Question:
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
Solution:
Carrier wave frequency, fc – 20 MHz = 20 × 106 Hz
Bandwidth = 2fm = 3 × 103 Hz
fm = 1.5 × 103 Hz
1/fc = 0.5 × 10-7
1/fm = 0.7 × 10-3
(i) 1/fc << RC < 1/fm
Therefore, it can be demodulated
(ii) 1/fc << RC << 1/fm
Therefore, it can be demodulated
(iii) 1/fc < RC
Therefore, it cannot be demodulated