[Test-Equipment] Residual FM measurements (was HP 8656A question)

[John Miles]

OK, I didn't feel like going to bed just yet, so: a comparison at
http://www.speakeasy.net/~jmiles1/gens.gif shows all three generators
running at 100 MHz.  The vertical scale at midscreen is roughly 50 Hz per
horizontal division, based on the observed slope of the analyzer's 100-Hz
filter at 10 dB/division.

Blue is the HP 8656A; green is the HP 8640B; purple is the HP 8662A.  Both
of the latter generators are at or below the analyzer's own FM noise floor
(a clean 10 MHz crystal reference standard looks almost the same), but the
8656A is well above it.

I suspect that the peak-to-peak nature of these graphs means that they
aren't directly comparable to RMS deviation figures, but I don't know how
you'd go about converting them.  Also, the 8656A's manual specifies a
certain post-detection (video) filter passband to be used for the
measurement, but no video filtering was switched on for these measurements.
That being said, the observed deviation is roughly in agreement with the
8656A's specs.

Needless to say, a +/- 30 Hz peak variation in frequency is readily audible
in a CW receiver, especially at low beat frequencies.

A similar test at 990 MHz can be seen at
http://www.speakeasy.net/~jmiles1/gens1g.gif.  Blue is the 8656A; purple is
the 8662A; and green is a homebrew DDS/PLL hybrid synthesizer based on a $25
Mini-Circuits VCO.  Here again, the 8656A is showing its age.  You might not
want to use it as a reference for a clean microwave source, but it's fine
for most ham-related applications.

The biggest complaints I have about the 8656A isn't its phase noise, but its
fan noise!  I keep mine on a separate power strip, so I can shut the whole
thing down, including its always-on fan, when I'm not using it.

-- john KE5FX


----- Original Message ----- 
From: "Gottfried Ira" <ira@atnet.at>
To: <test-equipment@mailman.qth.net>
Sent: Monday, July 21, 2003 10:19 PM
Subject: Re: [Test-Equipment] HP 8656A question


> The 8656A is notorious for its excessive phase noise.
> I have access to a HP8656A at work which shows similar FM in the low
> frequency ranges as well. It seems to be normal for this this type
> of generator.
> When you slap it slightly with your hand, you hear the
> PLL unlock an regain lock in the receiver.
> The 8656B und 8657A/B are supposedly (somewhat ?) better.
>