Top 100 Communication System Interview Questions

Que.76 What is constellation diagram? What does it signify?

Answer:

• The constellation diagram approach in communication is similar to the vector diagram approach in vector algebra.

• Constellation Diagrams are basically graphical representations of the complex envelope of each possible signal. The x-axis represents the in-phase component and the y-axis represents the quadrature component of the complex envelope The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how well a receiver can differentiate between them when random noise is present. The greater the distance, the greater would be the ability of a particular modulation scheme(demodulator on the receiver side) to distinguish between the signals in case the signal gets contaminated due to different noise present in the channel.

• Also, these diagrams can be used to calculate the energy of the signal on the modulation side.

• The constellation diagram is useful because it displays both the ideal (reference) signal and the actual measured signal on the same plot. The ideal signal locations of a constellation diagram are pre-defined generically depending on the modulation format chosen.

Que.77 What is the quadrature null effect? How it can be avoided?

Answer: QNE is a phenomenon occurred due to the difference in phase and frequency of carrier generated by local oscillator and input signal. Various receivers are used to synchronize phase and frequency which leads to complex circuitry. Hence, to avoid this situation in DSB-SC, SSB-SC is used where QNE does not occur.

Que.78 What is the need of using VSB modulation?

Answer:

• The process of suppressing one of the sidebands, along with the carrier, and transmitting a single sideband is called a Single Side Band Suppressed Carrier system, or simply SSBSC or SSB. In the case of Single Sideband modulation, when a sideband is passed through the filters, the bandpass filter may not work perfectly in practice. As a result of which, some of the information may get lost. But in DSBSC two of the sidebands that are transmitted carry the same information, which is a waste.

• Hence to avoid this loss, a technique is chosen, which is comprised between Double Sideband single carrier modulation and Single Sideband modulation, called the Vestigial Sideband technique. The word vestige which means “apart” from which the name is derived

Que.79 What is Vestigial sideband modulation?

Answer:

• Vestigial Sideband Modulation or VSB Modulation is the process where a part of the signal called a vestige is modulated, along with one sideband.

• Along with the upper sideband, a part of the lower sideband is also being transmitted in this technique. A guard band of a very small width is laid on either side of VSB in order to avoid interferences. This VSB modulation is mostly used in television transmissions. The transmission bandwidth of the VSB modulated wave will be the total message bandwidth and the width of the vestigial sideband. 

Que.80 What is the use of a mixer circuit in communication?

Answer:

• A frequency mixer is a nonlinear electrical circuit that can produce new frequencies from two applied signals. Also, a frequency mixer in its purest form takes two applied signals and creates new signals equal to the difference and sum of the originating frequencies. Furthermore, it is possible to generate other frequency components in a standard frequency mixer. 

• In general, mixers, as a whole, are in use to shift signals from one frequency range to another, and this process is called heterodyning. The overall need for heterodyning is to alleviate transmission inconvenience or for additional signal processing.

• For example, a vital component of a superheterodyne receiver is a mixer, and its use centers around moving received signals to a standard, intermediate frequency (IF).

• Moreover, frequency mixers are in use to modulate a carrier signal in radio transmitters as well. 

• A superheterodyne receiver, or superhet, is a type of radio receiver that uses frequency mixing to convert an applied signal to a fixed IF, which makes the signal more convenient to process than the initially applied frequency.