frequency difference detector (fdd) and a baereboelgemoduleret receiver containing such a fdd

摘要

A frequency difference detector (FDD) of a balanced quadricorrelator type comprises a front end formed by quadrature related first and second mixers (12, 14) in which an input signal is mixed down to base band. After the in-band signal components have been selected by low pass filter (18, 20) the signals VI1, VQ1 are applied to third and fourth mixers (30, 32). In applying these signals VI1 and VQ1, they are differentiated by one or more orders and applied to respective first inputs of the third and fourth mixers (30, 32) and to respective second inputs of the fourth and third mixers. When differentiating the signals VI1 and VQ1, it is necessary that there is one order of difference between the signals at the first and second inputs of each of the third and fourth mixers (30, 32). The outputs of the third and fourth mixers are applied to a subtracting circuit (38) which provides a frequency difference output signal. An a.g.c. circuit is disclosed. The FDD disclosed can form part of an FM demodulator for a carrier modulated receiver.