FM to Am

In a parallel LC circuit the impedance curve has segments below and above the resonant frequency where the impedance is almost a linear function of frequency. If the LC circuit is excited with AC current source at frequencies within those linear-impedance segments, the voltage that will be developed across the LC circuit is approximately a linear function of frequency. Thus, when an FM current with frequency range within those segments is applied to the LC circuit an amplitude-modulated voltage will be obtained. In this circuit the resonant frequency of the LC circuit is above the FM carrier, the FM signal then falls at the segment with a rising slope.