@AndrewBCN
That was a very interesting document (downloaded and added to my ublox folder). I read it all from start to end. The CEP scatter plot for the M8N closely matched that of my very first (genuine as it happened) M8N module. The subsequent fakes (needed to replace the original on account I managed to blow up the PPS output with a 12v jolt) also showed similar CEP map deviation plots.
It was interesting to note the almost constant observed sawtooth correction rate (which
does depend on how close the tcxo just happens to be to its nominal 48MHz frequency as was later acknowledged in that document).
If you program the PPS to 10MPPS and monitor this frequency (or better yet, the third harmonic) with an HF receiver using narrow band FM, you can quite clearly hear these saw tooth corrections as ticks which IME can vary at a rate of from 4 per second to one every 25 seconds or longer, depending on the tcxo's temperature (cooling or warming the whole module will vary the rate - a finger tip touch is usually sufficient to reveal this sensitivity to temperature).
Fortunately, since I'm not interested in such short term (1 to 100 seconds) stability, the saw tooth corrections disappear below the noise at more sensibly chosen 1000 seconds or longer integration intervals as far as its use as a GPSDO frequency standard goes.
Whilst I still had the M8N based MK I working to compare with the MK II GPSDO, I had the distinct impression that the M8N had a much greater phase shift modulation (up to 50ns at minutes long time scales versus the 6 or 7 ns pk-pk shifts of the M8T). However, I didn't possess a GPS antenna splitter to properly compare them at that time and by the time I'd made one up, I'd already stripped the MK I to test its "Five Volt" 13MHz OCXO at potentially destructive voltages without endangering the rest of the component parts.
As it eventually turned out, this "Five Volt" ocxo did prove to be a 12 volt unit that just happened to be able to function perfectly fine from a 4.8 to 5.2v supply (mind you, it took a few seconds before it would start outputting a
sine square wave on 4.8v which reduced to just a second when powered from 5.2v).
It had been my very first, one and only, ocxo that, in the absence of a datasheet, I didn't care to risk burning out before getting some use out of my 4 quid purchase - hence my playing safe and assuming it might possibly be a five volt only part.
By the time I was ready to sacrifice it on the altar of 'scientific research', I'd already gained a few clues to indicate that the experiment wasn't likely to end in tears. Indeed, I raised the test voltage to 14 volts to put its 12 voltedness beyond all doubt.
I've been meaning to lash up a second MK II on solderless breadboard using another 10MHz ocxo with an M8N module to compare against the performance of the M8T using an antenna splitter to 'level the playing field'. Previous tests had involved an indoors antenna alongside of the external antenna, which setup had only served to confuse these initial comparative test results. Hopefully this time round, I'll be able to get a more definitive answer to the question, "Does replacing the M8N with an M8T make as much of an improvement as I think it does?"
BTW, if anyone's interested in my DIY Rb lab reference project, I've retired the analogue temperature controller in favour of an Arduino nano setup, the details of which I posted into a seemingly moribund Rubidium oscillator based topic as per the link below.
https://www.eevblog.com/forum/blog/eevblog-235-rubidium-frequency-standard/msg3652885/#msg3652885 Although there haven't been any responses so far, the number of times that the attached images have been viewed suggest it's not an entirely dead topic. I'm not the first one to have made such a necro-posting as you'll see if you look about half a dozen postings upthread.
Whether I'll get any replies as the first necro-posting received remains to be seen. It's early days yet