Oh, and turn of "auto" from trigger voltage, many counters tries to fix
it way quicker than the PPS occurs. The 53131/132 has this issue with PPS.

Cheers,
Magnus
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[Bob Camp]

Hi

Ok, basic setup (most of which you have already):

Set up the counter in time interval mode. Start on channel 1 and stop on channel 2.
Set both channels to DC coupling and whatever impedance works with your pps signal
Manually set the trigger level to about 1/2 the zero to peak level of the PPS on each channel.

Drive one channel with the GPS PPS
Drive the other channel with the pps out of the SRS-725
Feed the output out the serial port to a terminal program for logging.

You will see a series of delta time numbers. If you are getting deltas under 1 us, check out
the cable delay command. It can be used to “move” the pps on the uBlox. You can offset it
so that the numbers are always > 1 us and not going to both sides of zero. It gets really confusing
if things keep flipping back and forth all the time.

For real fun, run another terminal session and log the sawtooth correction data (if you have a T
version uBlox).

Lots of Fun.

Bob

Post by Peter Membrey
Hi guys,
I'm looking to measure and characterize the 1PPS output of a uBlox MAX M6Q using a Keysight 53230A. Specifically, I'm mostly interested in the period of the signal, how close it is to exactly 1Hz and how that signal changes over time.
The Keysight is using the 10Mhz output of a SRS FS-725 Rubidium Frequency Reference (which is being disciplined by a PPS from a microsemi S600 GPS) as an external reference.
* DC
* 50Ohm
* Bandwidth filter enabled
The default gate time is 100ms, which if I leave it configured for that, it will never show a reading. I can get it to start giving a reading if I set the gate time to 3 seconds but for reliable readings, I need to set it to at least 5 seconds.
1. I would like to capture the period of each pulse - is this doable with my set up?
2. What does gate time actually mean? I've read a number of descriptions, but I'm still not 100% clear on what it means in this context and what impact it has
3. How exactly does the gate time work in this case? I mean if it captures data over 5 seconds and gives one reading - what actually is that reading? An average? One of the samples?
4. Should I be doing anything differently?
Ideally, what I'd like to do is capture the period of each pulse individually so that I can do an Allan variance on the data over a period of time.
I've googled around and I've read numerous guides, but they mostly seem to cover faster signals (like 10Mhz sine wave) rather than the much slower PPS. I asked a very similar question on the EEVBlog forum, and the suggestion was to use time-stamp capture with the rise time, but unfortunately, it never seems to see the signal, even though the gate indicator flashes every second.
Thanks in advance for any advice or guidance!
Kind Regards,
Peter Membrey
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

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[Charles Steinmetz]

Post by Bob Camp
Drive the other channel with the pps out of the SRS-725
Feed the output out the serial port to a terminal program for logging.
You will see a series of delta time numbers. If you are getting deltas under 1 us

Note also that typically, only one edge of a PPS pulse is held to a high standard of accuracy (almost always the leading edge, which can be either the rising or falling edge depending on the design). So, while it might seem convenient to just flip the trigger polarity on one input to get a TI that is always positive, you don't want to do that -- each input should trigger on the designated "timing edge" of the PPS. Then, if the TI is too short for comfort, adjust the position of one of the pulses -- either by using the cable delay setting of the GPS as detailed by Bob, or by using a short piece of coax to the source with the earlier PPS pulse and a longer cable to the later source. (If you use a long cable run, remember that the propagation velocity of coax changes with temperature, so do your best to minimize temperature variations.)

Best regards,

Charles

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