[time-nuts] Effects of filter delay

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 23 18:15:48 EST 2006


Peter Vince wrote:
> Please forgive what is probably a naive question from this newbie, 
> but a loop filter has considerable delay (half the number of samples 
> for a typical(?) symmetrical FIR or block-averaging filter).  This 
> must surely limit the usefulness of very long averaging times?  And 
> perhaps it could exacerbate day/night changes by effectively applying 
> a "correction" of the wrong phase if averaging for a whole day was 
> attempted.  Presumably you need a "crystal ball" subroutine to try 
> and predict what the trend is going to be in the future in order to 
> get your correction correct for "now"?
>
> Peter
>
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>   
Peter

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



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