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 <finndmik...@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 <joegw...@comcast.net> wrote: > >> On Sun, 05 Apr 2020 12:00:02 -0400, time-nuts-requ...@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 <kb...@n1k.org> >>> To: Discussion of precise time and frequency measurement >>> <time-nuts@lists.febo.com> >>> Subject: Re: [time-nuts] PRS-10 PPS output >>> Message-ID: <1131af5e-3444-4e48-b5d9-ecb36457d...@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. >>>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > <pps_fail.png>_______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.