LOW IF RECEIVER

摘要

A low IF receiver in which an input signal is frequency translated in quadrature related mixers (24, 26), filtered in respective low pass channel filters (38, 40), derotated to produce a wanted signal and its image (52, 54), is at switch-on operated as a zero IF receiver in order to determine which of the adjacent channels to the wanted channel has the smaller interferer and the local oscillator frequency supplied to the mixers (24, 26) is then adjusted to bring that interferer into the channel bandwidth of the low pass channel filters (38, 40). More particularly at switch-on the local oscillator (28) frequency is shifted by half a channel in order to place the wanted signal symmetrically about DC with the unwanted adjacent channel interferers disposed on each side. Simultaneously the bandwidth of the low pass filters (38, 40) is widened to include both the unwanted interferers and also a DC notch is inserted into the signal paths by high pass filtering means (34, 36). Each of the interferers is produced on a respective output (52, 54) of a derotate stage (50) and in a decision stage (56) it is determined which is the smaller of the interferers and on that basis a control stage (58) adjusts the local oscillator frequency by half a channel bandwidth in the positive or negative direction and simultaneously the bandwidths of the low pass filters (38, 40) are reduced and the high pass filtering means are removed thereby eliminating the notch in the frequency of the output of the mixers (24, 26).