[time-nuts] More GPSDO measurements

[Matthias Welwarsky]

Hello, fellow time-nuts.

finishing the next PCB revision of my GPSDO cape project, after assembling 
some components that JLCPCB SMT service doesn't stock I did a test run to 
compare it to the previous revision. The measurement setup is identical to my 
previous test, 53131A with MV89A OCXO, frequency mode with 2s gate time.

There are only a few changes, I routed the OCXO power rail  through an LC 
filter to decouple it a bit from the digital noise generated by the 
Beaglebone. Another change was to add a 1PPS output, using an additional timer 
of the AM3358 in PWM mode, clocked by the 10MHz OCXO output. This will give a 
nice, stable 1PPS signal.

I also changed the analog output stage to be just a unity gain buffer for the 
DAC instead of a 6dB gain stage. This is probably the most notable change as 
it doubles the DAC resolution. It also shows in the results, see the attached 
picture. Green curve is the new rev2 board, blue curve is the best run of the 
previous revision I could do, magenta is the Rb GPSDO.

Unfortunately I did a bad pick with the OCXO. I bought three MV85 OCXOs from 
Ebay, two of them obvious fakes, the third one appeared to be genuine and it 
seems to be quite OK with regards to short term stability, but it drifts 
heavily and doesn't seem to find equilibrium. This means the GPSDO needs to  
kick it back on track quite frequently, preventing using of larger time 
constants (now 300s). But still, the overall stability is better than the 
previous revision, which I find quite remarkable. I'm pretty sure that I could 
now beat the noise floor of the measurement setup with a working OCXO. I've 
ordered a brand new OCXO from Digikey, I have another PCB revision to launch 
which will hopefully be the final one and the new OCXO will then go on the 
final PCB. The final PCB revision will feature a slightly more elaborate 
analog output stage which will allow down-scaling and biasing the DAC output 
to further extend the DAC resolution.

Regards,
Matthias
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