[time-nuts] Spec An for phase noise measurements

John Miles jmiles at pop.net
Wed Jan 23 03:24:59 EST 2008

> 	It is quite surprising that the 859xE series is spec'd about
> 10-15 db worse on phase noise than the comparable 856xE instruments.   I
> wonder if this difference exists in the performance

Yes, and I just documented it.  See my earlier post with the .GIF

> or just marketing
> domains and what the actual differences are in the implementation that
> accounts for it.

Usually phase noise in a synthesizer comes down to one question: "What's the
N factor?"

The older HP analyzers, dating back to the 8554/8555 plugins for the 141T,
stabilized their LOs by locking them with a sampler to a harmonic of a 1-MHz
VCXO.  You can imagine what N=2000-4000 or more does to the phase noise.  I
don't know if the 8590s used a similar synthesizer topology but it wouldn't
be surprising.  They may have been designed in a hurry; Tektronix came out
with reasonably-good portable analyzers before HP did, and took a lot of
business that historically belonged to HP.

The 8560s use a completely-different synthesizer architecture with a
much-lower N factor.  I think they call it a "roller oscillator," where the
synthesizer runs at something like 1/10 the frequency of the YTO.

> 	This was poorly phrased - I perfectly well know why one is
> interested in phase noise on LOs and other signals closer to the carrier
> than 9 KHz (no question there for me at least) but what I meant is why
> one would want to use a general purpose wideband SA for making
> measurements between 0-9 KHz or so... when FFT based technology is
> available and cheap and very high dynamic range with very tight filters.
> (Obviously this means high end soundcard type devices on a PC platform
> these days),

It comes down to convenience, I think.  (1) Most people already have an RF
SA, many of which are perfectly usable down to 100 Hz or 1 kHz; (2)
Commercial phase-noise packages don't talk to PC sound cards; and (3) Most
people don't care about PN at offsets that their SAs won't cover directly.
Add to that (4): Newer SAs are FFT-based anyway, and don't need an auxiliary
FFT analyzer at all.

-- john, KE5FX

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