An audio signal is modulated by a carrier wave of 20MHz such that the bandwidth required for modulation is 3kHz. Could this wave be demodulated by a diode detector which has the values of R and C as

(i) R = 1 kΩ, C = 0.01μF.

(ii) R = 10 kΩ, C = 0.01μF.

(iii) R = 10 kΩ, C = 0.1μF.

Let the frequency of the carrier wave be and the frequency of message signal be . For the modulated signal to be demodulated, the following equation has to be satisfied:

Now, it is given that

Carrier frequency,

Message signal frequency,

(Note that 2 × message signal frequency = bandwidth)

Now,

Also,

(i)

Here, and

Clearly,

Hence, this diode detector can demodulate the signal.

(ii)

Here, and

Clearly,

Hence, this diode detector can demodulate the signal.

(iii)

Here, and

Clearly,

Hence, this diode detector cannot be demodulate the signal.