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

Bob Camp kb8tq at n1k.org
Mon May 2 22:49:48 UTC 2016


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

> On May 2, 2016, at 7:09 AM, Peter Membrey <peter at membrey.hk> wrote:
> 
> 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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