I just got a new BGTBL GNSSDO, the successor to the GPSDO, with an used (OEM-)specific OCXO.
Just would like you to know, that the parts configuration and placement does and may vary depending on the original OCXOs used. This might make it almost "impossible" to replace a defective (original) OCXO with another one without adding additional parts of unknow specs, dependig what OCXO was used originally by BG7TBL. The major problem is, that there are no schematics "in the wild" or supplied, which could be used. The latest incarnation of the GPSDO, now called GNSSDO, has a new motherboard (2021-06-10) to adapt to all kinds of (used) OCXOs which might be available right now on the used market. Unfortunately many rare OEM OCXOs are not documented, even when searching the net intensively, and may feature - eventually - a different PIN layout or other variations in specs, which make it almost impossible to find a direct replacement in those cases. Mine i.e. got an OEM specific C-MAC OCXO, which seems to have reached (almost) "End-of-life" (tuning voltage 8.212 82VDC, multiplier is X3 already) but my fully functional replacement OCXO (Oscilloquartz 8663-XS) does fit mechanically but not electrically exactly for this reason, because some parts are missing on the board (solder pads are unpopulated, no values given etc.) for substituting the OCXO. I already looked around for another replacement OCXO (same kind), but no luck...
Sending it back from Europe costs a lot too and does not make this feasiblack boxle.
If you look at page 1 of this thread, you will find an ever changing list of different board layouts, which seems to continue "endlessly" up to the current date. Many OCXOs have an identical PIN layout and usage, but some vary quite a bit not only by size.
In the end an OCXO just needs a power supply and tuning voltage. It shouldn't be hard to retrace the circuit which drives the OCXO tuning voltage. Most likely a DAC and an opamp. Power supply is a matter of measuring which voltages are available where (likely 5V and 12V). And some retracing to determine where the filtering components are and then to move these if necessary. Only things you need to find out are a DMM with continuity and a piece of paper to draw on.
Many people here in this thread have been obsessed with swapping out the u-blox GPS module in their GPSDO for a much more expensive and supposedly more precise one, and supposedly that would make their BG7TBL GPSDO perform "better" or "more accurate".
Of course most people don't even have access to the equipment required to back these claims with solid data.
But one person (John Ackermann N8UR) went ahead and did a rather good job of collecting the data on a variety of u-blox receiver modules, and the verdict is quite clear imo. They all perform more or less the same.
https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf
A very good read, and perhaps that can settle the question.
Many people here in this thread have been obsessed with swapping out the u-blox GPS module in their GPSDO for a much more expensive and supposedly more precise one, and supposedly that would make their BG7TBL GPSDO perform "better" or "more accurate".
Of course most people don't even have access to the equipment required to back these claims with solid data.
But one person (John Ackermann N8UR) went ahead and did a rather good job of collecting the data on a variety of u-blox receiver modules, and the verdict is quite clear imo. They all perform more or less the same.
https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf
A very good read, and perhaps that can settle the question.
Thats not 100% true. The F9 series is half an order of magnitude better.
The final question - for me - is: do any improvements of the GPS chip or OCXO provide either significant higher accruracy and/or stability of the generated 10 MHz signal, because that is the main purpose of my unit ?
The final question - for me - is: do any improvements of the GPS chip or OCXO provide either significant higher accruracy and/or stability of the generated 10 MHz signal, because that is the main purpose of my unit ?
The output phase noise is determined mainly by the OCXO parameters. So choosing a better OCXO can give a noticeable improvement if a better one can be found.
That seems to be my "problem" right now, because the BG7TBL installed C-MAC OEM-OCXO 48S1164 (DC1212=> December 2012) drifts slowly downward (constant voltage) or needs a slowly increasing voltage to keep its frequency constant.
...
If we are talking about GPSDO, OCXO aging has no effect on the output parameters.
...
It need remember that after new power on it looks like aging more fast... because it typically drives back to the original aging curve much faster than the original aging is. As told, time when it reach original aging curve is mostly unknown but after start long time used and long time cold stored OCXO this aging after new power up may be quite fast in first hours and days. (until it reach nearly this original aging)
...
...
Within 2 days (48h) the tuning voltage drifted (for almost constant frequency measured by my Fluke PM6685) from 5.24322 up to 5.24756 VDC.
...
...
It need remember that after new power on it looks like aging more fast... because it typically drives back to the original aging curve much faster than the original aging is. As told, time when it reach original aging curve is mostly unknown but after start long time used and long time cold stored OCXO this aging after new power up may be quite fast in first hours and days. (until it reach nearly this original aging)
...
In OCXO jargon this is called "retrace". And yes, OCXO aging is a complex phenomenon affected by various variables. I wouldn't worry too much about OCXO aging, though, because it is exactly for that reason that GPSDOs operate with a closed loop (either PLL or FLL or a hybrid PLL/FLL) to control the OCXO frequency.
Yes as I told "Even if DOCXO is "wolds best" it have aging curve and retract if stored without power."
(I have used tens of years term retract independent of "Merriam-Webster")
And as you told it is normal, no need worry so much. But when someone wonder how it drift perhaps faster than he think is ok, reason may be this retract related phenomenon.
In cheap GPSDOs as example these many BG7TBL produced are, there is not so much what average user can do for optimize Discipline / LOOP parameters. As can do example in quite cheap old TrimbleThunderbolt and some others.
...
Within 2 days (48h) the tuning voltage drifted (for almost constant frequency measured by my Fluke PM6685) from 5.24322 up to 5.24756 VDC.
...
cheap primary reference that is good enough.
it doesn't satisfy the level of time-nuts and volt-nuts metrology obsessions, but it's far beyond the requirements of almost any casual hobbyist. after all, most hobbyists don't even have a frequency reference at all.
if someone comes up with a better, cheaper gpsdo solution than bg7tbl, they should easily be able to take over the market for such devices.