1

### JEE Main 2017 (Online) 8th April Morning Slot

A signal of frequency 20 kHz and peak voltage of 5 Volt is used to modulate a carrier wave of frequency 1.2 MHz and peak voltage 25 Volts. Choose the correct statement.
A
Modulation index = 5, side frequency bands are at 1400 kHz and 1000 kHz
B
Modulation index = 5, side frequency bands are at 21.2 kHz and 18.8 kHz
C
Modulation index = 0.8, side frequency bands are at 1180 kHz and 1220 kHz
D
Modulation index = 0.2, side frequency bands are at 1220 kHz and 1180 kHz

## Explanation

Modulation index (m) = ${{{V_m}} \over {{V_0}}} = {5 \over {25}} = 0.2$

Given, carrier wave,

Fc = 1.2 $\times$ 106 Hz = 1200 kHz

and frequency of modulate wave,

Fm = 20 kHz

$\therefore\,\,\,$ Side frequency bands are

F1 = 1200 + 20 = 1220 kHz

F2 = 1200 $-$ 80 = 1180 kHz
2

### JEE Main 2017 (Online) 9th April Morning Slot

A signal is to be transmitted through a wave of wavelength $\lambda$, using a linear antenna. The length l of the antenna and effective power radiated Peff will be given respectively as :

(K is a constant of proportionality)
A
$\lambda$, Peff = K $\left( {{1 \over \lambda }} \right)$2
B
${{\lambda \over 8}}$, Peff = K $\left( {{1 \over \lambda }} \right)$
C
${{\lambda \over 16}}$, Peff = K $\left( {{1 \over \lambda }} \right)$3
D
4

### JEE Main 2018 (Online) 15th April Morning Slot

The number of amplitude modulted broadcast stations that can be accomodated in a $300$ $kHz$ band width for the highest modulating frequency $15$ $kHz$ will be :
A
$20$
B
$15$
C
$10$
D
$8$

## Explanation

Given, modulating frequency = 15 kHz

$\therefore\,\,\,\,$ Bandwidth of one channel = 2 $\times$ 15 = 30 kHz

$\therefore\,\,\,\,$ No of stations = ${{300} \over {30}}$ = 10