JPS6435387A - Spectrum analyzer - Google Patents

Abstract

PURPOSE:To obtain a high-accuracy variable and BPF for resolution setting by mixing mutually 90 deg. out-of-phase signals with a signal passed through a fixed band BPF respectively and inputting them to a digital filter, and calculating the sum of squares of them. CONSTITUTION:The fixed band BPF is used as the BPF 30 of the final stage of a superheterodyne type spectrum analyzer and mixers 40 and 41 mix the outputs of an oscillator 38 which outputs the two 90 deg. out-of-phase signals S2 and S3 of frequency equal to the center frequency of the BPF 30 with the output of the BPF 30. The outputs of the mixers 40 and 41 are connected to variable band width digital filters 50 and 51 through low-pass filters 43 and 44 and AD converters 46 and 47. Then their outputs are connected to a computing element 53 to calculate the sum of their squares and a peak holding circuit 54 holds the peak, thereby obtaining a filter whose band width, i.e. resolution is settable with high accuracy.