Hi If you have a FS740 and measure it’s performance …. you likely will take anything the manual says a lot less seriously ….. Their ADEV performance in the real world is a bit underwhelming.
Bob > On May 2, 2022, at 1:42 AM, Markus Kleinhenz via time-nuts > <[email protected]> wrote: > > Hi Erik, > > I just found a hint that what you are seeing may be correct after all: > > The Manual of the Stanford Research Systems FS740 says: > > /*Predictive Filtering*// > //The superior short term stabilities of the OCXO and Rb timebases > enable the usage of// > //predictive filtering to improve the stability of the FS740 by up > to 3 times over traditional// > //methods. Predictive filtering uses state space methods to predict > the phase of the local// > //timebase relative to GNSS. The technique is quite similar to > Kalman filtering. The// > //benefit is that the FS740 can average the GNSS signal much more > effectively, resulting// > //in a significantly more stable signal with a much shorter time > constant than would be// > //possible with traditional filtering./ > > And has the ADEV Plots I attached. The GPS curve they printed is in the > realm of a sawtooth corrected M8T (<1e-12 @ tau=10ks) [See the Plot from > John Ackermanns Ublox evaluation]. But especially the Rb option seems to > surpass the reference in a parallel fashion. > > My two cents on simulated 1PPS Signals: > > One has to be careful when only using ADEV as the only characteristic > for modeling the 1PPS Signal as it combines White PM and Flicker PM in > one slope. So you may create an artificial signal which is pure WPN and > in turn is best predicted by something like the kalman filter. > > Regards, > > Markus > > Am 29.04.2022 um 19:24 schrieb Erik Kaashoek: >> Thanks for confirming something is still wrong. :-( >> I've extended the simulation to contain a full Kalman filter working >> with 2 state parameters: phase and frequency. >> The biggest impact I can see is when increasing Kp above the optimal >> value the PPS noise normally starts to impact the output phase and the >> ADEV at tau 1 becomes worse >> The Kalman filter seems to be able to filter the noise from the PPS >> better so with equally high Kp the ADEV at tau =1 is about a factor 4 >> better >> Unfortunately the high Kp of 0.1 is far from optimal and setting Kp to >> 0.01 gives overall a better performance and the Kalman filter no >> longer seem to have a visible impact. >> Octave code for the simulation and the used data files are attached. >> Also 3 plots are attached showing optimal Kp, high Kp with no filter >> and high Kp with Kalman filer >> I'm still seeing some weird stuff in the ADEV plots. >> Erik. >> >> On 29-4-2022 16:53, André Balsa wrote: >>> Hi Erik, >>> Mathematically, no, a GPSDO cannot have a lower uncertainty (ADEV) >>> than the >>> minimum observable uncertainty (ADEV) of the combined oscillator >>> (disciplined clock) and PPS (disciplining clock) from the GPS receiver. >>> Unless there is some magic trick to remove the uncertainty in a clock >>> that >>> I am not aware of. ;) >>> >>> On Thu, Apr 28, 2022 at 10:03 PM Erik Kaashoek <[email protected]> >>> wrote: >>> >>>> I'm doing some simulations to understand the impact of a filter >>>> between the >>>> TIC measurement and the PI controller steering the Vtune of the OCXO. >>>> With a well tuned PI controller without filter the best ADEV I can >>>> get is >>>> just above the minimum ADEV of an actual measured OCXO and an actual >>>> measured GPS PPS. >>>> When I add an alpha-beta filter, similar to a first order Kalman filter >>>> with a manually tuned Kalman gain, and using similar Kp, Ki, the >>>> overall >>>> performance does not change (much) >>>> However with the filter its is possible to increase the Kp, Ki with a >>>> factor 10 and when I use in the simulation instead of a measured PPS an >>>> artificial PPS created from noise with the same ADEV as the GPS PP >>>> but with >>>> a very constant phase (different from the varying phase of a GPS >>>> PPS) the >>>> ADEV of the GPSDO output in my simulation seems to drops below the >>>> ADEV of >>>> the PPS. Am I correct to assume this is a hint there is still something >>>> wrong in the simulation or was my initial assumption about the possible >>>> range of the GPSDO ADEV wrong? >>>> Erik. >>>> > > <FS740_ADEV.PNG><UBLOX_QERR_ADEV.PNG>_______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
