Spectrum analyzer having image frequency eliminating device

Abstract

A frequency spectrum analyzer includes a display for showing frequency spectrums in a frequency domain versus power level form, a local oscillator which is driven by a sweep signal to repeatedly cause a frequency sweep of a local signal wherein the local oscillator and the display operate in synchronism with the sweep signal, an intermediate frequency filter whose center frequency is an intermediate frequency for transmitting intermediate frequency signals, a frequency mixer for mixing an input signal to be measured and the local signal and generating the intermediate frequency signals whose frequency is a difference of the input signal and fundamental and harmonic components of the local signal, a detector for rectifying the intermediate frequency signal from the intermediate frequency filter wherein an output of the detector shows frequency spectrums of a true frequency spectrum of the input signal and image spectrums produced by the mixer, a sweep offset control for alternately causing a frequency shift in the frequency sweep in the local signal wherein the frequency shift is equal to f.sub.if2 .times.2/N, where f.sub.if2 is the intermediate frequency and N is a harmonic number of the local signal, and an image removal processor for monitoring the output of the detector and identifying the true frequency spectrum of the input signal and the image spectrums on the basis of positional changes between the frequency sweep without the frequency shift and the frequency sweep involving the frequency shift in the local signal.