Poor Man’s Spectrum


By Joseph J. Carr

June 1999 Nuts & Volts Magazine


Ever wondered what to do with an old, worn out, low-frequency oscilloscope? I’ve seen a large number of 500-KHz to 5-MHz oscilloscopes on sale at hamfests and other places for a prayer and a dime (or some such low price). But what are they used for? Any self-respecting oscilloscope will have a 20 MHz or higher bandwidth, which is why the older models are basically in the dec­imal dust category. Hmmmm ... what to do with one of these “boat anchors?”

There is a solution. A number of years ago, I wrote a column in a magazine called Ham Radio. One of the projects I built for the column was a Spectrum Analyzer kit offered by Murray Barlowe of Science Workshop. 

            I recently ran across the Science Workshop web site, and found to my delight that the project is still being offered, but with some upgrades. I obtained the pieces from Science Workshop and did it again.

The Poor Man’s Spectrum Analyzer (PMSA) got its start back in 1978 when Murray demonstrated it at the Dayton Hamfest. The PMSA (Figure 7) uses cable television tuners as the front end. Two frequency ranges are available: 0~500 MHz, and UHF above 500 MHz. These tuners are voltage controlled so that the television designer can build phase locked loop (PLL) tuning. The fact that they are voltage tuned means that they can be swept through their frequency ranges. When a sawtooth waveform is applied to the tuning voltage input on the tuners, the sweeping takes place.

The cable TV tuners output the converted signal on an IF frequency close to 53 MHz (normal to TV receivers). The SW-6006 main board is a receiver that will down convert the 53-MHz IF signal to a low IF of 10.7 MHz, and then demodulate it. The bandwidth of the output signal is approximately 250 KHz, so there is a resolution problem that may be a problem on crowded bands. The problem does not prevent you from using the device to check the output of a transmitter for spurs and har­monics, however. Fortunately, there is a Switched Filter Upgrade kit (SW-6010) that permits selection of 250 KHz, 55 KHz, or 15 KHZ bandwidths.

It should also be possible to use a regular VHF receiver as the IF, and get even better resolution. A number of general coverage receivers will go to the required range. Alternatively, there are ham radio six-meter band converters that can be used as well. However, my experience with the Science Workshop product indicated that the SW.6006 worked satisfactorily for my purposes.

The sawtooth waveform needed to sweep the tuner through the range (with Center Frequency, Sweep Width, and Sweep Rate controls) is provided by the Ramp Board (SW-6001).

One of the problems with the original PMSA that I built was the necessity of calibrating the frequency dial. I used a chart comparing 0-999 micrometer dial (connected to the tuning voltage potentiometer) read­ings to frequency. The current offer­ing has a Frequency Readout Board (SW-6007) that converts the pre­-scaler output of the cable-TV tuner to drive a digital voltmeter that serves as the frequency readout.

Frequency can also be measured using an optional Marker Generator (MSG-100). This is also a cable TV tuner in which the voltage tuned local oscillator is used to provide a signal to mix with the front-end signal. It puts a “pipper” on the oscilloscope display that indicates frequen­cy. By measuring this frequency with a digital frequency counter, we can make accurate determinations of frequency.

Also available is a Tracking Generator upgrade (SW-5900). This type of Instrument is basically a signal generator that tracks the Spectrum Analyzer center frequency. It can therefore be used to make frequency response measurements of circuits and devices. The tracking generator was a really neat addition to the system because it permits test­ing that is not possible with the PMSA alone.

The various pieces of the PMSA are available from Science Workshop in either kit form or wired and tested. Add a “K” to the kit number for kit and “W” for wired.

The bottom line is that the Poor Man’s Spectrum Analyzer is one way to get a reasonably performing Spectrum Analyzer without laying out a huge amount of cash. Will it do everything that a $30,000.00 professional model will? Nope, but that’s not the question. The real question is: Will it do the jobs that you need it to do? NV