HIGH SENSITIVITY FM SIGNAL DEMODULATION SYSTEM

摘要

The present invention relates to an improved demodulation system which improves the noise characteristics for the demodulation of a wideband television FM signal. A variable bandpass filter for improving the C/N ratio is provided at the output of a reference bandpass filter. This variable bandpass filter, of which the center frequency and bandwidth are controllable, is inserted in an FM signal path along with the bandpass filter. The bandwidth of the variable bandpass filter is controlled so that it is wide when the input carrier level is high, and is narrow when the input carrier level is low. The center frequency of the variable bandwidth filter is controlled so that it follows the instantaneous frequency change of the color sub-carrier component (3.58 MHz) of the FM signal, the latter being derived by a narrow-band bandpass filter coupled with the output of a discriminator also residing within the circuit and as a result, control of the center frequency of the variable bandpass filter is achieved. This system is advantaseous over prior art systems in that it will follow the instantaneous frequency change of FM input signal to the system, and improve the impulse noise characteristics which reduces the demodulated waveform deformation.