An FM receiver that is suitable for reducing a transmission bandwidth W F of a bandpass filter to remove adjacent-channel interference, and increasing W F to prevent audio distortion. A detection output signal S OUT is inputted to an HPF 122 when a reception electric field strength signal S M-DC indicates an intermediate or stronger electric field. In a weak electric field, an AC component signal S M-AC , which is extracted from an intermediate signal S IF1 prior to detection and which has fewer high-pass noise components than S OUT , is inputted to the HPF 122 . A control circuit 120 detects a case as an adjacent-channel interference state when a large amount of high-pass components passes through the HPF 122 , and reduces W F of an IFBPF 70 in order to remove adjacent-channel interference. When a small amount of high-pass components is transmitted, W F is increased in order to minimize audio distortion.
1. An FM receiver, comprising:
an intermediate signal generating circuit for subjecting a reception signal to frequency conversion in which a carrier frequency of a received target FM signal is shifted to a predetermined intermediate frequency, and generating an intermediate signal;
a bandpass filter for allowing a transmission bandwidth to be variably set, and transmitting the received target FM signal that has been converted to the intermediate signal;
a detection circuit for subjecting the received target FM signal that has passed through the bandpass filter to detection, and generating a detection output signal;
a bandwidth controller for controlling the transmission bandwidth of the bandpass filter;
a strength signal generating circuit for generating, on the basis of the intermediate signal, an electric field strength signal according to the strength of a reception electric field; and
a fluctuation component signal generating circuit for generating a fluctuation component signal according to fluctuation in an amplitude of the intermediate signal; wherein
the bandwidth controller comprises:
a high-pass filter, which has a cut-off frequency that exceeds an audio band;
a switch circuit for selecting either the detection output signal or the fluctuation component signal according to the electric field strength signal, and inputting the selected signal to the high-pass filter; and
a control circuit for controlling the transmission bandwidth on the basis of whether a weak high-pass component state or a strong high-pass component state is in effect, with the weak high-pass component state occurring when the output level of the high-pass filter is equal to or less than a predetermined reference value, and the strong high-pass component state occurring when the output level exceeds the reference value; wherein
the switch circuit outputs the fluctuation component signal when the reception electric field strength is in a predetermined weak electric field state, and outputs the detection output signal when the reception electric field strength exceeds the weak electric field state; and
the control circuit sets the transmission bandwidth to a predetermined wide bandwidth in the weak high-pass component state, and sets the transmission bandwidth to a narrow bandwidth, which is narrower than the wide band width, in the strong high-pass component state.
2. The FM receiver of claim 1 , further comprising:
a signal meter circuit composed of the strength signal generating circuit and the fluctuation component signal generating circuit; and
the signal meter circuit smoothing a signal to be measured according to the intermediate signal, generating the electric field strength signal, subjecting the signal that is to be measured to envelope detection, and generating the fluctuation component signal.
3. The FM receiver according to claim 1 ; comprising:
the bandwidth controller having:
a low-pass filter, which receives the detection output signal, and allows the audio band to pass through; and
a low-pass component input control switch circuit, which is selectively switched on in the weak electric field state, and which is capable of inputting a signal from the low-pass filter to the control circuit;
the control circuit setting the transmission bandwidth to the narrow bandwidth when the weak high-pass component state is both in the weak electric field state and in a weak low-pass component state in which an output level of the low-pass filter is equal to or less than a predetermined reference value.
4. The FM receiver of claims 1 , comprising:
the bandwidth controller having:
a high-pass filter output capacitor for smoothing a transmission signal of the high-pass filter, and generating a voltage that is the output level of the high-pass filter; and
a pulse noise cancelling circuit for detecting the appearance of pulse noise, and preventing the high-pass filter output capacitor from being charged by the pulse noise.
5. The FM receiver of claims 1 , comprising:
the FM receiver being of a double-conversion format in which a first intermediate signal is inputted as the intermediate signal to the strength signal generating circuit and the fluctuation component signal generating circuit, and a second intermediate signal, which is obtained by down-converting the first intermediate signal, is inputted to the band pass filter.