JEE Main · 2013 · 07 Apr 2013 · Offline · Semiconductors

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.

(A)10.62 kHz
(B)5.31 MHz
(C)5.31 kHz✓ Correct
(D)10.62 MHz

Answer: (C)

Solution

A diode detector can follow the modulation envelope only up to the cut-off set by its

R C

filter,

f_{c} = \frac{1}{2 π R C}

. With

R = 100 k Ω

and

C = 250 pF

f_{c} = \frac{1}{2 π ( 1 0 ^{5} ) ( 250 \times 1 0 ^{- 12} )} \approx 6.4 kHz

. The highest modulation frequency it can faithfully detect must stay below this, and of the choices only

5.31 kHz

does.

JEE Main · 2013 · 07 Apr 2013 · Offline · Single correct option

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