[time-nuts] Spec An for phase noise measurements

John Miles jmiles at pop.net
Mon Jan 21 14:44:11 EST 2008

> I am considering getting a new spectrum analyzer so I can make better
> phase noise measurements than with my 8596E.  I've looked at the 8566B
> and the 8562 and 8563 since I need coverage to at least 6 GHz.  The
> 8566 is huge and ancient, though, so I think I'm leaning away from
> that one.  Anybody have other suggestions?  Some of the Advantest
> units seem to be reasonably priced on ebay, but it is hard to figure
> out what their phase noise performance is.

After working with a quadrature PLL/LNA, I feel very strongly that anyone
who's serious about PN measurement should go this route.  I wasted a lot of
time myself, sitting around wishing I could afford a quieter spectrum
analyzer.  It's simply the wrong question to ask.

It's true that the HP 8590s are among the noisiest spectrum analyzers out
there, but the difference between the phase-noise floors of an 8596E and an
8560E is only about 20-25 dB.  The difference in cost is several thousand
dollars.  If you invest in a nice 8662A instead -- or even an 8640B! -- you
can use your *existing* 8596E to make measurements 30-40 dB below what even
the 8560E series can do.

This is a seriously-nice thing to be able to do at home.  You can use a
quadrature PLL to do things like compare different crystal-oscillator
circuits and look at DDS phase-noise floors... and lots of other things you
would never be able to do with a spectrum analyzer alone.

I'll see if the list will let me attach a small .GIF showing the difference.
The 11729C's noise floor is about 6-10 dB below the green trace, which is a
comparison of a couple of decent-quality 100 MHz OCXOs.  Note how much
higher the direct measurement floors of your current spectrum analyzer and
an 8560E are.  For $25 in parts, you could make the measurement in the green
trace with your 8596E!  Spend the extra money on signal generators, and/or
collecting VCXO references at various frequencies.

> Also, what is the difference between the A, B, and E models on the
> 8560 series?  The A models are much cheaper on ebay.

'A' models go down to 10 Hz RBW, while 'E' models have 1 Hz RBW.  The 'E'
models also have better phase noise than the 'A' models from what I've seen.
8560E-series analyzers are the quietest I've tested at most offsets.  I
don't know offhand what the 'B' model designation implies.

Still, you should keep what you already have, and add a quadrature PLL and
LNA to it.

-- john, KE5FX

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