Hi
> On Mar 19, 2020, at 3:32 PM, Tobias Pluess <[email protected]> wrote: > > Dear colleagues, > as you may remember I am still working on my own GPSDO project. > So far I have populated all the components on the PCBs I made, and I have > written some very basic software to test everything. Currently, I am > running following simple test: > I set my DAC to a constant value (5 Volts at the EFC input of the OCXO), > and then I send the phase measurement to the PC via RS-232 where I can log > it. More on that later. > Here is my schematic (not much has changed since last time): > > https://hb9fsx.ch/files/gpsdo/gpsdo_schematic.pdf > > The GPS phase detection happens on page 3. I use the TDC7200 as > interpolator. > Inside the STM32 microcontroller, I let a timer run with the 10MHz clock. > The timer overflows to 0 when it reaches the value 9999999. > Further, the 1PPS pulse from my GPS module triggers the interpolator, and > is delayed by max. 2 clock cycles. The delayed signal stops the > interpolator and is used as a capture event of the timer. > So, when the OCXO has the right frequency, the captured value would always > be the same. To determine the phase, I use the following code: > > if(captured value >= 5000000) > phase = 10000000 - captured value; > else > phase = -captured value; > > with the aid of this bit of code, my phase value is always perfectly in the > interval -5000000 to +5000000. My test today was to let the OCXO run with a > fixed voltage at its EFC input, while I recorded the resulting phase data. > My recorded data is here: > > https://hb9fsx.ch/files/gpsdo/phase.txt > > Now I made two plots (here: https://hb9fsx.ch/files/gpsdo/phase.png). The > upper is the raw phase data, and I also fitted a linear function to it > (i.e. if the OCXO has a constant frequency but has some offset from 10 MHz, > the phase would drift linear into one direction - at least I expect it to > do so). > The lower plot is what results when I subtract my linear fitted function > from the measured phase. I am wondering whether this makes some sense? One > can see some wiggles and ripple, especially between sampel 0 and 3000. Is > this the GPS jitter I see there? > Further, I also imported this phase data into Timelab and determined the > ADEV. Here is my plot: > > https://hb9fsx.ch/files/gpsdo/timelab.png > > This trace is kind of what I expect, but since I use a OSA/UCT 8663 double > oven OCXO, I would have expected it to be much lower. Something in the > 1e-12 regime perhaps? is that realistic? As one can see at tau=800s, the > ADEV is around 5e-11, so is this really all I can expect from this very > sophisticated OCXO? (However I should maybe add that the OCXO is brand-new > and has never been used before, so maybe it has some initial aging?). > I have no plots about the temperature, but this is quite constant. > For the GPS module, I used an active outdoor antenna at the window ledge. > Not optimal, but better than an indoor desktop antenna, I guess. That all looks like the raw / uncorrected for sawtooth output of a GPS module. Bob > > Is this some plausible data I have here, or do I measure complete crap? > > It would also be great if someone would look at my code. Maybe I am doing > something wrong when I read out the TDC7200? > The code is here https://hb9fsx.ch/files/gpsdo/code/ and the most > important function is perhaps get_tic(). > > I am looking forward to any good hints on this! :-) > best > Tobias > HB9FSX > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
