double balanced mixer 3 by u03bc produces a balanced condition the following equation holds from the above equation for the feedback loop and if equ3 then the equation below is obtained equ4 in the above equation the first and second terms are about 1 2 mv and 0 03 mv respectively in other words the rf voltage can be controlled with an accuracy level of 4 u00d7 10 6 of that of the reference voltage to be controlled additionally the u voltage can be controlled with an accuracy level of 10 5 of that of the reference voltage to be controlled because the former is a dc voltage or has a sawtooth waveform while the positive peak value of u vcosmt of the voltages to be applied to the quadrupole mass spectrometer is compared for calibration in the above embodiment the negative peak value may alternatively be compared with a negative calibration voltage of u o v o then the polarity of the calibration diode and that of the error signal amplifier will have to be inverted alternatively the positive or negative peak value of the voltage u vcos u03c9 t to be applied to the other pair of electrodes of the quadrupole mass spectrometer may be compared with u.