直交変調復調形サーボアナライザ

抄録

A method of measuring the frequency response of a linear system is proposed; it provides the real and imaginary parts of the frequency response simultaneously by driving the system with a sinusoidal test signal.
The measuring device is based on a modification of the so called servo-analyzer. That is, the output signals of integrators corresponding to the real and imaginary parts of the frequency response are modulated by sine and cosine waves respectively, and the sum of them is fed back negatively to the input of the device.
The name “Quadrature Modulation and Demodulation Type Servo-analyzer” refers to the fact that the device has sine-and cosine-wave demodulation circuits in the forward paths in addition to the modulation circuits in the feed-back paths.
The outputs of the QMD type servo-analyzer are free from the fluctuation caused by the harmonics as is shown in the characteristics of a quadrature modulation and demodulation type filter (or two-path filter) which is the subject of recent interest in circuit theory.
When the observed signal is disturbed by noise signal the stochastic approximation method is applied to surpress the fluctuation of the results.
An on-off type correlation analyzer proposed by the authors before was modified and a very simple QMD type servo-analyzer was constructed.
The QMD type servo-analyzer can be used as a narrow band pass filter in which the center frequency is fixed by an external signal. Thus it can be used as a filters in the frequency response measuring system without test signal.