FM & PM

Q.

In half-wave rectification, what is the output frequency if the input frequency is 50 Hz. What is the output frequency of a full-wave rectifier for the same input frequency.

Q. A photon of energy 8 eV is incident on a metal surface of threshold frequency $1.6\times {10}^{15}Hz$, then the maximum kinetic energy of photoelectrons emitted is $(h=6.6\times {10}^{-34}Js)$

1. 4.8 eV
2. 2.4 eV
3. 1.4 eV
4. 0.8 eV

Q. In half-wave rectification, what is the output frequency if the input frequency is 50 Hz. What is the output frequency of a full-wave rectifier for the same input frequency?

Q. A baseband signal of $3.5\text{MHz}$ frequency is modulated with a carrier signal of $3.5\text{GHz}$ frequency using amplitude modulation method. What should be the minimum size of antenna required to transmit the modulated signal?

1. $21.4\text{mm}$
2. $42.8\text{m}$
3. $42.8\text{mm}$
4. $21.4\text{m}$

Q.

What are the different types of amplitude modulation?

Q. The threshold frequency for a photosensitive metal is $3.3\times {10}^{14}\text{Hz}$. If light of frequency $8.2\times {10}^{14}\text{Hz}$ is incident on this metal, the cut-off voltage for the photoelectric emission is nearly :

1. $5\text{V}$
2. $2\text{V}$
3. $3\text{V}$
4. $1\text{V}$

Q. A) Find the maximum frequency of X-rays produced by $30kV$ electrons.

B) Find the minimum wavelength of X-rays produced by $30kV$ electrons.

Q. The figure shows variation of $R,X_L$  𝑎𝑛𝑑  $X_C$ with frequency $f$ in a series $L,C,R$ circuit. Then for what frequency point, the circuit is inductive?

1. All points
2. $B$
3. $C$
4. $A$

Q.

A variable frequency a.c. source is connected to a capacitor. How will the displacement current change with decrease in frequency?

Q. The frequency of a FM transmitter without signal input is called

1. Lower side band frequency
2. Upper side band frequency
3. Resting frequency
4. None of these

Q. In the non-resonant circuit, what will be the nature of the circuit for frequencies higher than the resonant frequency?

1. Inductive
2. None of the above
3. Capacitive
4. Resistive

Q. The modulation index of an FM carrier having a carrier swing of 200 kHz and a modulating signal 10 kHz is

1. 5
2. 10
3. 20
4. 25

Q. In frequency modulation

1. The amplitude of modulated wave varies as frequency of carrier wave
2. The frequency of modulated wave varies as amplitude of modulating wave
3. The amplitude of modulated wave varies as amplitude of carrier wave
4. The frequency of modulated wave varies as frequency of carrier wave
5. The frequency of modulated wave varies as frequency of modulating wave

Q. In an FM system a 7 kHz signal modulates 108 MHz carrier so that frequency deviation is 50 kHz. The carrier swing is

1. 0.71
2. 350
3. 7.143
4. 8

Q. The carrier frequency generated by a tank circuit containing 1 nF capacitor and 10 $\mu$ H inductor is

1. 1592 MHz
2. 1592 kHz
3. 159.2 Hz
4. 1592 Hz

Q. A 500 Hz modulating voltage fed into an FM generator produces a frequency deviation of 2.25 kHz. If amplitude of the voltage is kept constant but frequency is raised to 6 kHz then the new deviation will be

1. 4.5 kHz
2. 54 kHz
3. 27 kHz
4. 15 kHz

Q. A series ac circuit consist of an inductor and a capacitor. The inductance and capacitance is respectively $1$ henry and $25\mu F$. If the current is maximum in circuit, then angular frequency will be

1. $100rad/s$
2. $200rad/s$
3. $50rad/s$
4. $200/2\pi rad/s$

Q. How does the sign of the phase angle $\varphi$, by which the supply voltage leads the current in an LCR series circuit, change as the supply frequency is gradually increased from very low to very high values?

Q.

What are the two types of angle modulation?

Q. Find the root mean square (rms) voltage for the given variation of voltage.

1. $V_0\sqrt{\frac{5}{24}}$
2. $V_0\sqrt{\frac{1}{2}}$
3. $V_0\sqrt{\frac{7}{24}}$
4. $V_0$

Q. 74. Ratio of amplitude of two coherent waves is 3:2 .Ratio of maximum intensity to the minium intensity of their interference pattern is. (1) 9:4 (2) 3:2 (3) 25:1 (4) 25:9

Q.

If voltage of X-ray tube is doubled then intensity of X-rays will :-

1. halved

2. remains constant

3. doubled

4. quadrupled

Q. An oscillator is producing FM waves of frequency 2 kHz with a variation of 10 kHz. What is the modulating index

1. 0.67
2. 1.5
3. 0.2
4. 5

Q.

What is the Threshold Frequency?

Q. In the LC circuit shown below, $C=1\mu \text{F}$. When the capacitor is fully charged up to $100\text{V}$, the switch $S$ is closed at $t=0$. The current in the circuit oscillates at a frequency of ${10}^3\text{Hz}$. The value of $L$ will be approximately equal to



Take ${\pi }^2\approx 10$

1. $0.025\text{H}$
2. $0.015\text{H}$
3. $0.045\text{H}$
4. $0.085\text{H}$

Q.

Obtain the resonant frequency ${\omega }_r$ of a series LCR circuit with $L=2.0H,C=32\mu F$ and $R=10\Omega$ . What is the Q-value of this circuit?

Q. A p-n photodiode is fabricated from a semiconductor with band gap of 2.8 𝑒V. Can it detect a wavelength of $6000nm$?

Q. For an $LCR$ circuit driven at frequency $\omega$, the equation read $L\frac{di}{dt}+Ri+\frac{q}{c}=v_i=v_m\sin \omega t$

1) Multiply the equation by $i$ and simplify where possible.
2) Interpret each term physically.
3) Cast the equation in the form of a conservation of energy statement.
4) Integrate the equation over one cycle to find that the phase difference between $v$ and $i$ must be acute.

Q. Obtain an expression for the frequency of radiation emitted when a hydrogen atom de-excites from level n to level (n – 1). For large 𝑛, show that this frequency equals the classical frequency of revolution of the electron in the orbit.

Q.

What is a perfect modulation?