Does anyone know what happens to the pulse width of the Neo-6M Timepulse signal if the frequency is re-configured?
At the default 1PPS, the pulse width is 100ms.
I am guessing that the pulse width is fixed for very low frequencies but changes to a 1:1 mark space ratio square wave above 10Hz?
Edit: I just discovered this post from "Johnny B Good" in another thread...
https://www.eevblog.com/forum/metrology/what-do-you-want-from-a-gpsdo/100/====
"I mention this on account this new found interest in test and measurement kit has set me off on a seemingly never ending quest for better and more accurate gear, aka a GPSDO I can rely on and be proud of. Lately, in earlier postings here, I've been bemoaning my problems with getting a 5 volt 13MHz OCXO to phase lock against my cheap NEO-6M's 1kHz PPS (it's a make do for now substitute for the original M8N module I damaged with an injection of 12 volts into its PPS line a couple of weeks back).
All this time I'd been blaming first the XOR gate phase detector circuit and then the 4046 phase detectors for not doing 'their job' and, only now between this and the previous post, discovered a rather strange fact about the PPS signal from a NEO-6M (or at least this NEO-6M) when it's programmed to output a 1ms second pulse with a 50% duty cycle (1KHz sq wave).
Up until just prior to this post, I'd only ever quickly checked out the PPS and divider chain 1KHz output waveforms, preferring to monitor the 13MHz OCXO and FY6600 outputs when testing my crude PLL setup. It's only since I decided to take a much closer look in detail at the 1KHz signals going into the 4046's phase detectors, whilst monitoring the output voltage, that I've finally discovered the root cause of my crudely lashed up on prototyping bread board GPSDO test setup's failure to lock onto the GPS reference. It turns out to have been all to do with the NEO-6M's strange habit of slowly wittling away at the positive going pulse, shrinking its width until it disappears up its own backside to instantly reincarnate itself as a full fat sq wave, remaining so for the following five or so minutes before starting another cycle of shrinkage over the next 5 or so minutes.
If it hadn't been for the fact that I'd upgraded the FY6600 to an OCXO clock, I doubt I'd have been able to hand adjust the tuning pot on the OCXO to keep it close enough to the leading edge of the 1KHz PPS pulse to observe such weirdness (I'm not sure the original 0.1ppm 50MHz TCXO upgrade would have been enough of an improvement to have pulled off such a stunt).
It's no wonder the XOR gate failed to produce any meaningful failure behaviour let alone actually function as intended other than for relatively brief moments when it seemed to be locking in an offset fashion. The type 2 phase detector in the 4046 should have gotten round the non-square waveform issue but I'm guessing it was triggering on the trailing falling edge which would have been slowly happening at earlier and earlier times in the cycle for half of this 10 or so minute cycle before jumping back to a sq waveform for another 5 or so minute round of fun and games. No wonder it was slowly driving me round the bend these past few evenings (including midnight to 5 or 6 in the am if truth be known
![Banging Head |O](https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif)
).
Anyway, that's just a taster of the sort of process that sucks you ever deeper into the mad mad world of DIY test gear and electronic projects in general. BTW, I think, now I've got the use of the type 2 phase detector in that 4046 which doesn't rely on the waves being square, I'll have a go at reprogramming the NEO-6M to generate the narrowest pulse possible (10 microsecond, afaicr) and see how it behaves at that setting.
"
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It indicates the pulse width of the Timepulse is separately configurable to the frequency (as low as 10uS miniumum).
However, from the report above, I am a concerned about how the mark-space ratio varies over time.