Erik, On 2022-03-03 21:36, Erik Kaashoek wrote:
The GPSDO I'm building started with frequency locking but now I'm adding phase locking so the time stamping counter can be on GPS time. A first version works with a PI controller setting the vc-tcxo Vtune DAC based on the phase difference of the 10 MHz with the PPS phase. Due to tolerances the tcxo frequency range is big and is set by a 16 bit DAC where 1 bit is about 2e-11 frequency change. Once the DAC value is close to the correct frequency the loop catches nicely but if the setting is far off catching takes a long time. A possible solution is to use the frequency error to set the DAC close to the optimal frequency for catching. Speed of catching is important as the design is intended to only be switched on when needed. Does anyone have pointers to info on how to do quick catching in such a control loop?
The track-in time scales proportionally with the square of the frequency error and inversely cube of the loop bandwidth. The formula becomes garbled in ascii-math and I am too tired to convey it in readable form.
The lesson is: Make bandwidth high for quick lock-in so frequency acquisition is achieved, then switch over to a different bandwidth of the loop from that point.
The second lesson is: Pre-setting the frequency from previous learning can quick the process up.
Notice that the PI-loop will lock if the frequency error is within capture range, with is in effective that the oscillator can be steered to align.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
