[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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