Hi all,
I have build the GPSDO following the original schematic on perf board, although made some small changes. I'm using the Bliley NV47A1282. My version seems to have a finicky output because the slightest loading distorts the sine wave. After trying a couple of things I resorted to using another HCT4046 to create a square wave with an almost 50% duty cycle. I also used 100R in series between the HC390 and the Arduino Nano and another 100R in series with the HC390 10MHz input.
To create a mid-point for the DAC range at the mid-point of the Bliley frequency range, I used the 1PPS signal of my (genuine) NEO-M8T and the free running Bliley output on my scope, and adjusted it with a trimmer to get a steady, non moving drift, creating a manual lock. I then replaced the trimmer setting with a resistor in my case of 200K. The total DAC range now has a adjustment range of 7.9Hz. I verified this all with my counter.
I have noticed two problems.
The first problem is that using H1 to determine the gain does not work for me. The ns results drift linearly way out of range. Same with h65353. Setting the DAC value at mid-point creates a repeating but steady range of numbers, as expected. To calculate the gain, I used the delta of the Bliley in Hz, which is 7.9 Hz, or 790ppB resulting in a gain of 83, if my math is correct.
The second problem is that with the standard lowest offset of 200, I can't get a lock, and if so, only for very brief periods. The ns value is too far out of range, beyond -100. I could cheat by set a higher lockPPSlimit in the code, but then a Lock is reported while there is too much jitter.
I modified the code to allow me to set the offset as low as 10. At an offset of 100, I got a lock, but still with too much drift. When I changed the gain from 83 to 830, the resulting lock has been running for 24 hrs without a glitch and with minimal jitter, I think. I have steadily increased the damping and I'm now running at a damping level of 10. I'm not sure if the damping counteracts the higher gain that I needed to set, but even at a damping factor of 0.5, everything seemed to work fine. See the attached scope capture of the 5MHz signal and the signal at the anode of the Schottkey Diode.
Can anybody enlighten me on these two issues?
Tks, Paul