Disclosed is a frequency converter with an excellent frequency characteristic,
having a minimized number of multipliers. A digital down-converter includes a decimator/mixer,
an interpolator and a channel filter. The decimator/mixer performs quadrature conversion
from a real signal to complex signals, frequency conversion by a frequency K,
and 1/(MI)-fold decimation on a signal obtained by sampling an RF/IF signal
S(i) by an A/D converter. The interpolator is comprised of I-fold up-samplers and
lowpass filters, and performs I-fold interpolation on the outputs from the decimator/mixer.
The channel filter is comprised of lowpass filters having a band characteristic
given to a communication channel, and outputs band-rejected baseband signals i(j)
and q(j) by filtering the outputs of the interpolator.