Automatic frequency sweeping method for controlling a frequency synthesizing type autotuner

An automatic frequency sweeping method for controlling a frequency synthesizing type autotuner for broadcasting receivers, in which said autotuner includes a phase locked loop for controllably generating a local oscillation signal, a mixer receiving a RF received signal and said local oscillation signal for generating an intermediate frequency signal, an intermediate frequency counter receiving said intermediate frequency signal through a gate, and a controller coupled to said phase locked loop, said intermediate frequency counter and said gate and also coupled with a key input used for providing an automatic frequency sweeping instruction, said controller responding to said automatic frequency sweeping instruction given through said key input so as to cause said phase locked loop to increment or decrement the frequency of said local oscillation signal, said controller operating to open said gate so as to allow said intermediate frequency signal to be inputted to said intermediate frequency counter so that said intermediate frequency signal is counted for a period in which said gate is opened, said controller also operating to examine whether or not a count value of said intermediate frequency counter is within a predetermined count range and to cause said phase locked loop to further increment or decrement said frequency of said local oscillation signal if said count value of the intermediate frequency counter is not within said predetermined count range, and said controller operating to cause said phase locked loop to stop the further increment or decrement of said frequency of said local oscillation signal if said count value of said intermediate frequency counter is within said predetermined count range, the method comprising a first intermediate frequency counting method in which said intermediate frequency signal is counted by opening said gate for a relatively short first period of time and the presence of the received signal is judged when said count value of said intermediate frequency counter is within a first range corresponding to a relatively wide intermediate frequency width and a second intermediate frequency counting method in which said intermediate frequency signal is counted by opening said gate for a second period of time longer than said first period of time and the presence of the received signal is judged when said count value of said intermediate frequency counter is within a second range corresponding to an intermediate frequency width narrower than said relatively wide intermediate frequency width, wherein when said automatic frequency sweeping instruction is given through said key input, a high speed frequency sweeping is ordinarily executed by using said first intermediate frequency counting method, and when the presence of said received signal is judged by said first intermediate frequency counting method, a more precise judgment is made in accordance with said second intermediate frequency counting method.