Solved Example: 9927-01

Magnetic permeability is maximum for ______ material.

A. Diamagnetic

B. Paramagnetic

C. Ferromagnetic

D. None of the above

Correct Answer: C

Solved Example: 9927-02

The relative permeability of a ferromagnetic material is:

A. Less than one

B. More than one

C. More than 10

D. More than 100 or 1000

Correct Answer: D

In electronics and information theory, noise refers to those random, unpredictable, and undesirable signals, or changes in signals, that mask the desired information content. In radio, this noise is called static; in television, it is called snow. White noise is a complex signal or sound covering the entire range of component frequencies, or tones, all of which possess equal intensity. In other words we can say that, noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits.

Noise generated by electronic devices varies greatly, as it can be produced by several different effects. Thermal noise is unavoidable at non-zero temperature, while other types depend mostly on device type (such as shot noise, which needs steep potential barrier) or manufacturing quality and semiconductor defects, such as conductance fluctuations, including noise. In communication systems, noise is an error or undesired random disturbance of a useful information signal in a communication channel. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however, typically distinguished from interference, (e.g. cross-talk, deliberate jamming or other unwanted electromagnetic interference from specific transmitters), for example in the signal-to-noise ratio (SNR), signal-to-interference ratio (SIR) and signal-to-noise plus interference ratio (SNIR) measures.

Noise is also typically distinguished from distortion, which is an unwanted systematic alteration of the signal waveform by the communication equipment, for example in the signal- to-noise and distortion ratio (SINAD). In a carrier-modulated passband analog communication system, a certain carrier-to-noise ratio (CNR) at the radio receiver input would result in a certain signal-to-noise ratio in the detected message signal. In a digital communications system, a certain Eb/N0 (normalized signal-to-noise ratio) would result in a certain bit error rate (BER). SOURCES OF NOISE It is important for the analyst who uses a particular instrumental method to be aware of the sources of noise and the instrument components used to minimize this noise because noise determines both the accuracy and detection limits of any measurement.

Solved Example: 9166-01

White noise is characterized by:

A. Gaussian probability density function

B. Power spectral density. $\dfrac{\eta}{2} \delta(f)$

C. Autocorrelation function depends only on delay $\tau$

D. Autocorrelation $\dfrac{\eta}{2} \delta(\tau)$

Correct Answer: D

Solved Example: 9166-02

Which type of noise is produced in a microwave tube due to random nature of emission and electron flow?

A. Rutherford noise

B. Partition noise

C. Shot noise

D. Becquerel noise

Correct Answer: C

Solved Example: 9166-03

The white noise spectral density:

A. Changes with frequency

B. Remains constant

C. Changes with BW

D. Changes with amplitude

Correct Answer: B

Solved Example: 9166-04

Following are few statements regarding noise in communication system:

a) Atmospheric noise, shot noise, solar noise are examples of external noise sources

b) Noise temperature is useful in dealing with UHF noise

c) Thermal agitation is the only source of noise in receiver

Choose the correct answer:

A. Only b)

B. a) and b)

C. b) and c)

D. a) and c)

Correct Answer: A

Solved Example: 9166-05

The noise caused by random variations in the arrival of electrons or holes at the output electrode of an amplifying device:

A. White noise

B. Flicker

C. Shot Noise

D. Transmit time noise

Correct Answer: C

Solved Example: 9166-06

Spectral density of the white noise is:

A. Uniform

B. Exponential

C. Gaussian

D. Poisson

Correct Answer: A

Solved Example: 9166-07

One of the following types of noise assumes importance at high frequncies:

A. Thermal noise

B. Shot noise

C. Transit-time noise

D. Flicker noise

Correct Answer: C

Permittivity is a measure of resistance that is encountered when forming an electric field in a medium.

In simple words permittivity is a measure of how an electric field effects and is effected by a dielectric medium.

$\epsilon_0$ (permittivity of space): The electric field generated in vacuum. It is constant value $\epsilon_0$= 8.85 x 10$^{-12}$ F/m

The relative permittivity εr often called the dielectric constant, is equal to the ratio $\epsilon_r = \dfrac{\epsilon}{\epsilon_0}$ which is greater than unity and represents the increase in charge storing capacity by insertion of the dielectric medium between the plates. The dielectric constant is one material property that is of prime consideration for capacitor design.

dielectric constant or permittivity of a material is defined as the “ratio of capacitance of a capacitor with that material as dielectric between the conducting plates, to the capacitance of the same capacitor with vacuum as dielectric medium.” $\epsilon_r =\dfrac{\epsilon}{\epsilon_0}$. The relative permittivity of vacuum is 1.00 and that of air is 1.00058 which is taken as unity. Gases have a relative permittivity slightly higher than unity, while polar liquids and ionic solids have high values of permittivity.

Solved Example: 9926-01

Relate the dielectric strength of insulating material with its thickness.

A. Dielectric Strength = Breakdown voltage $\times$ Thickness of the dielectric

B. Thickness of the dielectric = Dielectric Strength * Breakdown voltage

C. Dielectric Strength = Breakdown voltage / Thickness of the dielectric

D. Dielectric Strength = Thickness of the dielectric / Breakdown voltage

Correct Answer: C

Solved Example: 9926-02

What happens when moisture gets into the insulating materials?

A. It increases dielectric constant

B. It increases the insulation resistance

C. It increases the dielectric strength

D. It increases the dielectric loss

Correct Answer: D

Solved Example: 9925-01

Simens is the S.I unit of ________.

A. Conductivity

B. Conductance

C. Resistivity

D. Resistance

Correct Answer: B

Solved Example: 9925-02

The magnitude of current density in a superconductor depends on:

A. Temperature

B. Both temperature and magnetic field strength

C. Temperature for some time and then on magnetic field strength

D. Magnetic field strength

Correct Answer: B

Solved Example: 9925-03

With the rise of temperature, the conductivity of a semi-conductor:

A. Remains unchanged

B. Decreases

C. Increases

D. Nothing is certain

Correct Answer: C