[time-nuts] PRS-10 PPS output

[Bob kb8tq]

Hi

Backing up a little bit …. the PPS in on these telecom Rb’s is designed to 
easily get the part set on frequency. It’s not designed for a GPSDO application. 
What you are seeing is consistent with that “application target”. 

Bob

> On Apr 7, 2020, at 6:32 AM, Mike Ingle <finndmike62 at gmail.com> wrote:
> 
> Hi all,
> 
> Thank you for your feedback.  I found the schematics for the PRS-10 online,
> and the output is 5V HCMOS.
> 
> signal :
> 
> locked -->  hc08 in    hc08 out  -->  hc14 in   hc14 out --> 4x 240 ohm
> resistors -> output pin
> PPS    -->  hc08 in
> 
> Which brings me to my next question.  I have been running this used PRS-10
> FW version 3.21 with apparently (from lady heather) 126000 hours lifetime.
> Since Friday when it came in the mail.  It has the bench heat sinks and the
> accessory output board.  It is being disciplined by a blox5T PPS signal.
> Occasionally the PPS is missing from the PRS-10 (1 in 1000 or so) .  I
> presume due to loss of lock, as the and gate would inhibit PPS when lock is
> zero.
> 
> First some over-all results, then some background on my testing methodology.
> 
> The PRS-10 seems to lock to PPS-in such that the PRS-10 PPS out is 1.84us
> after PPS in.  The +1.84us happens to be where the time tag (TT) goes to
> zero.  I suspect that something might be wrong with the Time Tag setup.
> The unit steers itself until the TT=0ns, at which point the TT no longer
> returns data.  The PPS out from this point seems to wander by about 60ns or
> so (delay from GPS PPS in to PPS out).
> 
> Why do I think that I am missing PPS from the PRS-10?
> 
> Test 1.
> 
> GPS PPS -> scope ch2 (1M input on BNC T) -> PRS-10 PPS in
> PRS-10 PPS out -> scope ch4
> 
> scope setup to trigger on ch2 PPS in and look 1.84us (200 ns/div) later at
> the PRS-10 PPS out.  On infinite persistence, one clearly sees that some
> PPS out times  remain at ground.
> 
> OK so is the problem that the GPS had outliers of more than +/- 1.2us (the
> scope display at 200ns/div), or that the PRS-10 did not create a PPS?
> 
> Test 2.
> 
> Trigger on PRS-10 PPS-out and set up the scope delay to 1s, to observe the
> next PPS after the trigger.  Set up a mask test.
> This also indicates a failure.
> 
> My goal was to get a good low noise and accurate reference oscillator.  I
> bought the PRS-10 for 550 euro, and probably should have just purchased a
> new one, but live and learn...
> It probably does not need to be GPS disciplined, but I wanted to get it
> calibrated before setting it to free-running.
> 
> At this point, to get further, I probably need a time interval counter,
> which I can set up using my 4ch 14 bit 500MSPS  A/D boards pulse stretching
> input, which should give 5ps RMS time stamping.  Unfortunately, right now I
> only have one "good" quality timebase, the PRS-10.  I do have an old
> TrueTime XL-AK GPSDO, but the 10MHz out is noisy, with strong 100Hz spurs
> (2X line freq in Germany), and my boards on-board 50Mhz tcxo.
> 
> The two time bases I neglected to mention, are the 10M ref out from my
> Rigol DG4062, and the 10M ref out from my Siglent SSA 3021X.  I cannot
> comment on their respective quality.
> 
> Possible test config 1
> prs-10 10MHz -> ref in -> LMX2581 (creates 500MHz sample clk for AD9684 14
> bit 500MSPS A/D)
> PRS-10 pps out -> pulse stretcher ->  ch0 500MSPS A/D
> ublox pps out -> ch1 500MSPS  ( does not require pulse stretcher, as the
> leading edge is 48ns, and should interpolate to sub 50ps easily)
> 
> possible test config 2
> 50MHz tcxo -> LMX2581 (creates 500MHz sample clk for AD9684 14 bit 500MSPS
> A/D)
> PRS-10 pps out -> pulse stretcher ->  ch0 500MSPS A/D
> ublox pps out -> ch1 500MSPS  ( does not require pulse stretcher, as the
> leading edge is 48ns, and should interpolate to sub 50ps easily)
> PRS-10 10MHz -> ch2 500MSPS
> 
> I think that test config 2 is probably the best until I can get a good free
> running 10MHz reference.
> With the shown setup I can store the arrival time/ rising edge zero
> crossing of all of the inputs to a file for analysis.
> 
> Any further suggestions on test setup?
> 
> --mike
> 
> 
> 
> 
> [image: pps_fail.png]
> 
> 
> 
> On Sun, Apr 5, 2020 at 6:43 PM Joseph Gwinn <joegwinn at comcast.net> wrote:
> 
>> On Sun, 05 Apr 2020 12:00:02 -0400, time-nuts-request at lists.febo.com
>> wrote:
>> Re: time-nuts Digest, Vol 189, Issue 9
>> [snip]
>>> ----------------------------------------------------------------------
>>> 
>>> Message: 2
>>> Date: Sat, 4 Apr 2020 14:08:02 -0400
>>> From: Bob kb8tq <kb8tq at n1k.org>
>>> To: Discussion of precise time and frequency measurement
>>>      <time-nuts at lists.febo.com>
>>> Subject: Re: [time-nuts] PRS-10 PPS output
>>> Message-ID: <1131AF5E-3444-4E48-B5D9-ECB36457D014 at n1k.org>
>>> Content-Type: text/plain;     charset=utf-8
>>> 
>>> Hi
>>> 
>>> Pulling the output of a normal CMOS gate to ground through a small
>>> resistor is not generally a good idea. That?s what an attenuator or far
>>> end termination of the coax is doing. It?s not specifically outlawed in
>> the
>>> spec, but it's still not what they are designed to do. Also the output
>> level
>>> is going to be pretty wimpy run through an attenuator.
>>> 
>>> One way to 'fix' the problem is with a 50 ohm series resistor at the
>> source
>>> end. That only works to the degree that the output impedance of the gate
>>> is very low when in saturation. How true this is?. that depends.
>> 
>> Well, the coax 1PPS outputs I've had to deal with are all claim in
>> their datasheets to be able to drive a 50-ohm load, so I didn't worry
>> about overloading the output circuitry with a standard attenuator.
>> Their source impedance seemed to be closer to 200 ohms, as I recall.
>> 
>> The PRS-10 datasheet (page 59) says:  "The LOCK/1PPS function may be
>> configured via RS-232. The factory default is a low level to indicate
>> lock, with a 10μs pulse to +5V at 1PPS, with the leading edge being
>> defined as the 1PPS timing reference. This BNC output is a CMOS logic
>> output via a 1kΩ resistor."
>> 
>> So, a 50-ohm load (or even a dead short) should not hurt anything.  And
>> we can predict the peak voltage over a 50-ohm load driving a 50-ohm
>> coax is 5(25/1000)= 0.125 Vpeak.
>> 
>> For the TTL outputs in the days of yore, the series 50-ohm resistor was
>> standard practice.
>> 
>> Joe Gwinn
>> 
>> 
>> Context:
>>> 
>>>> I've [JMG] had this issue with coax 1PPS outputs across the board.
>>>> What I generally do is to attach a coaxial 50-ohm attenuator between
>>>> instrument coax output connector and the 50-ohm cable it will drive.
>>>> The attenuator matches the output impedance to the coax impedance.  I
>>>> use attenuators between 3 dB and 10 db.  The actual attenuation is not
>>>> as marked, because of the mismatch between the ~1K output impedance and
>>>> the 50-ohm input of the attenuator.  I suppose that a 75-ohm attenuator
>>>> may work as well or better to drive 50-ohm cable, but have not tried
>>>> it.  All the ringing et al are suppressed because the length of the
>>>> mismatched part of the path is maybe an inch or two.
>>>> 
>> 
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