To provide a quadrature detector wherein unnecessary frequency components
are not produced, the following procedure is carried out: an analog input
signal is converted into a digital signal f1(t), and the signal f1(t) is
delayed by a sampling time .tau. to form a signal f2(t). Then, letting
the reference frequency be .omega.0, the I- and Q-components in
quadrature detection by the following expression:
I={f2(t)*sin .omega.0t-f1(t)*sin .omega.0(t-.tau.)}/sin .omega.0.tau.
Q={f2(t)*cos .omega.0t-f1(t)*cos .omega.0(t-.tau.)}/sin .omega.0.tau.
The signal processing based on the above expression is carried out by a
circuit comprising delaying means, multiplying means, and subtracting
means.
