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
