Hi Sounds like you might want to try a different TCXO. Or even try an OCXO. Even with a good TCXO, 1x10^-9 accuracy is a stretch. I’m assuming we’re talking frequency accuracy so the usual “one sigma” time accuracy not the overriding number.
GPS module “output jitter” forces you to a fairly long filter time constant to achieve better than that. Hitting the usual “>99% of the samples at a 10 second gate" sort of frequency accuracy spec drives you to numbers out past 100 seconds. The TCXO (even a good one) in a fairly normal ambient gets hit by all sorts of small thermal changes. You can try this or try that, in a real world portable setting … there is “thermal rumble”. The TCXO will struggle to stay on frequency as it sees that rumble. Bob > On Feb 19, 2022, at 3:24 AM, Erik Kaashoek <e...@kaashoek.com> wrote: > > Thanks for all the excellent info > The TCXO is actually a VC-TCXO at 10MHz intended for use in a cheap GPSDO > where the ambition is to have 1e-9 frequency accuracy so the 2e-9 jump was > just too high. > The VC-TCXO had its own low noise voltage regulator but the Vtune was > connected to a variable voltage divider between GND and the Vcc of the > VC-TCXO through a low pass filter so even with its own voltage regulator any > change in current in the TCXO can change the supply voltage and feedback > through the variable voltage divider. > It was expected that only slow changes in current could happen because of > temperature changes as the temperature control is very fast, loop bandwidth > well within one second, and these where filtered out by a low pass filter > after the variable voltage divider. > To test if the jump up and down was caused by a current to Vtune feedback the > Vtune was set to Vcc, 1/2 Vcc, 1/4 Vcc and GND. > The result was interesting: > Vtune | Max jump > Vcc | 6e-9 > 1/2 Vcc | 3e-9 > 1/4 Vcc | 1.5e-9 > GND | no jump > This suggests the jump is indeed caused by feedback from the current changes > through the variable voltage divider into Vtune and there is a digital > circuit inside the VC-TCXO with changing current consumption causing the > 107.34 seconds periodicity. > To confirm the feedback was indeed the cause the variable voltage divider was > connected to a 3.7 V battery instead of Vcc and indeed, no more frequency > jumps! > The ADEV of this cheap VC-TCXO with Vtune at 1/2 Vcc is not bad: > 0.1 s | 2e-10 > 1 s | 1.5e-10 > 10 s | 0.9e-10 > 100 s | 2e-10 > With the Vtune at GND the ADEV is even much better so there is still some > more investigating to do. > Some DAC's with internal voltage references have been ordered to test if it > is possible to connect the DAC to the same Vcc (to save cost) and still have > a stable but variable Vtune without feedback . > If this does not work the DAC will need its own voltage regulator. > Again thanks to all the people that replied, I'm learning a lot. > Erik. > > > > _______________________________________________ > 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.
