PURPOSE: To obtain a stable tracking filter with a few number of spurious signals and to execute the switching of an RF signal to a high/low frequency at high speed and with high quality by providing a discriminator having a magnetic material in a magnetic field different a little from a tuning magnetic field.

CONSTITUTION: A magnetic field coil 308 generates a uniform tuning magnetic field on the entire of four YIG resonators. The resonator 302 switches the RF signal to a low frequency analyzer of a filter mixer or a succeeding stage by cooperating with a switch 307 as a first step. The resonator 303 is operated as the second stage of a filter, and the resonator 304 is operated as the third stage of the filter and a fundamental mixer which mixes the RF signal with a sweep LO signal and generates an IF output signal. The resonator 305 compares a LO frequency with a filter tuning frequency as the discriminator and generates an error signal in a magnetic coil driving circuit. An upper coil 312 generates an offset magnetic field, and other three resonators tracks the LO, however, it is tuned to an RF frequency.