[time-nuts] Re: Validating GPSDO control loop with simulation and measurement. Is this amount of difference between measurement and simulation to be expected?

[Bob kb8tq]

Hi

> On Feb 25, 2022, at 10:18 AM, Erik Kaashoek <erik at 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>_______________________________________________
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