Transmitter-receiver employing automatic channel hunting and selection

A channel hunting and selection system for a two-way transmitter-receiver combination of the type wherein the receiver operates over a selected one of a plurality of given RF channels, said receiver having a mixer for receiving RF signals and the output of a variable frequency local oscillator to provide a first IF signal output indicative of said channels, and said receiver having a demodulator for detecting the output of said mixer to provide the information bearing modulation signal of a modulated IF signal, said system comprising, in combination:

• a second local oscillator for providing a second IF signal the frequency of which is substantially equal to the frequency of said first IF signal;
• means for modulating said second IF signal with a given address signal;
• means for coupling said modulated second IF signal with said first IF signal at the output of said mixer and at a predetermined level above the noise level of said first IF signal;
• means coupled to the detected output of said receiver for detecting said given address signal and when the level of said second IF signal significantly exceeds the level of said first IF signal for providing an idle-channel output signal indicative thereof;
• and,
• means responsive to said idle-channel output signal for controlling said variable frequency local oscillator to a fixed frequency corresponding to a selected one of said given channels.