Selective Calling In Cb Radios

Base CB radio sets equipped with the present tone call unit can be left switched on all day without producing noise or conversations between other CB users, even if the signals trip the set squelch threshold. The receiver"s AF amplifier is only enabled when a specific tone call is received from another station, which may be fixed or mobile. It is also possible to have the selective tone decoder automatically turn on the transmitter in the base station to acknowledge ad- equate reception of the caller (auto-answer facility). The block diagram of the selec- tive tone call system appears in Fig. 1. The upper blocks repre- sent the tone transmitter, the lower the receiver, while those in the centre and to the right form the interface to the CB radio. 1 II V Circuit description Ub fe-0-10 miM 100n 252 BV The circuit diagram of the tone transmitter section is shown in Fig. 2. The main blocks in this circuit are a Schmitt-trigger based clock oscillator, N2, a counter, IC2, and an astable...