Question

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

1. R = 1 kΩ, C = 0.01 µF.
2. R = 10 kΩ, C = 0.01 µF.
3. R = 10 kΩ, C = 0.1 µF.

Answer 1

`f_m = 1.5  kHz, 1/f_m = 0.7 xx 10^-3 s`

`f_c = 20  MHz, 1/f_c = 0.5 xx 10^-7 s`

i. RC = `10^3 xx 10^-8  = 10^-5 s`

So, `1/f_c` << `RC < 1/f_m` is satisfied

So it can be demodulated.

ii. RC = `10^4 xx 10^-8  = 10^-4 s`

Here too `1/f_c` << `RC < 1/f_m` 

So this too can be demodulated

iii. RC = `104 xx 10^-12 = 10^-8 s`

Here `1/f_c > RC`, so this cannot be demodulated.