| 摘要 |
PURPOSE:To suppress noise components generated in the output by providing a cut-off frequency variable low-pass filter in the succeeding stage of an FM demodulator circuit and lowering the cut-off frequency in case that the FM signal input is small of frequencies of IF signals are separated by a prescribed frequency or more. CONSTITUTION:Signals obtained by converting FM signals applied to input terminal A to IF signals are applied to limiter amplifier 101 and are not only amplified but also subjected to amplitude limiting, and the output of amplifier 101 is applied to FM demodulator 301 and is demodulated to sound signals. Signals demodulated by FM demodulator 301 are outputted to output terminal B through cut-off frequency variable low-pass filter 401. Meanwhile, the output of amplifier 101 and signals demodulated by demodulator 301 are applied to signal detector 501 to detect the strength of FM signals of amplifier 101 and detect whether output signals of demodulator 301 are prescribed IF signals or not, and the passing band of filter 401 is controlled by the output of detector 501, thereby reducing the noise included in signals at output terminal B. |
