PM Decoder sample and hold circuit

摘要

An AM stereo receiver decoder is shown. An AM detector produces the stereo L+R signal and a PM detector produces the L-R signal. The PM detector is created from a conventional FM detector that employs an input limiter driving a balanced multiplier. The limiter also drives a tuned circuit which provides quadrature drive to the multiplier. An integrator connected to the FM detector converts the response to a PM decoder. A large value inductor is simulated to appear across the integrator so as to create a low modulation frequency resonance at a subaudible frequency thereby providing a controlled pilot tone response. The inductor is simulated by the action of a first Gm amplifier driving a capacitor which drives a second Gm amplifier having an output coupled back to the input of the first Gm amplifier. The capacitor is switched by means of a series connected switch that disconnects the capacitor when the AM exceeds a predetermined value. This means that when the L+R negative modulation peaks exceed some predetermined value the simulated inductor acts to short out the L-R signal channel. Since the switch is in series with the capacitor, the charge cannot vary when the switch is open, thereby creating a sample and hold action.