Abstract: First, second and third manual control switches are arranged side by side in close proximity to each other, the first switch for initiating the upward sweep of frequency at a low sweep rate, the third switch for initiating the downward sweep of frequency at the same sweep rate, and the second or intermediate switch for switching the sweep rate to a higher rate. A counter having up- and down-counting modes of operation is provided, the modes of which are under the control of the first and third switches. The up-down counter is supplied with pulses at a low repetition frequency when either the first or third switch is operated and with pulses at a high repetition frequency when the second switch is operated. The local oscillations of a radio receiver are under the control of the output of a comparator connected in a phase-locked loop including the local oscillator of a radio receiver, a programmable counter and the comparator.