Season's greetings to one and all.
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BTW, I did swap the OCXO out that evening and had it up and running for initial tests after checking the 'other usual suspects' I'd had in mind as possible contenders for the sudden EFC transient and 3% phase shift in the PLL's 100KHz input that I'd provoked with my finger tapping test.
However, after letting the EFC voltage settle to its new value of 2.4210, I did another finger tapping test and got an unpleasant surprise as it suddenly shifted half a cycle or so at 10MHz before stabilising. It hasn't responded to the many vibration tests over the past two days since, so I suspect I may simply have relieved some pent up stress in the "Stress Compensated Cut" crystal inside my "NOS" OCXO. I guess only time will tell but apart from that little 'surprise', it seems to be behaving itself just fine.
The EFC has dropped another 8mV and I'm expecting it to come to a halt and start going up again based on my previous experience with the 13MHz unit. I'm assuming that the 13MHz unit's retrace behaviour is typical - it may well not be. I'm just going to have to keep my eye on the EFC voltage (as I normally do) for the next few days and see what happens before I start running any power cycle tests.
John
Following up on my OCXO swap out just over six weeks ago (how the time just flies by!), here's an update on how the replacement has performed in that time.
From my experience with my MK I GPSDO build using the 13MHz OCXO which had exhibited a rising VFC trend, indicating a reducing frequency with age, I had assumed the replacement 10MHz OCXO I'd used in the MK II GPSDO might follow a similar trend (both being CQE units) with the initial reducing VFC being simply a retrace effect similar to what I'd seen with the MK I,
It would seem that ageing can go in either direction, even with same brand units. In this case, the replacement OCXO has relentlessly continued its downward VFC trend to the point where it slowly settled to 2.4018v from an initial 2.4210v just over six weeks back (I never did get round to power cycling it since I was hoping to see the trend halt and finally reverse). However, early this afternoon, It had suddenly dropped to 2.3998v +/-100uV or so.
After confirming my meter connections were still ok and that I was observing a real VFC reading which was behaving as if the PLL was still in control (I was in the middle of my RFS boxing up project so decided to forgo the half hour or so to set it up to compare the GPSDO output against), I tapped the GPSDO with my fingers and observed an immediate upward change in the VFC voltage (by around 10mV or so) before it eventually settled down around the 2.4010v mark where it has been hovering ever since, +/- the usual 2 or 3 tenths of a millivolt due to ionospheric effects.
So, after some six weeks of monitoring the replacement OCXO's behaviour without any untoward 'events' other than that initial response to my finger tapping at the start of the test run, I got to see a less dramatic repeat of the instability I'd experienced with the original OCXO, leaving me in a similar situation I'd had to start with.
The only thing I can say this time is that it does seem to be an OCXO issue rather than some obscure intermittent fault elsewhere (dry joint or whatever). I guess 'll just have to accept this as the downside of disciplining an OCXO which can't truly be guaranteed to be completely free of such anomalous behaviours no matter how good the quality is supposed to be. The only thing to be said in this case is that there isn't a microcontroller algorithm to be 'caught on the hop' since, for a start, there isn't a microcontroller to begin with.
The "Take Away" for those of you struggling with the Lars design, is that any odd events you might see in your data may well simply be down to your OCXO 'being an OCXO, unruly behaviour from time to time included". At the current rate of ageing, it seems to have settled to around 1mV drop per week in the average VFC voltage so I'll keep it powered until the new year when I expect it to finally drop below the 2.4000 volt mark before I try power cycling it to observe its retrace behaviour.
My own 'Take Away' from this is that these OCXO based GPSDO's need a little TLC to help with their disciplining. I'm pretty much resigned now to the need to administer a 'gentle love tap' every other day or so as part of its routine (percussive) maintenance to forestall any sudden frequency jumps (I'd been a little lax of late in providing the required TLC).
John
Update 2021-02-13 :-
The above speculation turned out to be completely off the mark (i.e. a load of "Bullshit"
). Those rogue phase shifts had started to become more frequent over the past few weeks, indicating that a second 'jumpy' OCXO showing identical behaviour to the first would be unlikely in the extreme, therefore pointing the finger of suspicion to a dry joint or a bad connector contact elsewhere.
I opened it up to check, specifically, for a dry joint between the centre pin of the PCB mount SMA socket and the ref input pin on the 74HC74046. I couldn't provoke the symptoms other than for the briefest of responses to finger pressure stress testing (which in hindsight points to insufficient spring contact pressure in the SMA socket itself).
I'd had great difficulty through difficult access to get a fine tipped soldering bit onto the pin to effect a good solder joint and this had been my prime suspect. Thorough and careful examination with a jeweller's loupe and varying angles of illumination revealed an almost text book perfect joint, much to my surprise. Careful examination of the socket contacts however, suggested the lack of contact pressure as the most likely issue so I used the tip of a pin to close the two contact fingers up.
Further vibration testing before sliding the board back into the enclosure failed to provoke a response but immediately after sliding it back in and temporarily fitting a couple of the screws to test the virtually fully assembled unit, resulted in a slow climb of VFC voltage free of the normal PLL variations seen during a normal startup. It seems I had disturbed a very weak solder butt joint between the adjacent PC1 output pin and the one and only 2M resistor I had to make up the 1200s TC filter. I resoldered this joint, adding additional solder to give it the much needed structural strength it had lacked from my initial soldering.
Whilst this makes it another possible source of trouble, I think it's an unlikely rank outsider since I don't think it became an intermittent solder joint until after I'd disturbed it when pushing it slightly to one side to gain better access to the adjacent ref input pin. Besides which, the symptoms hadn't exactly matched the original symptoms I'd experienced. My money is still on it being a low spring contact pressure in that SMA socket which, provided I don't disconnect it again, should now no longer cause any further trouble (fingers crossed
).