Automatic frequency scanning radio receiver

Claim 1

An automatic frequency scanning FM radio receiver comprising: a phase-locked loop frequency synthesizer acting as a local oscillator circuit tunable to different radio frequencies and having a voltage-controlled oscillator, a programmable counter for dividing the output frequency of said voltage-controlled oscillator, the dividing factor of said programmable counter being changeable in integral steps by a program input code signal applied thereto, a reference oscillator, a phase comparator for comparing in phase an output signal of said reference oscillator with an output signal of said programmable counter, and a loop filter connected between an output of said phase comparator and a control input of said voltage-controlled oscillator, the output frequency of said voltage-controlled oscillator being changed in steps in response to change in the dividing factor of said programmable counter provided by the program input signal; a mixer circuit coupled to said voltage-controlled oscillator for mixing an incoming radio frequency signal with the output signal of said voltage-controlled oscillator to produce an intermediate-frequency FM signal; a scan clock pulse source; scan counter means coupled to said scan clock pulse source for counting scan clock pulses to supply said programmable counter with the program input code signal whose code numbers change in integral steps, whereby said frequency synthesizer is caused automatically to scan channels having frequencies corresponding to the code numbers within a radiobroadcasting frequency band, said scan counter means comprising an up/down counter; a tuning detection means for detecting that the receiver is tuned to a broadcast signal having an input level above a predetermined level, said tuning detection means comprising a frequency discriminator connected to receive the intermediate-frequency FM signal, a first circuit means for detecting the level of noise appearing at the output of said frequency discriminator, a second circuit means for detecting the level of a direct-current voltage indicating the degree of tuning of the receiver which appears at the output of said frequency discriminator, and a third circuit means producing a control signal to disable said scan clock pulse source from producing the scan clock pulses when the noise level detected by said first circuit means is below a squelch level and the DC output level of said frequency discriminator detected by said second circuit means indicates that the receiver is completely tuned; means for disabling said scan clock pulse source from producing the scan clock pulses when the receiver is completely tuned to said broadcast signal in response to said control signal of said tuning detection means; one-step manual scanning control switch means connected to said scan counter means and to said disabling means for disabling the scan clock pulse source from producing the scan clock pulses, said one-step manual scanning control switch means, when operated, actuating said disabling means and changing the count number in said scan counter means by one step for each operation thereof, and having a first switch for increasing the count number in said scan counter means by one step for each operation thereof and a second switch for decreasing said count number by one step for each operation thereof; and a control means for reversing the counting direction of said up/down counter when the count number in said up/down counter attains a value corresponding to the minimum frequency in said frequency band.