[time-nuts] Effects of filter delay

[Dr Bruce Griffiths]

SAIDJACK at aol.com wrote:
>  
> 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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>   
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