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Abstract

A stereo decoder for use in a stereo FM radio receiver includes a phase-locked loop; for detecting the 19 KHz pilot signal in the FM composite signal; for generating the 38 KHz subcarrier for use as a reference signal to a double balanced gating circuit that combines he L-R and L+R components of the stereo composite signal to produce the left audio and right audio signals separately; and for providing a 19 KHz reference signal for use in the pilot detector and pilot indicator circuit. The phase-locked loop includes a mixer to which a 19 KHz reference and the composite signals are applied, a low pass filter switchable from a first bandpass of about 300 Hz to a second bandpass of about 10 Hz connected to the output of the mixer a voltage controlled oscillator connected to the output of the filter that has a natural operating frequency of 38 KHz and a divide-by-two circuit connecting the oscillator to an input of the mixer. The bandpass filter is normally (in the pilot search mode condition) switched to the 300 Hz bandpass and when the pilot indicator circuit is actuated the bandpass filter is switched to the 10 Hz bandpass (pilot locked mode).

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