[time-nuts] Phase noise measurement (was - no subject)

John Miles jmiles at pop.net
Fri Aug 20 22:53:27 UTC 2010


> Would anyone else like to suggest a known good low phase noise
> buffer amplifier?  Maybe something from a Fred Walls paper?

You can always build HF isolation amps by rigging MMICs and attenuators
together, but this will not reliably get you below -160 dBc/Hz.  Bruce G.
has given some good advice in this regard, with some circuit designs at
http://www.ko4bb.com/~bruce/IsolationAmplifiers.html and elsewhere.  I'm a
fan of this version (also from Bruce):
http://www.ke5fx.com/norton.htm

This one has the advantage of simplicity.  No weird parts, nothing that is
likely to be out of production or hard to find, and dirt cheap.  I've
measured the broadband floor at near -170 dBc/Hz at 10 MHz, and its noise
contribution at 100 Hz is below what the 3048A can see.  These figures are
adequate to measure any 10811-class OCXOs.

A practical PN measurement system for 10811-class oscillators can be made by
building two of those amplifiers and using them to drive pretty much any
random double-balanced mixer found on eBay with +10 dBm LO specs or more.
Both ports should be driven strongly to reject AM artifacts and avoid
degrading the excellent noise floor offered by the amps.  I'd hit the LO
port with +10 to +12 dBm and the RF port with at least 0 dBm.

Then, see the Wenzel app note here (
http://www.wenzel.com/documents/measuringphasenoise.htm ) to lock the two
oscillators in quadrature and amplify the resulting baseband output.  Any of
several sound-card FFT programs can be used to generate an output graph,
although if you want absolute calibration in dBc/Hz you need to be prepared
to sweep the actual test setup from mixer output to FFT input to watch for
various sources of flatness error.

A combination of an AD7760-EVAL board and a Digilent Nexys2 can be used to
construct an excellent baseband digitizer for the DC-1 MHz spectrum, but
most of the time a good-quality 192-kHz sound card is fine for this sort of
work.  Most good crystal oscillators reach their broadband floor by 10 kHz,
so there's no real need to go out to 1 MHz or more.

-- john, KE5FX




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