| 摘要 |
A decoder for obtaining the L-R information in an AM stereo radio receiver. The decoder is basically a four-quadrant multiplier that has one pair of inputs driven from a limiter that operates from the receiver intermediate frequency amplifier. The limiter also drives a tuned circuit which in turn drives the other two multiplier inputs in phase quadrature with the first input pair. The multiplier output is frequency modulation responsive which, when integrated, produces a phase modulation response. The integration is produced by a capacitor connected across the multiplier output terminals. The decoder also includes a large parallel connected inductor that resonates the integrating capacitor at a frequency at the low end of the audio range. This inductor acts as a d-c short at the decoder output and sets the decoder response to the setaudible stereo pilot signal. The inductor is simulated with integrated circuit components. A first transconductance amplifier coupled to the multiplier output, drives a capacitor which in turn drives a second transconductance amplifier that operates current sources which act as load elements for the multiplier. The resultant feedback loop creates an inductive reactance that simulates a large inductor.
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