PURPOSE: To change the frequency sensitivity by connecting a high-pass filter to a balanced modulator and a low-pass filter to other balanced modulator, taking the number of order of each filter as unity, making a cut-off frequency coincident and changing the cut-off frequency.

CONSTITUTION: In taking the number of order of the high-pass filter 31 and the low-pass filter 32 as unity and making their cut-off frequency coincident, the relative phase difference of both is almost 90°. The phase difference of outputs of the balanced modulators 14, 15 is 90°, and in connecting the filters 31, 32 to the balanced modulators 14, 15, the outputs from both filters 31, 32 are made to the in-phase. In multiplying the in-phase outputs at a multiplier 19, the frequency discriminating characteristic is a cubic curve taking a reference signal frequency fr as the center and having a pole at a frequency deviated by an approximate cut-off frequency fp, and since the cubic curve is kept while keeping the pole to a constant value even if the cut-off frequency fp is changed, the sensitivity of discrimination is changed.