Noise reduction in an FM receiver

摘要

Apparatus to reduce noise in a stereo FM broadcast received via an antenna (10), the apparatus comprising: a frequency translator (20) configured to translate the received stereo FM broadcast to an intermediate carrier frequency, a demodulation unit (60) configured to demodulate the translated FM signal so as to form left-plus-right and left-minus-right AM signals, a filter (110) configured to form a filtered signal by filtering one of the AM signals, so as to suppress a sub-band of that signal containing unwanted tones, a summing unit (130, 140) for summing the filtered signal and the other of the left-plus-right and left-minus-right signals to produce a stereo audio signal. The invention is particularly concerned with removing an audible tone (of 5232Hz) produced by a phase locked loop (PLL) reference frequency (of 32768Hz) used to generate a local oscillator (LO) signal. It has particular application in mobile telephones and other handheld devices incorporating FM radio receivers, in which a mono audio signal must be extracted from a stereo FM signal.