[time-nuts] ensemble oscillators for better stability

John Miles jmiles at pop.net
Mon Dec 31 00:23:47 UTC 2012

> Bruce,
> The Tsc5125A and miles Timepod show a phase noise floor 3dB above the
> noise floor of the two oscillators (if two are used with identical noise

Bruce is actually talking about a semi-undocumented trick with the TimePod
that allows it to act like an E5052 or TSC 5120A-01 with two internal
references.  Basically, you remove the SMA jumpers from the TimePod's input
jack panel and feed two independent reference sources that are very close to
the same frequency to the Ch0 IN and Ch2 IN jacks.  Then you connect your
DUT to the REF IN jack, and flip the channel equations from "0-1" and "2-3"
to "1-0" and "3-2" to reverse the roles of the input and reference sources. 

That allows the cross-correlation algorithm that normally removes the ADC
noise from a PN measurement to remove the uncorrelated noise from your two
references as well.  It's a nifty technique, but it is important that the
two references be close to the same frequency for various reasons, some of
which I haven't adequately investigated.   So it falls into the "Unsupported
technique/Use at your own risk" category.

> It would be nice if that worked, then we all could use $10 TCXOs instead
> $1000 Wenzel references.

That's basically how the E5052A/B and TSC 5120A-01 work -- if you look at
their phase noise floor specs, they are much better than what can be
obtained from any one reference source.  The E5052A/B instruments actually
use the technique to remove the noise from two VHF-microwave downconverters.
That was my original intent in adding those SMA jacks -- see
http://www.miles.io/timepod/VHF_test_jig.jpg and
http://www.miles.io/timepod/VHF_example_400_MHz.png , where a pair of HP
8662A synthesizers is used to downconvert a much quieter 400 MHz source for
measurement at 9 MHz.

In principle you can indeed use a pair of $10 TCXOs as references to measure
arbitrarily low-noise DUTs, but you may need to run the measurement for days
or weeks.  Measurement time is the price of the free lunch in this case.

-- john
Miles Design LLC

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