Consider the frequency demodulation scheme in Figure in which the incoming FM signal s (t) is passed through a delay line that produces a phase-shift of ?/2 radians at the carrier frequency ?_c. The delay-line output is subtracted from the incoming FM signal, and the resulting composite signal is then envelope-detected. This demodulator finds application in demodulating microwave FM signals. Assuming that s (t) = A_c cos (2??_ct) + ? sin (2??_mt) analyze the operation of this demodulator when the modulation index ? is less then unity and the delay T produced by the delay line is sufficiently small to justify making the approximations cos (2??_mT) = 1 and sin (2??_mt) = 2??_mt

FM FM wave Outout signal Delay-line Envelape detector ) s()