[time-nuts] Re: Catching range of GPSDO

[Magnus Danielson]

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