This document describes in detail the principle and construction of our audio-frequency Spectrum Analyzer. A spectrum analyzer is a device that takes an electrical signal (usually non-DC) as its input and displays the frequency spectrum of the signal as an output. Spectrum analyzers are classified according to the frequency range of signals that they can analyze. There are audio spectrum analyzers (such as this one), which analyze the frequency spectrum of an audio signal (20 Hz — 20 kHz). Then there are video spectrum analyzers (which can show the spectrum of video signals),RF spectrum analyzers and so on.

Spectrum Analyzer block diagram

Figure 1: Spectrum Analyzer operation

A spectrum analyzer can be a powerful tool for signal analysis and troubleshooting. Unfortunately for many users however, the high price of a commercial spectrum analyzer presents a major problem – LCD versions cost many hundreds of dollars, and a good oscilloscope model can run into thousands. But once again the IC Operational Amplifier comes to the rescue, allowing construction of a sophisticated “manual” analyzer that gives a reasonably good response, all for just the cost of a few components.

Commercial spectrum analyzers work by splitting the input signal into various frequency bands over the instrument’s frequency range through the use of electronic filters, designed to pass only signals lying within the band specified. The filtered signal is then passed on to a peak detector whose output is the instantaneous peak envelope of signal at that frequency, or in other words, the amplitude of that frequency component. However, such analyzers are quite complex and require separate filter-detector blocks for each frequency band. But they display a real-time spectrum containing all the frequency components.

Our spectrum analyzer, however, will work on a slightly different (and cheaper!) principle, called the Superheterodyning Principle. It is described in detail on the next page.