AM

Q. The amplitude of upper and lower side bands of $\text{A.M.}$ wave where a carrier signal with frequency $11.21\text{MHz}$, peak voltage $15\text{V}$ is amplitude modulated by a $7.7\text{kHz}$ sine wave of $5\text{V}$ amplitude are $\frac{a}{10}\text{V}$ and $\frac{b}{10}\text{V}$ respectively. Then the value of $\frac{a}{b}$ is _____.

Q.

An amplitude-modulated wave is represented by, $C_m=10(1+0.2\sin 12560t)\sin (1.11\times {10}^{4}t)V$, the modulating frequency in $kHz$ will be

Q. Two waves of equal amplitudes and wavelengths but differing in phase are superimposed. Amplitude of resultant wave is maximum when phase difference is

1. Zero
2. $\frac{\pi }{3}$
3. $\pi$
4. $\frac{\pi }{2}$

Q. An amplitude modulated signal is given by
$V\left(t\right)=10[1+0.6cos(2.2\times {10}^{4}t\left)sin\right(5.5\times {10}^5\left)t\right].$ Here $t$ is in seconds. The sideband frequencies (in $kHz$) are $[Given\pi =\frac{22}{7}]$

1. $1785$ and $1715$
2. $178.5$ and $171.5$
3. $91$ and $84$
4. $892.5$ and $857.5$

Q. A carrier wave $V_c\left(t\right)=160\sin (2\pi \times {10}^{6}t)\text{V}$ is made to vary between $V_{max}=200\text{V}$ and $V_{min}=120\text{V}$ by a message signal $V_m\left(t\right)=A_m\sin (2\pi \times {10}^{3}t)\text{V}$. The peak voltage $A_m$ of the modulating signal is _______$\text{V}$.

Q.

An amplitude modulated signal is given by $v\left(t\right)=10[1+0.3\cos (2.2\times {10}^4)]\sin (5.5\times {10}^5)t.$Here $t$ is in $seconds$. The sideband frequencies $(inkHz)$ are, Given $(\mathrm{\pi }=\frac{22}{7})$

1. $1785$ and $1715$

2. $892.5$ and $857.5$

3. $89.25$ and $85.75$

4. $178.5$ and $171.5$

Q. An amplitude modulated wave is modulated to 50%. What is the saving in power if carrier as well as one of the side bands are suppressed

1. 70%
2. 65.4%
3. 94.4%
4. 25.5%

Q. A stretched rope having linear mass density $5\times {10}^{-2}kg/m$ is under a tension of 80 N. The power that has to be supplied to the rope to generate harmonic waves at a frequency of 60 Hz and an amplitude of
$\frac{2\sqrt{2}}{15\pi }m$ is

1. 521 W
2. 215 W
3. 251 W
4. 512 W

Q. Three coherent waves having amplitudes 12 mm, 6 mm and 4 mm arrive at a given point with successive phase difference of $\frac{\pi }{2}$. Then the amplitude of the resultant wave is

1. 7 mm
2. 10 mm
3. 4.8 mm
4. 5 mm

Q.

The maximum peak-to-peak voltage of an AM wave is $16mV$, and the minimum peak-to-peak voltage is $4mV$. The modulation factor is equal to

1. 0.6

2. 0.3

3. 0.8

4. 0.25

Q.

Why PCM is called a modulation technique?

Q.

If $E_C=20\sin {10}^5\mathrm{\pi t}$ and $E_M=10\sin 400\mathrm{\pi t}$ are carrier and modulating signals respectively, the modulation index is

1. $56\%$

2. $30\%$

3. $50\%$

4. $48\%$

Q.

If a radio receiver amplifies all the signal frequencies equally well, it is said to have a high?

1. Fidelity

2. Distortion

3. Sensibility

4. Sensitivity

5. Selectivity

Q. In the explanation of photo electric effect, we assume one photon of frequency $v$ collides with an electron and transfers its energy. This leads to the equation for the maximum energy $E_{max}$ of the emitted electron as $E_{max}=hv-{\varphi }_{\theta }$ where ${\varphi }_{\theta }$ is the work function of the metal. If an electron absorbs $2$ photons (each of frequency $v$)

$A$: what will be the maximum energy for the emitted electron?


$B$: Why is this fact (two photon absorption) not taken into consideration in our discussion of the stopping potential?

Q. Write two basic modes of communication. Explain the process of amplitude modulation. Draw a schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating signal over a sinusoidal carrier wave.

Q. Compute the typical de Broglie wavelength of an electron in a metal at ${27}^{\circ }C$ and compare it with the mean separation between two electrons in a metal which is given to be about $2\times {10}^{–10}m$.

Q. A difference of $2.3eV$ separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes a transition from the upper level to the lower level?

Q. A source of frequency $f_0$ is moving on a circular path of radius R at constant speed of 10 m/s. At distance 2R from center of circular path is placed a detector. Frequency received by detector is $f$. Then,
(take speed of sound as 330 m/s)

1. maximum value of $f$ is $\frac{33}{32}{f}_0$.
2. maximum value of $f$ is less than $\frac{33}{32}{f}_0$.
3. time interval between receiving maximum and minimum frequency is $\frac{\pi R}{15}$.
4. time interval between receiving maximum and minimum frequency is $\frac{2\pi R}{15}$.

Q.

A charged $30\mu F$ capacitor is connected to a $27mH$ inductor. What is the angular frequency of free oscillations of the circuit?

Q. Sinusoidal carrier voltage of frequency 1.5 MHz and amplitude 50 V is amplitude modulated by sinusoidal voltage of frequency 10 kHz producing 50% modulation. The lower and upper side-band frequencies in kHz are

1. 1490, 1510
2. 
3. 
4. 1510, 1490

Q.

What is the frequency of light emitted when an electron in a hydrogen atom jumps from the 3^rd orbit to the 2^nd orbit?

Q. An amplitude modulated wave is modulated to 50%. What is the saving in power if carrier as well as one of the side bands are suppressed

1. 70%
2. 65.4%
3. 94.4%
4. 25.5%

Q. For an amplitude modulated wave, the maximum amplitude is found to be $10V$ while the minimum amplitude is found to be $2V$. Find the modulation index, $\mu$.
What would be the value of $\mu$ is the minimum amplitude is zero volt?

Q. Sinusoidal carrier voltage of frequency, 1.5 MHz and amplitude 50 V is amplitude modulated by sinusoidal voltage of frequency 10 kHz producing 50% modulation. The lower and upper side-band frequencies in kHz are

1. 1510, 1490
2. $\frac{1}{1490},\frac{1}{1510}$
3. $\frac{1}{1510},\frac{1}{1490}$
4. 1490, 1510

Q. The rms voltage of the wave form shown is

1. $10V$
2. $7V$
3. $Noneofthese$
4. $6.37V$

Q. A sinusoidal carrier voltage of frequency $10MHz$ and amplitude $200volts$ is amplitude modulated by a sinusoidal voltage of frequency $10kHz$ producing $40\%$ modulation. Calculate the frequency of upper and lower sidebands.

1. $10100Hz$, $9990Hz$
2. $1010MHz$, $990MHz$
3. $1010kHz$, $990kHz$
4. $10010kHz$, $9990kHz$

Q. A source of frequency $f_0$ is moving on a circular path of radius R at constant speed of 10 m/s. At distance 2R from center of circular path is placed a detector. Frequency received by detector is $f$. Then,
(take speed of sound as 330 m/s)

1. maximum value of $f$ is $\frac{33}{32}{f}_0$.
2. maximum value of $f$ is less than $\frac{33}{32}{f}_0$.
3. time interval between receiving maximum and minimum frequency is $\frac{\pi R}{15}$.
4. time interval between receiving maximum and minimum frequency is $\frac{2\pi R}{15}$.

Q. Sinusoidal carrier voltage of frequency $1.5MHz$ and amplitude $50V$ is amplitude modulated by sinusoidal voltage of frequency $10kHz$ producing 50% modulation. The lower and upper side-band frequencies in $kHz$ are

1. $1490,1510$
2. $1510,1490$
3. 1/1490, 1/1510
4. 1/1510, 1/1490

Q. Consider the following amplitude modulated (AM) signal, where $f_m<B$.
$x_{AM}\left(t\right)=10(1+0.5\sin 2\pi f_{m}t)\cos 2\pi f_{c}t$
The average side-band power for the AM signal given above is

1. $12.5$
2. $3.125$
3. $25$
4. $6.25$

Q. Define modulation index. Why is it's value kept, in practice, less than one?
A carrier wave of frequency 1.5 MHz and amplitude 50 V is modulated by a sinusoidal wave of frequency 10 kHz producing 50% amplitude modulation. Calculate the amplitude of the AM wave and frequencies of the side bands produced.