A Spectrum Analyzer for Your Laboratory

If you plan to equip an electronic laboratory, there is no doubt that you would incorporate, in addition to a soldering iron and the usual tools, these instruments:

• Multi-Purpose DVM
• Oscilloscope
• Signal Generator

You most likely would have a signal generator of some sort, and other items if your lab is a specialized one.

Not included in the above list is the Spectrum Analyzer. Why? Not because it isn't useful, but because until the advent of eBay, it was too expensive. So let me put in a plug for the Mighty Spectrum Analyzer, an indispensable addition to any but the most casual electronic laboratory.

What Is A Spectrum Analyzer?

Everyone is familiar with the Oscilloscope. In effect, it's a special type of voltmeter, one that measures voltage with respect to time, and thus can show you a "waveform." Any signal other than plain DC can be better observed by the oscilloscope, since it not only shows you the voltage, it shows you its variation. A spectrum analyzer is similar in that it shows you voltage, but instead of showing it with respect to time, it shows it with respect to frequency. Thus, if you were to connect a "line frequency" signal to an oscilloscope, it would show the familiar sine wave, with voltage on the vertical axis and time on the horizontal. If you connect the same signal to a spectrum analyzer, it will show a single vertical line whose height represents voltage, and whose position on the horizontal axis represents frequency.

Why is this a benefit? For a single sine wave, there isn't - the 'scope and 'analyzer provide the same information. But let's say that the 60Hz signal was distorted: Maybe it has low level harmonics. Maybe it has some data waveforms riding on it. These signals would most likely be invisible on the 'scope, or perhaps look like "fuzz." In either case you could not learn anything about them. With the spectrum analyzer, they stand out plainly. You can calculate distortion and you can see the characteristics of the data signal even in the presence of the enormous primary waveform.

Almost all electrical phenomena beyond charging and discharging batteries produce important data in the "frequency domain," and often an oscilloscope is simply inadequate to see what is going on. Spectral data if often far better suited for the task. If you think of an analyzer as a continuously tuning radio receiver that shows every station on the dial along with an indication of its strength, you'll see how useful it can be.

Why Doesn't Everybody Have a Spectrum Analyzer?

For the same reason that you may have a small fishing boat but not a yacht. There are rarely any yachts on eBay! However since the advent of eBay, it has been possible to get a used spectrum analyzer for a reasonable price. In the past, you might be lucky to get a used "panadaptor," a specialized and not very versatile spectrum analyzer, for a few hundred dollars. Now, you can get lab grade instruments for a few hundred to a few thousand, i.e., similar to the cost of a new oscilloscope.

What Kind Of Spectrum Analyzer Do I Need?

Certainly the very first question you need to ask yourself is "what frequency range must I measure." Although you can get a lab grade oscilloscope that will measure from low audio to many GHz, this is overkill if you're just working with audio equipment. Likewise, if you work with VHF and above, you probably don't need something that will distinguish between 60Hz and 90Hz. The type of job you will be doing is also important. There are general purpose analyzers, and there are specialized ones. For example, if you are working with acoustics, sometimes a "third octave" analyzer is the most useful. If you're a ham radio operator and not doing technical work, you can often get an accessory "panadaptor" if your rig doesn't already have a "spectrum scope." The picture below is from my 1962(!) QSL card, which is when I began my love affair with the spectrum.

Different trades have different requirements, e.g., the "service monitor" in the 2-way radio shop. Other than for brief mentions, I will be discussing general purpose analyzers only.

Spectrum Analyzer Specifications

A general purpose analyzer will inevitably allow the user to vary these critical functions and parameters:

FREQUENCY: You can select the start and stop frequency of your sweep, so that if you are examining a single radio station signal you don't have to see the whole band. Typically you will be able to vary the frequency sweep from the full range of the analyzer to just a few times that of the analyzer filter. "Zero sweep" is often provided to allow viewing of the time waveform of the signal or its modulation, just as you would with an oscilloscope.

SPAN, sometimes called DISPERSION: This in a sense is equivalent to frequency, although many analyzers require you to set them independently instead of allowing you to set start/stop frequencies. For example, you might set the frequency to 100MHz and the span to 10MHz, giving a view of much of the FM radio band.

RESOLUTION or BANDWIDTH: This is one of the most critical specifications, since it tells you how close together two signals can be before you can't tell them apart. It also tells you by implication the quality of the instrument. If you want to look at 60Hz sidebands on a 1MHz carrier, you would need a resolution of 30Hz or, even better, 10Hz. Most (originally!) inexpensive analyzers won't do this.

AMPLITUDE: Analyzers have an on-screen "dynamic range" of anywhere from 50dB to over 100dB in the very best. Specialized instruments, e.g., those that test cell phones, know they will be receiving a strong input signal of fixed characteristics and so a narrow amplitude range is OK. Lab instruments, which may have to look at tiny signals next to big ones, typically have wider ranges. The analyzer provides an attenuator to bring the signal into a good measurement range.

Those are only the basics. Some analyzers have special spectrogram displays, "markers" that will measure or track a signal, various forms of digital analysis, and "tracking generators" that will generate a signal at the analyzer's frequency so that you can measure a circuit's performance at different frequencies. This list doesn't even scratch the surface.

Additional Guides

EBay is loaded with spectrum analyzer bargains! Since I discovered this I have fulfilled my life-long fantasy to be able to measure signals. (Yes, I'm that kind of guy.) A number of analyzers appear frequently on eBay, others rarely. I've photographed some and will be writing additional guides on these. I will probably break them down as follows:

LOW-FREQUENCY Analyzers:  A guide to analyzers that are primarily designed for audio and low-frequency signals.

HIGH FREQUENCY Analyzers: A guide to analyzers primarily used for radio and microwaves

SPECIALIZED Analyzers: A guide to some devices that you might not even know are spectrum analyzers! This one will probably be a work in progress!

In these guides I will discuss specific units that you might find on eBay and offer my for-what-they're-worth recommendations. (You can find these guides by searching for Spectrum Analyzer in Guides. I tried to put links here, but eBay says: "Your link name has exceeded the maximum of 80 characters. Please enter a shorter name and try again." I did try!)