Patent ID: 6215829
Filing Date: 2001-04-10
Classification: H03J,H04B

Abstract:
An apparatus for receiving carrier wave signals, comprising:a frequency converting portion for frequency-converting an input signal so as to convert a carrier frequency of the input signal into an intermediate frequency signal having a predetermined intermediate frequency using a local oscillation signal whose frequency determines a receiving frequency and varies at a predetermined frequency interval;an intermediate frequency amplifying portion for amplifying the intermediate frequency signal;a frequency demodulating portion for frequency-demodulating the intermediate frequency signal obtained from the intermediate frequency amplifying portion with a demodulation characteristic having a center frequency substantially equal to the intermediate frequency;a DC level detecting portion for producing a first level detection output signal which has a predetermined level when a DC level of a demodulation output signal obtained from the frequency demodulating portion is not less than a predetermined level corresponding to the center frequency of the demodulation characteristic;an intermediate frequency level detecting portion for producing a second level detection output signal which has a predetermined level when a frequency component of the intermediate frequency signal obtained from the intermediate frequency amplifying portion has a level not less than a predetermined level, and when the frequency component belongs to a predetermined frequency range that includes the center frequency of the demodulation characteristic of said frequency demodulating portion; anda signal discriminating portion for discriminating between a desirable condition in which the input signal is a carrier wave signal desired to be received and an undesirable condition in which the input signal is an image frequency signal based on a strength of the first level detection output signal obtained from the DC level detecting portion and a strength of the second level detection output signal obtained from the intermediate frequency level detecting portion, wherein the receiving frequency is successively changed until both the first level detection output signal and the second level detection output signal are at the respective predetermined levels.