# Topic: Communications (Test 3)

Topic: Communications
Q.1
The raised cosine pulse p(t) is used for zero ISI in digital communications. The expression for p(t) with unity roll-off factor is given by The value of p(t) at t = 1/W is
A. −0.5
B. 0
C. 0.5
D.
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Q.2
A sinusoidal signal of 2 kHz frequency is applied to a delta modulator. The sampling rate and step-size Δ of the delta modulator are 20,000 samples per second and 0.1 V, respectively. To prevent slope overload, the maximum amplitude of the sinusoidal signal (in Volts) is
A. 1/2𝜋
B. 1/𝜋
C. 2/𝜋
D. 𝜋
Explaination / Solution:

For preventing slope overload, we should have
(slope of m(t) ≤ (slope of sampling)
For sinusoidal signal, if Workspace
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Q.3
The parallel branches of a 2-wire transmission line re terminated in 100Ω and 200Ω resistors as shown in the figure. The characteristic impedance of the line is Z0 = Ω and each section has a length of λ/4. The voltage reflection coefficient Γ at the input is A. -j-
B. -5/7
C. j-
D. 5/7
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Q.4
The longitudinal component of the magnetic field inside an air-filled rectangular waveguide made of a perfect electric conductor is given by the following expression The cross-sectional dimensions of the waveguide are given as a = 0.08 m and b = 0.033 m. The mode of propagation inside the waveguide is
A.

TM12

B.

TM21

C.

TE21

D.

TE12

Explaination / Solution:

We have the expression, and the cross sectional dimension is
a = 0.08 m, b = 0.033m
Now, we compare the equation for TEm,n mode whose Hz is given as  Hence, given mode is TE21 mode

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Q.5
The circuit shown in the figure is used to charge the capacitor C alternately from two current sources as indicated. The switches S1 and S2 are mechanically coupled and connected as follows: Assume that the capacitor has zero initial charge. Given that u(t) is a unit step function , the voltage vc(t) across the capacitor is given by
A. B. C. D. Explaination / Solution:  Workspace
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Q.6
A transmission line terminates in two branches, each of length as shown. The branches are terminated by 50W loads. The lines are lossless and have the characteristic impedances shown. Determine the impedance Zi as seen by the source. A. 200

B. 100

C. 50

D. 25

Explaination / Solution:

The transmission line are as shown below. Length of all line is  Workspace
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Q.7
In the design of a single mode step index optical fibre close to upper cut-off, the single-mode operation is not preserved if
A. radius as well as operating wavelength are halved
B. radius as well as operating wavelength are doubled
C. radius is halved and operating wavelength is doubled
D. radius is doubled and operating wavelength is halved
Explaination / Solution:
No Explaination.

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Q.8
Four messages band limited to W,W,2W and 3W respectively are to be multiplexed using Time Division Multiplexing (TDM). The minimum bandwidth required for transmission of this TDM signal is
A. W
B. 3W
C. 6W
D. 7W
Explaination / Solution:

Bandwidth of TDM is = 1/2 (sum of Nyquist Rate) = 1/2 [2W + 2W + 4W + 6W] = 7W

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Q.9
A speed signal, band limited to 4 kHz and peak voltage varying between +5 V and -5 V, is sampled at the Nyquist rate. Each sample is quantized and represented by 8 bits. If the bits 0 and 1 are transmitted using bipolar pulses, the minimum bandwidth required for distortion free transmission is
A. 64 kHz
B. 32 kHz
C. 8 kHz
D. 4 kHz
Explaination / Solution: Workspace
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Q.10
The modes in a rectangular waveguide are denoted by where m n and are the eigen numbers along the larger and smaller dimensions of the waveguide respectively. Which one of the following statements is TRUE?
A.  The TM10 mode of the wave does not exist
B.  The TE10 mode of the wave does not exist
C. The TM10 and TE10 the modes both exist and have the same cut-off frequencies
D.  The TM10 and TM01 modes both exist and have the same cut-off frequencies