Interview Questions: ECE (basic-1)

Some electronics and communication engineering basic skill topics and answers are given here.............

Electron

Nobody knows the shape, size, or just about anything about electrons besides their charge and mass. We do know they're very, very small. How small? So small we can't measure their diameter, we can only say they're smaller than some upper bound.

Flat channel

A channel which passes all spectral components with approximately equal gain and linear phase.

Co-channel interference

Frequency reuse implies that in a given coverage area there are several cells that use the same set of frequencies. These cells are called co-channel cells and the interference between signals from these cells is called co-channel interference.

Adjacent channel interference

Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference. Adjacent channel interference can be minimized through careful filtering and channel assignments. 

What is inter-modulation noise?

When a signal (having different frequency components) passes through a transmitting media, then due to non-linearity, some of the frequency components may combine to generate a different frequency component. This leads to distortion in the signal, which is known as inter-modulation noise. For example, a signal may be having frequency components fx and fyand due to non-linearity of the media they may generate a frequency component (fx+fy). Further a frequency of (fx+fy) may be already present in the original signal. This causes inter-modulation noise.

Major impacts on electromagnetic wave propagation

Reflection, diffraction, and scattering are the three basic propagation mechanisms which impact propagation in a mobile communication system.

       Reflection occurs when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave. Reflections occur from the surface of the earth and from buildings and walls.

       Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even when a line-of-sight path does not exist between transmitter and receiver.

       Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel. In practice, foliage, street signs, and lamp posts induce scattering in a mobile communications system.

Relation between EM waves & perfect conductor

Since electromagnetic energy cannot pass through a perfect conductor a plane wave incident on a conductor has all of its energy reflected.

How the effect of delay distortion can be minimized?

Ans: Delay distortion can be minimized by using an equalizer (a kind of filter). Delay distortion arises because different frequency components of the signal suffer different delay as the signal passes through the media. This happens because the velocity of the signal varies with frequency and it is predominant in case of digital signals.

What is Fading?

Fading is used to describe the rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance so that large-scale path loss effects may be ignored. Fading is caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times. These waves called multi-path waves, combine at the receiver antenna to give a resultant signal which can vary widely in amplitude and phase, depending on the distribution of the intensity and relative propagation time of the waves and the bandwidth of the transmitted signal.

Why is it necessary to limit the band of a signal before performing sampling?

It is necessary to limit the bandwidth of a signal before sampling so that the basic requirement of sampling theorem ( the sampling rate should twice or more than twice the maximum frequency component of the signal ) is satisfied. This is known as Nyquist rate. If it is violated, original signal can not be recovered from the sampled signal.

Coherent and non-coherent demodulation

Demodulation techniques may be broadly divided into two categories: coherent and non-coherent demodulation. Coherent demodulation requires knowledge of the transmitted carrier frequency and phase at the receiver whereas non-coherent detection requires no phase information. 

Between AM and FM, which one gives better noise immunity?

FM is more immune to noise than AM, since the power of transmission is independent of the modulation index.

Why do you need encoding of data before sending over a medium?

Suitable encoding of data is required in order to transmit signal with minimum attenuation and optimize the use of transmission media in terms of data rate and error rate.

Between RZ and NRZ encoding techniques, which requires higher bandwidth and why?

RZ encoding requires more bandwidth, as it requires two signal changes to encode one bit.

Generation of FM

There are basically two methods of generating an FM signal: the direct method and the indirect method. In the direct method, the carrier frequency is directly varied in accordance with the input modulating signal. In the indirect method, a narrow-band FM signal is generated using a balanced modulator and  frequency multiplication is used to increase both the frequency deviation and the carrier frequency to the required level.

Performance of a modulation scheme

The performance of a modulation scheme is often measured in terms of its power efficiency and bandwidth efficiency. Power efficiency describes the ability of a modulation technique to preserve the fidelity of the digital message at low power levels. Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a limited bandwidth. 

Nyquist caiteria

Nyquist was the first to solve the problem of overcoming inter-symbol interference while keeping the transmission bandwidth low . He observed that the effect of ISI could be completely nullified if the overall response of the communication system (including transmitter, channel and receiver) is designed so that at every sampling instant at the receiver, the response due to all symbols except the current symbol is .equal to zero.

What  is inside of fan regulator ?

It is TRIAC. It is a power-electronic device which changes the firing angle to get different voltage and that’s how we control our fan.

Over voltage is dangerous!

In case over-voltage occurs in a house , all your electrical equipment will get burned if they are in ON position. Electrical equipments are protected by MCBs (or fuses) only from short-circuits and over-loading and not from Over-voltage.The one way to control and prevent this is by installing an auto-cut voltage regulator in the main supply of the house.Over-voltage gets introduced in distribution power lines from sub-stations side in case over voltage relays installed in sub-stations fail to operate for some reason.

Transformer is only for AC

If you will supply a transformer with a battery (dc voltage source), u will end up burning the windings of transformer. Larger the size of transformer, larger the time it will take.

Electronic device don't get damaged via overcharging!

That your phone/laptop battery will not get damaged if you still have the device connected to charging even after reaching 100%. Because the internal circuitry in the devices will prevent overcharging. 

AC versus DC

AC is more efficient for transmission, but can't be stored as AC;  and DC  can be stored in batteries but is not efficiently transmitted.

Current is cut!

Generally we use to say 'current is cut', which is technically wrong. We cannot cut current. It should be 'power cut'.

Some multi-conductor transmission cables are twisted. why?

Instrumentation cables are twisted. (Twisted pair ). This twisting is done to cancel out the electromagnetic interference between the two.

What do you mean by Corona ?

Voltage between transmission lines is so high that it can cause partial breakdown of the air surrounding it, which results in a colorful field and a hissing noise called corona.