By Peter Leig
Full Disclosure Magazine, Box 903-FD25, Libertyville, Illinois 60048
Poor Manís Spectrum Analyzer
One of the most frequent yet difficult tasks confronting the counter-surveillance specialist is sweeping a premise for hidden transmitters or "bugs". Surveillance transmitters can operate on virtually any frequency from D.C. to daylight and utilize a number of different modulation schemes. A.M., F.M., and Single Sideband are all commonly employed, and checking each of these modes on every possible operating frequency with a manually-tuned receiver is a very time-consuming procedure. What is needed for an effect sweep is a device that can monitor a whole bond of frequencies simultaneously and display them real-time as a function of signal strength.
Such a device exists in the form of the Spectrum Analyzer. In its simplest form, it consists of three elements: a tuneable radio receiver, a sweep circuit which continuously and repetitively tunes the receiver from one end of the desired band to the other, and a display readout which shows signal strength at each frequency. The display is typically an oscilloscope; a device with a cathode-ray tube (CRT) for a readout. Somewhat similar in operation to a TV screen, the CRT has an electron beam sweeping across its face from left to right and back to repeat at a rapid rate. A voltage applied to a vertical drive circuit deflects the beam upward from the baseline in an amount proportional to the magnitude of the signal. If the signal is derived from the output of a radio receiver and the receiver is tuned across a band of frequencies, every time a transmitter signal is detected, the beam will deflect vertically indicating the strength of the incoming signal.
The remaining circuit to complete the analyzer is the sweep circuit, which automatically tunes the receiver and also drives the CRT beam horizontally at the same rate. The resulting display represents the band of frequencies being observed with the lowest frequency at the left edge of the CRT baseline and the highest on the right. At each frequency with an active transmission present, there will be a vertical blip corresponding to the strength of the transmission. Three basic adjustments control the operation of the analyzer. A center-frequency adjustment, which sets the middle of the display to a given frequency, a sweep width adjustment which sets how far above and below the center frequency will be displayed on the CRT, and a sweep rate which determines how fast the beam and tuning control will traverse the band. If such a circuit were connected to a FM receiver with the center frequency set at 98 MHz. and the sweep width set at 20 MHz. (from 10 mHz below center to 10 MHz above) the display would show all active FM Stations in the area simultaneously. All modulation formats am indistingishably visible and the vertical scale can be calibrated to indicate signal strength.
If the sweep width is set very wide, the whole electromagnetic spectrum from a few MHz to several hundred MHz is displayed. As the width is decreased, the "window" looks at smaller and smaller segments of the spectrum. At the narrow end, a single transmission can be spread out across the whole width of the display. Looking at a TV station transmission in this manner reveals the picture carrier, the sound sub-carrier and any additional sub-carriers individually. Obviously, such a device would greatly simplify the task of searching for hidden transmitters. Even though they are usually quite low in power, if they are transmitting from a relatively close location, their signal strength would be quite high relative to a distant commercial station. The magnitude of the signal would also fluctuate in step with the sound being picked up in the room and transmitted to the bug.
Tektronix, Hewlett-Packard, Avcom, and others manufacture commercial spectrum analyzers. But prices range from $3000 to $10,000 and up, placing them beyond affordability for most of us.
Fortunately, a rather inventive gentleman named Murray Barlowe has come to our aid. A Ham Radio operator, WA2PZO, Murray runs a company called Science Workshop (Box 310, Bethpage, NY 11714, (516) 731-7628). He sells a kit for well under $100 consisting of two circuit boards and a modified cable TV tuner assembly which can be combined with any garden variety oscilloscope to produce an extremely versatile spectrum analyzer. The kit contains printed circuit boards, tuner assembly and all parts except for controls, switches, connectors, and cabinet. The finished analyzer has outputs to drive an oscilloscope, and since these signals are low frequency, any inexpensive scope will do.
The two circuit boards are a sweep board, which performs the functions described above, and a main board which, along with the cable tuner, functions as the receiver. The basic package will tune signals up to 500 MHz, which should be adequate to detect most covert transmitters. The documentation is extensive and the parts and circuit boards are top quality. By supplying the kits without case and connectors, the cost is kept down and the builder can assemble the project in a configuration suiting his needs. I built mine in a box approximately 2" x 6" x 10" including batteries for the power supply. I used a small battery-operated scope of roughly the same size manufactured by Nonlinear Systems and the resulting package is compact, portable, and extremely useful. B & K and several other manufacturers make battery-powered scopes, but if you donít need portable operation, inexpensive line-powered units will do nicely. See Heathkit and others.
Three other optional kits add some nice frills to the basic unit. There is an additional tuner module, which extends frequency coverage to 900 MHz. if you want to cover a greater bandwidth. A digital frequency readout board is available which displays center frequency and a three-digit display. This board uses a really elegant yet simple method of measuring the tuned frequency and converting it to a digital readout. Iíve used this board on several other projects requiring frequency readout and it works very well. The final optional kit is a tracking generator. This is another modified cable tuner, which is used as a signal source whose output frequency follows (tracks) the tuning of the spectrum analyzer. Since the instantaneous output frequency of the tracking generator matches the input frequency of the analyzer, the overall system acts as a sensitive synchronous detector. This works great for measuring the frequency response and bandwidth of filters, amplifiers, mixers, attenuators, and the like. Even if youíve had only minimal experience in kit building, these projects are quite simple and lots of fun to build and package. All user-supplied parts are readily available from Radio Shack or other outlets, and you can casily customize the final unit to meet your individual requirements.
If you want to use the same tools that sky-high budget investigators use without mortgaging the homestead, try the Poor Manís Spectrum Analyzer. I think youíll like it.