[time-nuts] Effects of filter delay
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Dec 24 01:27:32 UTC 2006
In a message dated 12/23/2006 15:17:33 Pacific Standard Time,
bruce.griffiths at xtra.co.nz writes:
The FIR filter used in some GPSDOCXOs is not the optimum prefilter for
the control loop.
An exponential averaging (IIR) filter is better.
However an FIR filter has the advantage of being easy to do and FIR
filters are not subject to limit cycle oscillations due to roundoff
error that plague some IIR filters.
There is a maximum prefilter delay that can be used (for a particular
loop time constant) before the control loop becomes unstable.
The prefilter output is typically further processed (in a second order
loop) by a Proportional + Integral feedback stage whose output controls
the OCXO frequency via a ADC connected to the OCXO efc input. Even
better performance is achievable with a third order loop containing 2
cascaded integrators, however stabilising the loop can be more difficult.
Bruce
Hi Bruce,
surely you mean "DAC" driving the OCXO EFC :)
There are several problems with PI(D) controllers: the Temperature
coefficients of the OCXO's are not corrected well, they are only corrected after the
fact (after generating an error in the output frequency that can be sensed and
corrected by the PI loop). Also, aging must be taken into account,
especially for hold-over periods. So by the time the PI loop senses some offset, the
frequency may have, and is in the process of drifting quite significantly.
The best temperature correction is done by using the sensor inside the OCXO
that controls the oven, using this to store the required DAC settings for a
given temperature over long periods of time, correcting for aging, then
calculating the DAC offset versus temperature by linear interpolation etc. There
are ways to sense the OCXO internal temperature very accurately without having
access to the actual heater circuitry :)
Appearantly Kallmann filters are the best predictors for Temperature
coefficients etc.
Does anybody have any information of the performance of PID controlers
compared to simpler PI controllers (not doing the differential part)?
bye,
Said
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