An approximate spectrum analyser

Abstract

A simple hardware approximate spectrum analyser is described which is obtained by replacing the sine and cosine modulating functions used in a complex demodulator by a square-wave approximation. Measured approximate power spectra show a defined pattern of small sub-harmonic error terms. If the maximum input frequency is limited to F then no errors wall be incurred over the range F/3 to F. Experimental and theoretical results are presented describing the effect of sweep time on the measured speetrum. The simplicity of the hardware involved allows a parallel array of complex demodulators to be created and, when the individual elements of the array are swept over adjacent frequency bands, a greatly reduced measurement time is obtained.