Dr Bruce Griffiths wrote: > [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 > > Said
I should have stated that it is also necessary (among other things) to compensate for the heater circuit nonlinearity if one wishes to minimise the oven startup overshoot. The power in a resistive heater is I^2 R or V^2 /R so if one is controlling the heater voltage (or current) one should take the square root of the control output before applying it to the heater. A PWM heater is a linear system but tends to add unwanted noise. It is also possible to produce a linear heater system using the dissipation in power transistors but this technique has the disadvantage that it is difficult to distribute the heat optimally. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
