Communication Systems Lab Viva Questions

Viva Questions on Communication Systems Lab

1. Draw the block diagram of an analog communication technique. Discuss about the drawbacks of analog communication techniques?

The block diagram of an analog communication system is:

The main drawbacks of analog communication techniques are: we cannot trust analog communication for all time. The effect of noise is more on the transmitted signals. The power required for the transmission of the signal is very high, this will yields for high length transmitting antennas which makes the transmission section more complex. The circuit complexity and cost are high for analog communication.

2.  List the main advantages of digital communication techniques?

a. The main advantages of digital communication are:

b. It is highly reliable.

c. The effect of noise is very less, hence yields to high SNR.

d. The power consumption is very less.

e. The circuit complexity is very less because various Digital ICs are used.

f. It requires less cost.

g. The error detection and correction is possible.

3. Which types of digital modulation techniques you know?

The different types of digital modulation techniques are:

a. Amplitude shift Keying (ASK)

b. Frequency Shift Keying (FSK)

c. Phase Shift Keying (PSK)

d. Pulse code Modulation (PCM)

e. Differential Pulse Code Modulation (DPCM)

f. Delta Modulation (DM)

g. Adaptive Delta Modulation etc...

4.  With the help of a block diagram, explain the process of converting analog signal to digital signal?

For the conversion of analog signal to digital, first the analog signal is passed through an anti aliasing filter. Then this signal is passed to a Sampler circuit, then to a quantizer and  encoder.

5.  Discuss the functions of a sampler and quantizer?

We know that sampler and quantizer are the basic blocks of an analog to digital converter. A sampler circuit converts a continuous time signal into discrete time signal and the function of a  quantizer is to convert a continuous in amplitude signal into discrete in amplitude signal.

6.  Which types of Coding techniques are familiar to you?

The different coding techniques in communication system are:

a. Pulse code modulation,

b. Differential pulse code modulation,

c. Delta modulation,

d. Adaptive delta modulation

7.  What you know about the Sampling process?

Simply we can say that sampling process is similar to the process of cutting a bread into slices. Technically in terms of signals, we can say that the process of converting a continuous time signal into discrete in time signal is called as Sampling. In order to perform the sampling process, the analog signal is fed to a sampler circuit (switch).

8.  State sampling theorem?

Sampling theorem states that in order to reconstruct the continuous time signal from a discrete time signal, the sampling frequency taken should be more than or equal to twice of the maximum frequency of the continuous time signal frequency.

f_s  ≥ 2f_m

Violation of sampling theorem yields to aliasing effect. We cannot reconstruct the original signal from its samples, if sampling theorem is not maintained.

9.  What you know about Nyquist Rate in sampling process?

Nyquist rate is the sampling rate at which the sampling frequency is twice that of the maximum frequency component of the continuous time signal.

f_s  = 2f_m 

If nyquist rate is maintained for the sampling process, then that is said to be a perfect sampling process.

10.  How many types of samplings are you familiar. Explain?

The different types of samplings are:

a. Impulse Sampling:

The process of impulse sampling is done by multiplying the input signal x(t) with impulse train of period 'T'.

b. Natural sampling:

The process of sampling in which pulse have finite width equal to τ is known as natural sampling. Sampling process is done with respect to a carrier signal, which is in digital form.

c. Flat top sampling:

At the time of transmission of signal through channel, noise is introduced at top of the transmission pulse. This noise can be easily remove if the pulse is in the form of flat top. Thus, the top of the samples are flat, in other words, we can say that they have constant amplitude.

11.  Discuss in detail about the aliasing effect and explain how it is rectified?

At the time of sampling, if the sampling theorem is not satisfied; i.e if imperfect sampling occurs, the signals will be interfered in frequency domain. This is called aliasing effect in sampling. The aliasing effect can be rectified, if sampling theorem is satisfied;

f_s  ≥ 2f_m

The aliasing effect can also be rectified as first by passing signal from anti aliasing filter before sampling.

12.  How many types of analog pulse modulation methods you know. List all?

The different types of analog pulse modulation methods are:

a. Pulse Amplitude Modulation (PAM)

b. Pulse Width Modulation (PWM)

c. Pulse Position Modulation (PPM)

13.  What you know about Pulse amplitude modulation?

In pulse amplitude modulation, the height of the carrier pulse (amplitude) is proportional to amplitude of message signal. In other words, in PAM, the amplitude of the carrier and message signals are directly proportional to each other.

14.  What you know about Pulse width modulation?

In pulse width modulation, the width of the carrier pulse is proportional to the amplitude of message signal.

15.  Discuss about Pulse position modulation?

In pulse position modulation technique, the position of carrier pulse is proportional to the amplitude of message signal.

16.  Give the comparison of PAM, PWM, PPM?

The different parameters of PAM, PWM and PPM are shown below:

17.  Discuss about Amplitude shift Keying (ASK) ?

The ASK modulation technique represents digital data in carrier wave as variations in amplitudes. In other words, we can say that in ASK, '1' represented by transmitting a fixed amplitude carrier wave with constant frequency for the bit duration.

18.  Illustrate about Phase Shift keying (PSK) technique?

In PSK, the digital data is represented as variations in phase shift in the carrier wave. In other words, in PSK, '1' is represented by a 0 phase shift carrier wave and '0' is represented by a 180 phase shift carrier wave. Both the representation is for the bit duration with constant frequency.

19.  Discuss about Frequency Shift keying (FSK) ?

In FSK, the digital data is represented as variation in frequency in carrier wave. In other words, we can say that for '1' more than carrier frequency is used and for '0' less than carrier frequency is used.

20.  Discuss about Binary Phase Shift Keying (BPSK). Show the technique?

For BPSK modulation technique, for each bit of binary data (0 & 1), the carrier phase will be changed. The two different shifts are 0 and 180 degrees.

21.  Discuss in detail about Quadrature Phase Shift Keying (QPSK). Show the technique?

In QPSK, we know that for each two bits of binary data (00, 01, 10 & 11), carrier phase will be changed to four different shifts : (45, 135, -45, -135).

22.  Give the differences between Bit Rate and Baud Rate?

We know that the bit rate represents the number of bits per second. Baud rate can be defined as the number of symbols per second.  i.e., we can say that in communications the no. of bits transmitted per sec is known as Bit Rate (units bps). The number of times a signal (carrier) changes its state per sec is known as the Baud rate.

23.  What is bandwidth of BPSK signal?

The bandwidth of a BPSK signal is 2F_c, where F_c is the carrier frequency.

24.  Give the comparison of ASK, PSK and FSK ?

Bandwidth	ASK< PSK < FSK
Power	ASK <PSK = FSK
Probability of error	ASK > PSK > FSK
Signal to Noise Ratio	ASK < PSK < FSK

25.  Justify the reason behind calling Amplitude Shift keying (ASK) as ON-OFF keying?

Simply we can say that ASK acts like a switch. The reason is that when input data is than in ASK if the symbol is 1, then output is carrier and if input is 0, then the output is also zero. Hence its looks like a switch. This switches to ON state when input is 1 and to OFF state when input is zero.

Digital Signal Processing (DSP) Viva Questions and Answers

Viva Questions and Answers on Digital Signal Processing

1. Differentiate between a discrete time signal and a digital signal.

A discrete time signal can be defined as a signal, which is continuous in amplitude and discrete in time. In other words, a discrete time signal gives signal values only at particular (discrete) interval of time periods.

A digital signal represents signal as a sequence of discrete values. That is, a digital signal takes values from a given set of finite number of values. Digital signals are most commonly used in communication systems.

2. How we can represent a discrete time signal?

A discrete time signal can be represents in Graphical form, Tabular form, Sequence and as Functional representation.

3. Explain the process of sampling and define aliasing effect.

The process of converting a continuous time signal into a time signal is known as sampling. In order to convert, a continuous time signal is passed through a switch. The frequency of the switch is the sampling rate (F_s).

The Aliasing is an effect that occurs at the time of reconstruction of the sampled signal. This effect occurs, when the Sampling frequency (F_s) is less than two times the maximum frequency present in the signal component. F_s< = 2 F_m. The aliasing effect causes different signals to become indistinguishable.

4. What is the need of Nyquist rate in sampling process?

The sampling process is completely depends on the nyquist rate. The nquist rate is the sampling frequency, which is equal to twice of maximum frequency of the continuous time signal which has to be sampled. i.e, F_s= 2 F_m. If nyquist rate is maintained, we get perfect sampling. The sampling rate which is greater than the nyquist rate is called over sampling. The rate less than nyquist rate gives under sampling. A signal cannot be reconstructed from its samples, if it is under sampled.

5. State and discuss sampling theorem.

Sampling theorem states that, in order to reconstruct the continuous time signal from its samples (its discrete signals), the sampling frequency should be more than twice of  the maximum frequency present in the continuous time signal.

6. How you express a discrete time signal x(n) in terms of summation of impulses.

A discrete time signal can be represented as summation of impulse by,

7. Give the classification of discrete time signals?

A discrete time signal can be classified as: Causal and Non causal, Periodic and non periodic, even and odd, energy and power signals.

8. When you called a discrete time signal as a periodic signal?

A discrete time signal can be called as a periodic signal, if some set of samples of the signal repeats after a regular interval of time.

9. Describe about a discrete time system?

Basically a system is a device which takes a signal as input and gives another signal as output. A system is said to be a discrete time system, if the system's excitation and responses are both discrete time signals. In other words, a discrete time system takes a discrete time input signal and gives a discrete signal as output. The ratio of output signal to input signal is the transfer function of the system.

10. What is impulse response? Explain its significance.

The response of a system when the excitation is Impulse signal is called as impulse response. it also called as Natural response, free forced response.

11. Write the expression for discrete convolution.

The process of discrete convolution can be represented as:

12. Explain the classification of discrete time systems.

The discrete time systems are classified as causal, non causal, time variant, time invariant, linear, non linear, stable and unstable systems.

13. What you understand about time invariant system.

For a time invariant system, the system's operation is independent of time. In other words, we can say that if the delayed system response is equal to system's response for delayed input, then the system is known as time invariant system.

14. Distinguish between linear and nonlinear systems?

A system is said to be a linear system, if the system satisfies homogeneity principle and superposition principle. If a system doesn’t satisfy homogeneity and superposition principles, then the system is a not non linear system.

15. What you know about causality?

A system is said to be a causal, if the system's response should depend on present and past inputs only and not on the future inputs. That is causal systems are physically realizable systems. Non causal systems response depends on the future input values. Hence non causal systems are physically non realizable systems.

16. What you know about BIBO stability? Discuss the condition to be satisfied for stability?

A system is said to be BIBO stable, if the system's response is bounded (measurable) for bounded excitation. In other words, if the system’s output is measurable for the measurable input, the system is said to be BIBO stable.  For a system to be stable, the impulse response of the system should be absolutely summable.

17. Compare between FIR and IIR systems?

If the system's impulse response contains finite number of samples, then the system is a FIR system.

If the system's impulse response contains infinite number of samples, then the system is said to be an IIR system.

18. Discuss in detail about recursive and non recursive systems?

If the output depend on it’s one or more past outputs, then the system is said to be a recursive system. If the output is independent of output, then the system is said to be non recursive.

All systems with feedback are Recursive. Systems without feedback are non recursive. 

19. Discuss about the properties of linear convolution.

The properties of linear convolution are:

1) x(n)*y(n)= y(n)*x(n)

2) [x(n)+y(n)]*z(n)=x(n)*z(n)+y(n)*z(n)

3) [x(n)*y(n)]*z(n) =x(n)*[y(n)*z(n)]

20. What you know about circular convolution.

Actually linear convolution and circular convolution are same. Circular is for periodic signals.

21. Discuss the need of linear and circular convolution?

Actually convolution is mainly used for calculating the response of a LTI system for a given excitation.

22. How the process of linear convolution via circular convolution is performed?

Linear convolution is obtained when circular convolution with the length of linear convolution length (l+m-1) is performed.

23. Describe about sectioned convolution?

Sectioned convolution is performed, if any one of the given two sequences length is very high.  

24. List the two types of sectioned convolution?

The two different types of sectioned convolution are:

1) Over lap-Add method. 2) Over lap save method.

25. Differentiate the process of cross correlation and auto-correlation?

The measure of similarity between signals and its delayed version as a function of time delay is called as auto-correlation. Auto correlation is also known as serial correlation.

The measure of similarity between two signals as a function of time delay between them is known as cross-correlation. Cross correlation is also known as sliding dot product.

26. Discuss the properties of correlation?

The important properties of correlation are:

1) R12(T) ≠ R21(T)

2) R12(T) = R21*(-T)

3) if R12(T) = 0, if the two signals are orthogonal to each other

4) The energy spectral density can be obtained from the Fourier transform of the auto correlation.

Bipolar Junction Transistor (BJT) Viva Questions and Answers

Viva Questions and Answers on Bipolar Junction Transistor Experiment

1. Briefly discuss about a transistor?

We can simply say that, a “junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type”. A transistor has three terminals namely emitter, base and collector. A transistor is a current sensing device. Transistor was developed in the year 1948 at BELL laboratories. A transistor can be seen as two p-n junction placed back to back. The emitter is heavily doped, while the base is lightly doped. The collector is moderately doped. A transistor can be considered as the heart of electronic products. It is almost used in all electronic devices. There are mainly two types of transistors namely, pnp and npn transistors.

2. Discuss about a Bipolar Junction Transistor (BJT), explain about its terminals, types, and mention which type is better? Justify your answer?

Field Effect Transistor (FET) Viva Questions and Answers

Viva Questions and Answers on Field Effect Transistor Experiment

1. Explain the differences between a BJT and a JFET and compare them?

The main difference of BJT (Bipolar Junction Transistor) and FET (Field Effect Transistor) is BJT is a current controlled device, while FET is a voltage controlled device (Control by the change in voltage at the gate terminal).  The current in BJT is produced due to the movements of both electrons and holes (current flow is due to both majority and minority carriers). In FET, the current is produced due to either electrons or holes (current flow due to majority charge carriers only). The main advantage of BJT and FET is that in electrical and electronics circuits, both devices can be used as switches and amplifiers. As compared to BJT, FETs are faster switching devices. In BJT, the Current-Voltage (I-V) characteristics are linear, but in FET, it is non-linear.  For radiation, FET is good, while BJT is sensitive. The noise effect is more in BJT as compared to FET.

Intermodal and Intramodal Dispersion in Optical Fiber

Dispersion:

The spreading of light pulse as it propagates down the fiber is called Dispersion.

• Occurs due to the dependence of phase velocity of a wave on its frequency or wavelength

• The pulse broadening causes adjacent pulses to get overlapped and limit the maximum number of pulses sent per second, thus reducing the information carrying capacity of the fiber.

• To avoid overlapping of optical pulses the bit rate (BT) must be less than the reciprocal by broadened duration 2τ.

BT ≤ 1/2τ

• Two types of dispersion are,

1. Intramodal dispersion.

2. Intermodal dispersion.

1. Intramodal Dispersion

• Also known as ‘chromatic dispersion’ or ‘group velocity dispersion’ (GVD).

• When an EM wave travels through a medium of RI, ‘n’ the speed of wave is reduced from speed of light.

That is, v = c/n.

Hence the speed of light in a material depends on its RI which inturn is a frequency dependent parameter. As a result different spectral components of the light pulse travels at slightly different group velocities which causes group velocity dispersion or chromatic dispersion.

• Group Velocity and Phase Velocity:

Group Velocity (V_g) is the speed at which energy in a particular mode travels along the fibre.

Phase Velocity (V_p) of a wave is the rate at which the phase of a wave propagates.

V_p = ω/β

Where, β – propagation constant

• V_p and V_g depends on the frequency and the medium.

• Group delay is the time delay experienced by the spectral component of a signal as it propagates through the fibre.

τ_g = L/V_g

Where, L = fiber length

• If Δω is the spectral width of the pulse, the extend of pulse broadening for a fiber of length ‘L’ can be given as,

ΔT = dτ/dω. Δω

Substitute for τ,

We have, ΔT = d/dω(L/V_g). Δω

Substitute for V_g,

ΔT = d/dω(L/(dω/dβ)). Δω

= L d²β/dω². Δω

ΔT = L β₂. Δω

Where, β₂ = d²β/dω² = GVD parameter

• GVD parameter determines how much an optical pulse will broaden while propagating inside the fiber.

• Two types of intramodal dispersion are,

a. Material Dispersion

b. Waveguide Dispersion

Material Dispersion:

• Caused by variation of RI as a function of ‘optical wavelength’

• Since the group velocity of a mode is a function of RI, the various spectral components of a given mode will travel at different speeds depending on wavelength.

• The larger the spectral width the higher will be the pulse broadening. Since all wavelength in the spectral width (λ ± Δλ) of the optical source propagate with different speed.

• Prominent for LED Sources due to broader spectrum

Derivation:

We know, β = nk

β – propagation constant

k – wave vector = 2 π/ λ

diff w.r.to k,

dβ/dk = n

Group delay, τ_g = L/V_g = L/(c/n)

τ_g = L/c/( dβ/dk)

τ_g = L/c x dβ/dk

=L/c (dβ/dλ x dλ/dk)

We have, k = 2π/ λ

Differentiating we have,

dk = -2π/ λ² dλ

dλ/dk = - λ²/2π

Therefore, τ_g = - λ²L/2πc . dβ/dλ ------------------ (1)

This is the time delay experienced by the wave of wavelength λ and wave vector k as it travels through the fiber of length ‘L’.

Pulse spread,

σ_mat = dτ_g/dλ. σ_λ ----------- (2)

where, σ_λ is the RMS value of dλ

we know, β = 2πn(λ)/ λ

Substitute β in eq(1) we have,

τ_g = - λ²L/2πc . d(2πn(λ)/ λ)/dλ

Therefore, τ_g = L/c[n(λ)- λ dn(λ)/dλ]

Substitute τ_g in eq(2)

σ_mat = [d(L/c[n(λ)- λ dn(λ)/dλ])/dλ]. σ_λ

= σ_λ L/c |λ d²n/dλ²|

= σ_λ L D_mat(λ)

where, D_mat is the optical fibre material dispersion.

Dmat = λ/c. d2n/dλ2

For pure silica at ‘λ = 1.276, D_mat = 0’. Hence this wavelength is known as “Zero Dispersion Wavelength, λ_zθ”

Wave Guide Dispersion:

• Results from the variation in group velocity with wavelength for a particular mode

• One mode in the single mode fiber or each mode in a multimode fiber can have its own waveguide dispersion.

• For a mode distribution of light in the fiber varies for different wavelength. Shortr wavelengths are more confined to the core, whereas for larger wavelength, larger portion of power propagates through the cladding.

• Due to lower RI in the cladding that portion of light travels faster. Also since refractive index depends on wavelength different spectral components in a single mode have different propagation speeds. Thus difference in core-cladding spatial power distribution together with speed variation of various wavelengths causes change in propagation velocity for each spectral component resulting in dispersion.

• They are significant in single mode fiber.

Equation:

• Normalized propagation constant, b = a²ω²/v²

Where, a = co-radius

ω = decay parameter in cladding

v = v number

Sub. ω² = β² – K₂²

b = a²(β² – K₂²)/ a² K²(n₁² – n₂²)

b = β²/ K² - n₂²/ (n₁² – n₂²)

for small values of RI difference,

ie, for a weekly guided condition,

Δ = (n₁ – n₂)/n₁ << 1

and b = (β/ K – n₂)/(n₁ – n₂)

Therefore, β = (b(n₁ – n₂) + n₂)K

= Kb(n₁ – n₂) + Kn₂

= Kb(n₁ – n₂).n₁/n₁ + Kn₂

= Kb Δn₁ + Kn₂

= Kn₂[(bΔn₁/n₂) + 1]

Since, n₁/n₂ = 1

β = Kn₂[bΔ+ 1]

Group Delay, τ_waveguide = L/C dβ/dK

Sub for β,

τ_waveguide = L/C d(Kn₂[bΔ+ 1])/dk

= L/C d(Kn₂bΔ+ Kn₂)/dk

= L/C [n₂ΔdKb/dk + n₂]

Expressing in terms of V number, V

τwaveguide = L/C[n2 +n2ΔdVb/dV]

Pulse spread due to τ_waveguide

σ_wg = |d τ_waveguide/dλ|σ_λ

= |d (L/C[n₂ +n₂ΔdV_b/dV])/dλ|σ_λ

= n₂ L Δ σ_λ/ Cλ . V. d²(V_b)/ dV²

σwg = L|Dwg(λ)| σλ

where, D_wg(λ)= n₂ Δ/Cλ . V . d²(V_b)/ dV²

Also, D_wg(λ)= waveguide dispersion factor

2. Intermodal Dispersion:

• Also known as modal delay or modal dispersion

• Occur due to each mode having different group velocity at a single frequency

• Input light is made of a number of modes

• Different modes travel through fibre at different speeds in different directions. Hence, distance travelled by each mode is different. Thus time taken by each mode to cover the same distance is different. This results in pulse broadening.

• Increases with distance travelled through the fiber.

• Does not exist in single mode fibers

• Major source of dispersion in multimode fibers

Equation:

Let, T_min = Time taken by axial ray to travel a distance ‘L’.

ie, T_min = L/V = L/(C/n₁) = L n₁/C

Let, T_max = Time taken by meridional ray to travel distance ‘L’.

ie, T_max = (L/Cos θ)/V = (L/Cos θ)/(C/n₁) = L n₁/ (C x Cos θ)

Cos θ = Sin θ = n₂/n₁

Therefore, T_max = L n₁²/ C n₂

Time Delay = T_max - T_min

= L n₁²/ C n₂ - L n₁/ C

= L n₁²/ C n₂ (1 - n₂/ n₁)

= L n₁²/ C n₂ . Δ

= L/Cn₂ . (NA)²/2       .............. Since, NA = n₁√(2Δ)

Delay = δ Ts = L(NA)²/2Cn₂

Polarisation Mode Dispersion:

• The fibre birefringence on the polarisation states of the optical signal can cause pulse broadening. Polarisation is the electric field orientation of the signal. This can vary along the length of the fiber.

• Birefringence is the optical property of a material having a RI that depends on the polarisation and propagation direction of the light.

• Birefringence can be due to intrinsic factors (geometric irregularities, internal stress etc) and external factors (bending, twisting etc).

• Signal energy at a given wavelength has two orthogonal polarisation modes.

• Varying birefringence along the length of the fiber will cause each polarisation mode to travel at slightly different velocity. The resulting difference in propagation (Δτ_PMD) time between the two orthogonal polarization modes will result in pulse spreading. This is known as polarisation mode dispersion (PMD).

• Differential Time Delay,

Δτ_PMD = |L/V_gz – L/V_gy|

Where, V_gz and V_gy = group velocities of two orthogonal polarization modes.

• PMD varies randomly along the fiber, since causes of birefringence may vary with temperature, stress dynamics etc. Hence Δτ_PMD cannot be used directly to measure PMD. It limits the performance of the fiber in long-haul optical communication system operating at high bit rate.