[time-nuts] Re: Catching range of GPSDO

[Bob kb8tq]

Hi


> On Mar 3, 2022, at 3:36 PM, Erik Kaashoek <erik at kaashoek.com> 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.

Take a look at just how good the LSB ( or maybe even LSB’s … ) on your
DAC really are. Some are not all that great and others are quite impressive. 
Cost is indeed part of that. Summing a pair of lower resolution parts often 
turns out to be pretty cost competitive. 

With 2^16 steps a LSB of 2x10^-11 (and perfect linearity) you would get
a 1.3 ppm tune range. If it’s symmetric you would get +/- 0.65 ppm. That
probably is not “enough” to compensate for much aging on a low cost 
TCXO….. 

> 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 “normal answer” is to use a sequence of control loops. You start off with
a fast / wide band loop and then once it gets “good enough” you go to a 
somewhat narrower loop. Your state monitoring tells you when it’s got to an
adequately settled point and you step again. It is not uncommon to have 
anywhere from a few to a dozen or so of these steps. 

Bob

> Erik
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