[time-nuts] Spec An for phase noise measurements

John Miles jmiles at pop.net
Mon Jan 21 15:41:35 EST 2008


> John,
>
> I'm a little confused as to what you are suggesting.  An 8662A is
> about $1500, and the 11729C is about $3k.  What would I get for $25?

The parts needed to implement Wenzel's app note:
http://www.wenzel.com/documents/measuringphasenoise.htm

> I don't know exactly what is involved with the 11729 and how it makes
> measurements.

Take an hour and look through this HP app note (large file, but only about
50 pages):
http://www.thegleam.com/ke5fx/gpib/5952-8286E.pdf

It is not all that specific to the 11729B/C despite making frequent
references to it.

Basically, the 11729B/C is a decent implementation of the technique in
Wenzel's app note, plus some microwave plumbing that will, if you have a
clean 640 MHz source available, downconvert 1-18 GHz signals to less than 1
GHz, where they can be further mixed down to baseband with any convenient
DC-1 GHz source.  An 8662A is ordinarily used with the 11729 becaus it fills
both the 'any convenient DC-1 GHz source' and the 'clean 640 MHz source'
roles.

If you decide to homebrew a PLL/LNA box, the notes on baseband LNA design
that Bruce Griffiths has posted are better than Wenzel's, assuming you are
aiming for ultimate performance.  Modern low-noise opamps would be fine as
well, at least for a first attempt at getting a PN rig working.

> If I just connected a quadrature PLL and LNA, I would still need a
> very clean VCO at the same frequency, right?

Yes.  The reference VCO is what determines the PN floor in most cases,
unless you start spending thousands of dollars on low-noise crystal
oscillators.  8662A-class signal generators are good but not "great," which
is why it's also nice to have a bunch of varactor-tunable crystal
oscillators around.  Often you can structure your measurements to use them
instead of a noisier general-purpose reference generator.

You can also do residual tests by building two of whatever it is you're
testing, and beat them against each other in quadrature.

Finally, you can sometimes use a coaxial delay line to test a single source
effectively.  This is a more error-prone process; I haven't played with it
much and don't consider it of primary interest, compared to 2-source
phase-detector measurements.

-- john, KE5FX




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