Spectrum analyzer having image frequency eliminating device

Claim 1

A frequency spectrum analyzer for measuring frequency spectrums in an input signal, comprising: a display for showing frequency spectrums in a power level versus frequency domain form; a local oscillator which is driven by a sweep signal to repeatedly cause a frequency sweep of a local signal, said local oscillator and said display operate in synchronism with said 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 said local signal and generating said intermediate frequency signals whose frequency is a difference of said input signal and fundamental and harmonic components of said local signal, said intermediate frequency signal being supplied to said intermediate frequency filter; a detector for rectifying said intermediate frequency signal from said intermediate frequency filter, an output of said detector showing frequency spectrums of a true frequency spectrum of said input signal and image spectrums produced by said frequency mixer; a sweep offset control for alternately causing a first sweep having no frequency shift and a second sweep having a frequency shift in said frequency sweep in said local signal, said frequency shift being equal to f if2 ×2/N, where f if2 is said intermediate frequency and N is a harmonic number of said local signal to be used in said frequency mixer; and an image removal processor for monitoring an output signal of said detector and identifying said true frequency spectrum of said input signal and said image spectrums on the basis of positional changes between said first sweep without said frequency shift and said second sweep including said frequency shift in said local signal; wherein only said true frequency spectrum is superimposed on the same position of said frequency domain between said first and second sweeps.