Communications Device Theory and Design Question Answer 2022

Communications Device Theory and Design
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CT2 – Postponed/Resit Assignment 2018/2019
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[SECTION A]
Question 1:
Q1 (a)
Varactor diodes are found in radio and television receivers. Give one example of a
use or function of a varactor diode. A varactor diode has a capacitance value of 6 pF
when reversed biased by 9 volts. Calculate the capacitance value of the varactor
diode when the reverse voltage is reduced to 4 volts.
Solution:
We can use varactor diodes in RF circuits to vary the capacitance.
We know for a varactor diode, capacitance is inversely proportional to the square
root of the reverse voltage
Hence, C1/C2 = √ (V2/V1)
6 / C2 = √ 4 / 9
3 x 3 / 2 = C2
C2 = 4.5 pF
Hence, capacitance is 4,5 pF.
Q1 (b)
Calculate the emf generated in a current-carrying coil of 1000 turns and cross-
sectional area of 5cm2 when a flux density of 0.2T density in it is reversed, ie
changed to the opposite polarity, in a time of 10ms. Commenton the direction of the
induced emf.
Solution:
EMF generated = - N d φ / dt
Φ = BA = 0.2 x 0.0005 = 10-4
B = external magnetic field
EMF = 1000 x 10-4 / 0.01 = 10 V
The negative sign in emf equation shows that its direction is such that it opposes the
change in value of φ
CT2 – Postponed/Resit Assignment 2018/2019
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Q1 (c)
Figure Q1(c)
Compare, or graphically illustrate, the impedance and current characteristics of the
resonant circuits in figureQ1(c).
Solution:
The first circuit will show parallel resonance. In this case, the impedance is maximum
at the resonant frequency and the current is minimum.
The second circuit will show parallel resonance. In this case, the impedance is
minimum at the resonant frequency and the current is maximum.
The resonant frequency is given by f = 1 / 2∏ √LC.
Q1 (d)
The input amplifier of a satellite receiver has a noise figure of 3db and a power gain
of 6db. It is followed by a mixer stage of with a noise figure of 4db and a gain of
5.5db. Calculate the composite noise figure for the two stages.
Solution:
F1 = 2, F2 = 2.5, G1 = 4 ( in ratio )
Composite Noise figure = F = F1 + (F2-1)/G1 = 2 + ( 2.5 - 1) / 4 = 2 + 0.375 = 2.375
= 3.76 dB
CT2 – Postponed/Resit Assignment 2018/2019
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Q1 (e)
Figure Q1(e)
Given R2= 100Ω and a Op-Amp saturation voltage of +/- 14 Volts for the inverting
Schmitt trigger in figure Q1(e) above, calculate a value of R1to provide threshold
voltages of +/- 50 mV.
Solution:
UTP = V R2 / ( R1 + R2 )
0.05 = 14 x 100 / ( R1 + 100 )
0.05 R1 + 5 = 1400
R1 = 1395 / 0.05 = 27.9 kΏ
Q1 (f)
A modulation system with a basic signalling rate of 2400 baud may operate in either
DQPSK or V29 QAM mode. Calculate the information rate for both modes.
Solution:
Baud Rate = 2400
Information rate ( DQPSK ) = 2400 x 2 = 4800 bits / s
Information rate (V29 QAM mode ) = 2400 x 5 = 12000 bits / s
Q1 (g)
A linear PCM system has an 8 bit A/D converter and a ± 5V input range. Calculate
the quantisation noise power and the output S/N ratio for a sine wave input of 10V
peak to peak.
Solution:
Vp = Peak voltage = 5 V
Levels = 28 = 256 = L
CT2 – Postponed/Resit Assignment 2018/2019
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