Bob,
On 09/15/2014 06:23 PM, Bob Stewart wrote:
My daughter and I were discussing what she does for a living with time and phase as a
geophysicist and relating that to what I'm trying to do with my GPSDOengine. I explained
that I was trying to keep the frequency accurate and stable, while also keeping the phase
near a target of 180 degrees. And suddenly, I realized that that's not what I was
actually doing. For the I term of PID, I was integrating on phase position,
which was never going to work.
So, I made a change to start integrating on the phase change from second to
second (i.e. frequency error), and I think this is finally performing
correctly. The DAC is now very flat, and the phase is moving around as would
be expected from Bob's and Tom's comments on the LEA-6T. Here's an ADEV of the
GPSDO's OCXO vs the 10811 in my 5335A. It covers a timeframe from about
11:00PM last night to about 11:00AM this morning. This is with the PID running
as a PI controller with very small P and I gain values.
http://evoria.net/AE6RV/TIC/GPSDO.vs.HP10811.png
For a PID PLL you need to build the phase error properly one way or
another being
PE = phase_ref - phase_out
If you use a TIC, the time difference is simply a variant of the PE
value (phase_out to start and phase_ref to stop channels). It may be
useful to have a offset value that can be subtracted numerically.
A PID on this then becomes
f = (PE - PE_prev)/t0
PE_prev = PE
Vi = Vi + I*PE
Vf = Vi + P*PE + D*f
Integrating on the derivate of PE helps in one particular case, when the
frequency error is so large that it doesn't lock up easily. This is done
like this:
f = (PE - PE_prev)/t0
PE_prev = PE
Vi = Vi + I*PE + F*f
Vf = Vi + P*PE + D*f
As you increase the F factor, the rate of frequency learning of I goes
quicker, and steers the exponential relaxation of the frequency error
before the beat frequency becomes so low that the phase-lock takes over.
As we (me and Warren) had a long thread before, turning up F is the same
as turning up P.
P is proportional to the damping factor
P is proportional to the PLL bandwidth
I is proportional to the square of the PLL bandwidth
Sounds like you had too low P value.
I could write the exact formulas up, they are easy to derivate with
paper and pen. It is also textbook material.
Make sure you have a damping factor of at least 3.
Hope it helps.
Cheers,
Magnus
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