FM Demodulation

There are several ways to demodulate an FM signal. In this lab you will use a differentiator followed by an AM detector do demodulate and FM signal as shown in Figure 2

Figure 2: Frequency discriminator

In order to implement the above the frequency discriminator, we need to design a differentiator. One way to implement a differentiator is using an optimal equiripple linear-phase FIR filter. This filter is optimal because the weighted approximation error between the desired frequency response and the actual frequency response is spread evenly across the passband and evenly across the stopband. This results in minimizing the maximum error. Remez algorithm may be used to generate this filter. Below are samples of the output generated using the remez function. As seen in the figures if the differentiator is designed over a smaller frequency band the absolute error less that the filter over the entire range of frequencies.

Figure 3: Magnitude response FIR equiripple differentiator for $\omega =0$ to $\pi$

Figure 4: Absolute error of an FIR equiripple differentiator for $\omega =0$ to $\pi$

Figure 5: Magnitude repines FIR equiripple differentiator for $\omega =0$ to $0.4\pi$

Figure 6: Absolute error of an FIR equiripple differentiator for $\omega =0$ to $0.4\pi$