Buying Low Frequency Spectrum Analyzers on eBay

This is a companion to my guide to buying spectrum analyzers in general.  There is an introductory guide and a guide for High Frequency analyzers.  Please note that this doesn't pretend to be an exhaustive list.  It's weighted to laboratory-grade analyzers used for R&D.  There are any number of third-octave analyzers used in consumer hi-fi systems about which I know nothing and will not be covering.   

PC SPECTRUM ANALYZERS:  It is possible to download, sometimes for free, a program to run on your PC that takes a soundcard input and turns it into a spectrum display.  Beyond this paragraph I don't propose to discuss these.  First, they are rarely sold on eBay, being shareware, freeware, or software packages.  Technically, they have one great advantage:  Since they run on a PC, they typically have excellent analytical ability.  You can see, often at the same time, phase, amplitude, fractional octaves, spectrograms, "waterfall," and other displays.  They have one great disadvantage:  They are limited in frequency and stability by the PC soundcard, which is often of poor quality.  If you need analysis but not electrical precision, these are a good choice.

SWEPT ANALYZERS:  These are becoming obsolescent as computing power gets cheaper and A/D converters get better.  However because they are completely analog, these analyzers can occasionally boast superior specifications.  An eBay regular, the Hewlett-Packard 3580A can measure signals with 1Hz bandwidth and at very low level.  It was one of the first analyzers with a digital display, but the display only replaced CRT "persistence" rather than performing an FFT on a sampled signal.  This is an excellent analytical analyzer but very slow for wideband signals.  They typically go for $100-$300 on eBay.

Other swept analyzers, such as 'scope plug-ins, and older, clunky units from the '70s and earlier probably should be avoided at any price.

REAL-TIME and DIGITAL ANALYZERS:  These are analyzers that digitize the input signal and then, instead of measuring it's spectral components, calculate them using the Fast Fourier Transform and other mathematical procedures.  "Real-time" means that the calculation is keeping up with the signal rather than leaving out chunks.  Newer analyzers with more computational power can offer real-time throughout their range.  Older ones may be real-time up to 1kHz, for example, but not at higher frequencies.  With all the analyzers pictured below, please don't accept my thumbnail descriptions as the last word.  Detailed specifications are usually available on the web and you should definitely refer to them before buying!

The HP 3561A

This is a popular workhorse.  Originally selling for upwards of $10K, you can get one on eBay for under $1K, sometimes for half that.  (The odd looking display in the photo is due to the strobe reflections from the built-in EMI mesh.)  This is a very high quality instrument and covers frequencies up to 100kHz.  It has a built in stimulus generator and can resolve signals small fractions of 1Hz apart.  This is typicall for this class of analyzer.  One major limitation is that it is a single channel analyzer, which means that it can't compare two signals and derive functions from their phase or other differences.

The HP 35660A

This HP instrument has many of the features of the 3561A, above, although it isn't portable.  Its maximum frequency is 102kHz, but it does have two input channels and so can measure relative phase and other characteristics.  It, too, has a stimulus generator.  An option to look for:  Sometimes you can get one with built-in HP Basic, which allows you to write programs that run in the analyzer itself.  This unit often shows up on eBay in the $500-$1500 range.

The HP 89410A Vector Signal Analyzer

I have more to say about this analyzer in my guide for high-frequency analyzers.  It is one of the nicest units you can buy, and it is perfectly suited for low-frequency analysis as well as radio frequencies.  This is the low-frequency unit; it has a downconverter available which is pretty featureless.  This unit only shows up occasionally on eBay and sells for thousands of dollars when it sells at all.  But:  I'm writing this guide, and I love this unit, so here it is.

Analyzers Not Shown

These are the ones I have handy photos for.  Other manufacturers of this class of analyzer include

• Ono Sokki
• Wavetek
• Federal Scientific
• Rockland
• Tektronix

Try not to get a very old (pre-1980) digital unit.  They are hard to maintain and they don't have good computational speed.

Be sure to see my High Frequency Spectrum Analyzer guide for more info.