Hi > On Feb 25, 2022, at 10:18 AM, Erik Kaashoek <e...@kaashoek.com> wrote: > > Inspired by the gpsdo simulator written by Tom Van Baak I am trying to use > simulation to validate the PID loop parameters for a cheap and simple GPS > referenced timer/counter I'm building. > The following one hour measurements where done using a Rbd reference as input > to the timer counter(see Timelab.gif plot attached) > 1: Open loop (P=0, I=0) VC-TCXO versus the Rbd reference (green trace) > 2: PPS of the internal GPS versus the Rbd reference. (PPS trace) > 3: Rbd reference versus the closed loop (P=0.02, I=0) VC-TCXO (dark blue > trace) > 4: Rbd reference versus the closed loop (P=0.01, I=0,00005) VC-TCXO (red > trace) > All .tim files also attached. > The performance of the TCXO was remarkably good for a sub 1$ device, great > care went into a stable, low noise, Vtune. The noisy supply did not have a > big influence > The open loop TCXO frequencies where divided by 1e7 (SW PICdiv)and together > with the raw PPS frequencies loaded into an Excel speadsheet (see attached) > An identical PID controller (P=0.01, I=0.0005) was implemented in Excel that > took as input the frequency difference between the PPS and the TCXO and > calculated a correction for the TCXO frequency. The corrected TCXO > frequencies was exported from excel (PID output) and imported in Timelab > (PID, light blue trace) > As can be seen in the Timelab plot the controller implemented in Excel did a > better job then the controller in the timer/counter but the two actual closed > loop measurements used difference GPS and TCXO data (I do not own sufficient > good frequency counters to measure all frequencies at the same time). > The timer/counter was open on a table with minimum thermal isolation and > during the measurements some draft through door opening/closing could occur > My question: is the difference between the measured loop performance and the > simulated loop performance to be expected or am I making a big mistake > somewhere?
The TCXO *should* converge to match the PPS. It should do that at some point past the few hundred seconds range on your loop. You would need a longer data run to really know if this is happening or not. Something > 10 hours ( so overnight works well …) would be better. Another thing to look into: You can likely improve the performance of the PPS by 4X or more by using the sawtooth correction process. That would shift the whole graph to the left and turn a couple hundred seconds into sorta kinda a hundred seconds. Overnight runs would then become closer to a couple hours. Another check on things: The PPS should roughly drop off as 1/tau. It will depart from this once you get out a ways. Over the range you are looking at, that’s a pretty good approximation. Wobbles in the line might indicate some sort of gotcha in the data collection or (maybe) issues somewhere else. Bob > Feel free to ask more info or more measurement data > > Erik. > > > > > > <Timelab.gif><PID.xlsx><TCXO.tim><PPS.tim><TCXO GPS locked P=0.02.tim><TCXO > GPS locked P=0.01, I=0.00005.tim><PID > output.tim>_______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an > email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
