[EMAIL PROTECTED] wrote: > > In a message dated 12/23/2006 15:17:33 Pacific Standard Time, > [EMAIL PROTECTED] 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 > > > > > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Said
Yes I though DAC and typed ADC, I really should proof read a little more carefully. Even with predictive filters one still needs a feedback loop to correct prediction errors. A PII loop will correct linear frequency drift. Surely frequency ~ rate of change of phase so a PID filter in a phase lock loop just adds a frequency term to the PI loop. A properly designed PID loop should reduce overshoot in response to phase error steps, such as when initially closing the loop. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
