Group delay time measurement apparatus with automatic aperture value

Abstract

A frequency converter receives a signal to be measured and a local oscillation signal supplied from a local oscillator and ouputs an intermediate frequency signal. A phase detector detects a phase of the intermediate frequency signal. An A/D converter outputs a digital value corresponding to a phase detection output. A memory stores a plurality of aperture values. Each of the aperture values is read out with a predetermined frequency band which is incremented at a predetermined step. A first controller causes the local oscillator to oscillate signals having first and second frequencies separated from each other by the aperture value in the vicinity of a desired measurement frequency. An arithmetic unit calculates a group delay time of the signal to be measured in accordance with the two digital values and the aperture value corresponding to the respective phase detection outputs when the local oscillator oscillates the signals having the first and second frequencies. A second controller causes the memory to start or to stop reading and incrementing the aperture value when the group delay time is determined to be smaller or larger than a reference value.