Electronic Devices

A diode detector is used to detect an amplitude modulated wave of 60% modulation by using a condenser of capacity 250 pico farad in parallel with a load resistance 100 kilo ohm. Find the maximum modulated frequency which could be detected by it.

  1. 10.62 KHz
  2. 5.31 MHz
  3. 5.31 KHz
  4. 10.62 MHz

Answer

τ = RC = 100 × 103 × 250 × 10-12 sec

= 2.5 × 10-5 sec

The higher frequency which can be detected with tolerable distortion is

f = 1/(2πmaRC)

where ma = 0.6 and RC = 2.5 × 10-5 sec

f = 10.61 KHz

This condition is obtained by applying the condition that rate of decay of capacitor voltage must be equal or less then the rate of decay modulated singnal voltage for proper detection of modulated signal.

The correct option is A.