[time-nuts] Over optimistic PN measurements

John Miles john at miles.io
Tue May 27 19:00:42 EDT 2014

This type of artifact isn't unheard-of, but it's usually not hard to spot
because the divots in the PN trace don't look much like a real property of
the DUT.  At least with direct-to-ADC digital analyzers, the effect of
spectral collapse on the noise plot seems to be time variant in many
instances.  If you wait a while or repeat the measurement the artifact will
often resolve itself... but of course it might come back later.  It's good
that the NIST folks are trying to take some of the empiricism out of the
picture, as I've never been content with my own understanding of where some
of these effects really come from.

The spectral-collapse phenomenon is likely causing some headaches in some
R&D work I'm doing with multichannel downconverters.  The more hardware that
you add to the signal paths between the input splitter and ADCs, the more
opportunities there are for phase drift, various sources of EMI, and
differential delays to corrupt the cross spectrum.  As the paper points out,
these artifacts can potentially corrupt multiple FFT segments and be
mistaken for a genuine aspect of the measurement, if they share the same
noise slope and are stable enough.  The implications of crosstalk are pretty
scary in the latter regard. 

The longer paper points out that these artifacts are independent of the Syx
estimator used to represent the complex cross spectrum average as a positive
real that can be displayed on a log scale.  That's true in the long run,
since the I component will dominate the plot regardless of estimator after
the instrument noise falls out, but it's also true that plots made with the
vector magnitude estimator sqrt(<avg I>^2 + <avg Q>^2) look a lot better
when you watch them evolve in real time.  Using an estimator based on I
alone such as Rubiola's recommended max(<avg I>, FLT_MIN) gives you a 3 dB
SNR advantage during the time while the measurement is converging, when the
average I and Q magnitudes are similar, but it also exhibits many more
transient "divots" during that part of the measurement since it depends on
only one averaged variable rather than two.  Real-only estimators look bad
enough in practice -- and provide such a small advantage at the end of the
day -- that I chose to use the magnitude estimator in the TimePod/3120A
driver.  I suspect that most other commercial signal analyzers do this as
well, for the same reason.  Ideally the instrument would allow you to choose
the estimator... but look at all of the stuff the user would have to read in
order to understand how to use the feature. :)

-- john
Miles Design LLC

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of bruce at ko4bb.com
> Sent: Tuesday, May 27, 2014 3:32 AM
> To: time-nuts at febo.com
> Subject: [time-nuts] Over optimistic PN measurements
> There has been some controversy over the Phase noise of CMOS logic
> Perhaps the apparently anomalously low PN measures are due to the use of
> cross
> correlation in the phase noise measurement equipment and the occurrence of
> phenomena detailed in the recent NIST papers:
> http://tf.boulder.nist.gov/general/pdf/2697.pdf
> http://tf.boulder.nist.gov/general/pdf/2698.pdf
> Such collapse of the cross-spectral function may also be present in the PN
> shown in the datasheets for some OCXOs.
> Bruce

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