# FM Demodulation

I'm using a demodulation fm circuit in multisim. it takes 1 minute on the oscilloscope to complete a full waveform, and every waveform the amplitude is decreasing. I want to know why is the amplitude decreasing from the demodulated output? My fm signal is set to Voltage Amplitude =20 V, Carrier Frequency = 8 MHz, Modulation Index = 5 and Signal Frequency = 10 kHz.

• How can we possibly answer this without a description of your circuit and the signal you're feeding it? – Dave Tweed May 17 at 17:25
• With modulation index of 5, the delta_frequency is 5X the modulating_frequency. Thus the delta_frequency is 50,000 Hertz. Is your circuit designed to linearly response to 8MHz +- 25,000Hz? – analogsystemsrf May 18 at 2:17
• I'm not sure of my circuit if it is designed to respond linearly, this circuit is taken from national instruments: Understanding RF signals. – Ahsan May 19 at 10:34

## 1 Answer

Based on the schematic and the shape of the decrease it looks like the effect of 2 close but different frequencies on each other. The shape of the fm signal's envelope decrease looks like a sinusoid, rather than an exponential decay envelope.

In other words the fm signal frequency may be slightly off the frequencies of your filter. If you let the simulation run longer the signal may diminish and then come back again. Shown below is an example of a constant frequency that has a smaller, close frequency added to it.

For example, the resonant frequency of L1+C4 is 10.3 kHz.