a tuning circuit for a high frequency receiver in which the received frequency is determined by a voltage - controlled oscillator , the present tuning frequency is indicated by a counter connected to the oscillator and producing a representation of each digit of the decimal number identifying the current received frequency , representations of the digits of the number identifying the desired received frequency are fed in via a keyboard , and the decimal number representations are compared in a comparator to produce a control voltage that brings the oscillator frequency to the desired value , and in which the comparator is capable of comparing only one digit of the number representations at a time and receives the digits of corresponding significance in sequence , starting with the most significant digit , and the rate at which the control voltage is varied is made inversely proportional to the number of changes in the direction of the inequality of the digits of one of the number representations being compared relative to the corresponding digits of the other number representation being compared .