Abstract: A radio which has assigned frequencies or channels for operation is prevented from operating on other channels. The frequency synthesizer in the radio is a large scale integrated circuit in a single semiconductor chip which is controlled by input codes from channel select switches. The synthesizer includes circuitry which responds to input codes for illegal channels and produces an inhibit signal thereby preventing generation of unauthorized frequencies. The circuitry may include a read only memory which is addressed by the input code. Alternatively, logic circuitry may be provided which responds to codes for illegal channels. Due to the circuitry being in integrated form, the method of inhibiting unauthorized signal generation cannot be easily circumvented.