Apparatus and method for frequency channel selection in a radiotelephone system

In a mobile radiotelephone system having N separated radio frequency channels, apparatus for selecting the particular idle frequency channel having the best signal-to-noise ratio, said apparatus comprising:

• demodulating receiver means for receiving N different modulated carrier signals in each of said N radio frequency channels, said demodulator means under the control of switching means providing at an output a succession of N different output signals, and each output signal having idle and busy mutually exclusive states corresponding to the idle and busy state of the corresponding radio frequency channel;
• first detector means having a first input terminal connected to the output of said receiver means and having a first output terminal, said detector means providing a first logic signal when one of said receiver output signal concurrently exhibits an idle state and has at least a predetermined minimum signal-to-noise level, and said detector means providing a second logic signal at all other times;
• subscriber handset means having an input connected to said receiver means output and having a second output terminal, said handset means having on-hook and off-hook mutually exclusive states;
• a call-in-progress state detector for detecting the on-hook and off-hook states of said handset means, said state detector having an input connected to said second output terminal and also having a third output terminal providing an on-hook and off-hook output signal indicative of the on-hook and off-hook states respectively of said handset means;
• receiver switching means having an output and second and third input terminals, said output adapted for connection to said receiver means, said second input terminal connected to said first output terminal, and said third input terminal connected to said third output terminal, said switching means for cyclically switching said receiver means to each of said N frequency channels in response either to said first logic signal or to said off-hook signal;
• and
• gain control means having an output and fourth and fifth input terminals, said output adapted for connection to said receiver means, said fourth input terminal connected to said first output terminal, and said fifth input terminal connected to said third output terminal, said gain control means producing a control output signal for causing a decrease in the gain sensitivity of said receiver means from a predetermined maximum gain to a predetermined minimum gain and thereafter for causing an increase in said gain sensitivity to said predetermined maximum, said gain increase occurring at a rate substantially slower than the rate of gain decrease so that the period of time required for said switching means to switch said receiver means to each of said N frequency channels at least once is substantially greater than the time required for the gain sensitivity to increase from said minimum to said maximum, said control output signal being produced in response to either
• (a) a change in state of said first detector means output signal from said first logic signal to said second logic signal or
• (b) in response to a predetermined timed interval after a change in output state of said first detector means from said first logic signal to said second logic signal unless an off-hook output signal is produced by said call-in-progress detector, and
• said gain control means being inhibited from generating said control output signal whenever an off-hook indication from said call-in-progress detector means is produced.