[time-nuts] Spectrum Analyzer Specifications

[Charles Steinmetz]

Mike wrote:

>When using a spectrum analyzer to examine the output of a frequency 
>standard, what are the minimum specification needed? Bandwidth, 
>resolution, sensitivity, etc?

It very much depends on what you hope to see with it.  That said, 
virtually no SA will do anything useful in this application without a 
fair amount of preprocessing.  SA front ends do not have the dynamic 
range to handle the fundamental frequency and resolve any useful 
detail down at the oscillator noise level.  You will generally need 
to both filter out the fundamental with a very sharp filter and also 
use a very narrow SA bandwidth -- less than 1Hz -- to analyze the 
residue.  You may also need an ultra-low-noise amplifier to get the 
residual noise above the SA input noise.

The HP 3561A "dynamic signal analyzer" is supplied as part of the HP 
3048A phase noise measurement system, and has usable bandwidths down 
to the uHz range.  It is arguably the most useful SA for analyzing 
oscillators.  However, it only handles frequencies up to 115kHz, so 
it needs the rest of the 3048A to be useful -- or at least an offset 
oscillator (better than the one you are testing) and mixer to 
heterodyne the oscillator you are testing down to the useful range of 
the instrument, plus a sharp filter to attenuate the carrier.

Personally, I have rarely needed resolution bandwidths larger than 
300Hz for laboratory work (field work is different), and I frequently 
want RBWs considerably smaller than 1Hz.  I can highly recommend the 
HP 3588A, which operates in both sweep mode and also in FFT mode.

A tracking generator is nearly indispensable for much of what you 
will (probably) eventually do with an SA, so try to hold out for one 
with this feature.

Pay attention to ergonomics -- suffering with an SA that doesn't 
think like you do will drive you crazy.  For example, I prefer 
HP/Agilent SAs over Tek SAs because the HPs allow you to specify a 
start and stop frequency *or* a center frequency and span, while the 
Teks only allow you to specify a center frequency and span.

Also personally, I wouldn't waste money on any of the new plastic SAs 
that are flooding the market, or on any USB SA.  I would stick with 
used HP and Tek instruments (and *possibly* one or two others).

You can find operator's manuals (and service manuals) for the HP and 
Tek instruments on-line.  Study them carefully to figure out which 
ones have the features and ergonomics you want.  You will also pick 
up information that allows you to ask sellers detailed questions to 
increase the probability you will get a working unit.

I am all for having one or several SAs around the lab, but I fear 
that an SA may not be very useful for the particular tasks you are 
hoping it will perform.

Best regards,

Charles