Figure 10 12 illustrates the front-panel controls of a hypothetical, generic, spectrum analyzer. Below the front-panel illustration is a block diagram, greatly simplified, but adequate to offer an understanding of the basic principles.

Figure 10 12: Spectrum Analyzer, Simplified Illustration.

Central to the entire operation of the spectrum analyzer is the sweep generator, which creates a sawtooth Voltage for use in (1) moving the trace, from left to right, across the face of the cathode-ray-tube display, and (2) providing the Voltage-controlled local oscillator with a changing Voltage. The cathode-ray tube is generally the electrostatic-deflection type, common in test equipment. The sweep is free-running and unsynchronized, totally unrelated to the radar system timing. The sweep speed is adjustable; the choice of speed is dependent upon the user s objective, and the signal to be viewed. Generally, the slowest speeds would offer the best display, but the persistence of the cathode-ray tube makes them difficult to observe, and thus, undesirable. Over the years, direct-view storage tubes and memory devices have been used to overcome the undesirable effects of decaying brilliance.

The crt face is covered by a transparent graticule. The same sawtooth used for the crt sweep is modified before application to the Voltage-controlled local oscillator. One control, the FREQUENCY/DIVISION adjustment, is effectively a gain control for the sawtooth. As the sawtooth Voltage rise is increased, the rate of change in frequency applied to the first detector will also increase, and the local oscillator will span a...