Question

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

• A. $10.62 \,MHz$
• B. $10.62\, kHz$
• C. $5.31 \, MHz$
• D. $5.31 \, kHz$

Answer 1

Answer: (B)

$10.62\, kHz$

Answer 2

$\tau = RC = 100 \times 10^3 \times 250 \times 10^{-12}s$
$\, \, \, \, \, \, \, 2.5 \times 10^{-5}s$
The higher frequency which can be detected with tolerable distortion is
$f = \frac {1}{2\pi m_aRC} = \frac {1}{2\pi \, \times 0.6 \, \times 2.5 \, \times \, 10^{-5}} Hz$
$\frac {100 \times 10^4}{25 \times 1.2\pi}= Hz$
$= \frac {4}{1.2\pi} \times 10^4 Hz$
$= 10.61 kHz$