[time-nuts] Notes on tight-PLL performance versus TSC 5120A

John Miles jmiles at pop.net
Thu Jun 3 00:18:02 EDT 2010


> > The integration secret  (which is no secret to anyone but
> Bruce)  is to analog filter, Oversample, then average the
> Frequency data at a rate much faster than the tau0 data rate.
> >
> Which again is misleading as you specify neither the averaging method
> nor the analog filter.

I can't speak for the analog side as I never saw a schematic of the PLL, but
it may be worthwhile to point out that the averaging code in question is in
SOURCE_DI154_proc() in ti.cpp, which is installed with
http://www.ke5fx.com/gpib/setup.exe .  This is my code, not Warren's.  It
does a simple boxcar average on phase-difference data, the same as my TSC
5120 acquisition routine does.  Previous tests indicated that simple
averaging yields a good match to most ADEV graphs on TSC's LCD display, so I
used it for the PLL DAQ code as well.

I also tried a Kaiser-synthesized FIR kernel for decimating the incoming TSC
data, but found that its conformance against the TSC's display was worse
than what I saw with the simple average.  More work needs to be done here.

> When will you understand that phase differences and differences of
> average frequency (unit weight to frequency measures over the sampling
> interval zero weight outside) are equivalent.

One subtlety is the question of whether to average (or otherwise filter) the
DAQ voltage readings immediately after they're acquired and linearly scaled
to frequency-difference values, versus after conversion of the
frequency-difference values to phase differences.  I found that the best
agreement with the TSC plots was obtained by doing the latter:

	val = (read and scale the DAQ voltage)

	// val is now a frequency difference
	// averaging val here yields somewhat higher
	// sigma(tau) values in the first few bins
	// after tau0

	val = last_phase + (val / DI154_RATE_HZ);
	last_phase = val;

	// val is now a phase difference
	// averaging val here matches the TSC better

The difference is not huge but it's readily noticeable.

This is subtly disturbing because the RC filter before the DAQ *does*
integrate the frequency-difference data directly.  If it's correct to
band-limit the frequency-to-voltage data in the last analog stage of the
pipeline, it should be correct to do it in the first digital stage, I'd
think.

Further complicating matters is the question of whether the TSC 5120A's
filtering process is really all that 'correct,' itself.  When they
downsample their own data by a large fraction, e.g. when you select tau0=100
msec / NEQ BW = 5 Hz, there is often a slight droop near tau0 that does not
correspond to anything visible at higher rates.  To some extent we may be
attempting to match someone else's bug.

At any rate I've run out of time/inclination to pursue it, at least for now.
The SOURCE_DI154_proc() routine in TI.CPP is open for inspection and
modification by any interested parties, lines 6753-7045 in the current
build. :)  Warren has his hardware back now, and would presumably be able to
try any modifications.

-- john, KE5FX





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