amplitudes and phases of u03b1f 1 and u03b1f 2 especially u03b1f 1 and u03b1f 2 near the central frequency u03b2 t f 1 u03b2 t f 2 to compensate or offset the components of u03b1f 1 and u03b1f 2 the dsp unit 250 generates out of phase signals of u03b1f 1 and u03b1f 2 to cancel the components of u03b1f 1 and u03b1f 2 of amplified signals via a digital to analog converter dac 1 b 268 the 1 st stage output signal and the out of phase signals of u03b1f 1 and u03b1f 2 of the dac 1 b 268 are output to the 2 nd stage amplifier 270 in which the out of phase signals can be for example an invert phase so as to suppress the other signal component other than the target signal component with the frequency of u03b3 t f 1 u03b2 t f 2 thus the relative amplitude of u03b3 t f 1 u03b2 t f 2 with respect to other components increases as a result the amplitudes of u03b1f 1 and u03b1f 2 are not significantly amplified and may even be reduced due to the compensation process moreover the sub harmonic effect of electronic circuit is also suppressed hence it is possible to keep the total intensity of the output signal from the 1 st unit below the high level limitation of operation amplifiers in the 2 nd stage.