For the modulated signal x(t) = m(t) * cos(2πfct), the message signal m(t) = 4cos(100πnt) and the carrier frequency fc is MHz. The signal x(t) is passed through a demodulator, as shown in the figure below. The output M(t) of the demodulator is:
Ideal LPF with cut-off 510 Hz
y(t)
x(t)
cos(2π(fc - 40)t)
(a) cos(100πnt) (c) cos(920πnt)
(b) cos(540πnt) (d) cos(460πnt)