[time-nuts] GPSDO simulation tool

[Magnus Danielson]

Jim,

On 23/03/14 16:00, Jim Miller wrote:
>> To handle higher tau performance I think we want a higher degree loop.
>
>> Cheers,
>> Magnus
>
> Is a higher degree loop possible while maintaining stability? Commanding
> frequency while measuring phase is one pole, integrating the result of the
> phase comparison is a second pole and closing the loop will result in
> oscillation unless a zero is inserted (the P in PID).
>
> How would stability be maintained?

A third degree PLL is stable if the pole-pair is suitably located. The 
second degree PLL is kind of hard to bring into self-oscillation while 
that is much easier with the third degree, so more care needs to go into it.

When you think about it, putting an exponential averager just after the 
phase-detector (AVG1 in gpsim1) is in fact making the PLL a third 
degree, but since the time-constant needs to be constrained to be well 
below the loop time-constant "for stability reasons" it is in fact to 
avoid having the poles of this third-degree loop from deviating away 
into unstability.

I did a temporary hack on the PID code to convert the D-term into I^2 
term, by integrating the integrator output. First attempt was indeed 
quite resonant just to show that I was in the unsafe region. Backing 
down on the strength of the component sure did remove much of the 
resonance, but I did not see any appreciable improvement in filtering 
performance, so quick and dirty hacking isn't sufficient, darn.

Cheers,
Magnus