EEB/EDB 2043 COMMUNICATION SYSTEM
ASSIGNMENT 1
Due date: 18th June 2022 at 5:00 pm
Q1.
a. Determine the overall noise factor and noise figure for the three cascaded amplifiers
with the following parameters:
A1 = 10 dB, NF1 = 3 dB; A2 = 10 dB, NF2 = 6 dB; A3 = 20 dB, NF3 = 10 dB.
[4 marks]
b. Discuss on how can we reduce the overall noise factor.
[2 marks]
c. If an amplifier has a bandwidth B = 20 kHz and a total noise power
N = 2  10 −17 W , determine the total noise power if the bandwidth increases to
40 kHz and decreases to 10 kHz. Will the noise power increase or decrease if the
bandwidth increases? Justify your answer.
[4 marks]
Q2.
An AM signal has the following equation:
𝑣(𝑡) = [50 + 10 𝑠𝑖𝑛(25 × 103 𝑡)] 𝑠𝑖𝑛(455 × 103 𝑡) V.
Determine:
a. Upper and lower side frequencies.
[2 marks]
b. Modulation coefficient and percent modulation.
[2 marks]
c. Maximum and minimum positive peak amplitudes of the envelope.
[2 marks]
d. If an AM transmitter has a carrier power of 50W at carrier frequency of 12 MHz
and it is modulated at 80% by a 1 kHz sine wave. How much power is contained
in the sidebands? Convert the power level to dBm.
[4 marks]
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Q3.
An 800 kHz sinusoidal carrier signal is amplitude-modulated by a 5 kHz audio
signal to produce an AM envelope with maximum and minimum peaks voltages of
120Vp and 30Vp respectively. The transmitting antenna has an equivalent resistance
of 75Ω. Based on the given information,
a. Determine peak amplitude of the audio, carrier and sidebands voltages.
[3 marks]
b. Calculate the bandwidth of the modulated signal.
[1 mark]
c. Calculate the power of the carrier signal power and total sidebands power.
[2 marks]
d. Sketch and label the power spectrum of the modulated signal.
[4 marks]
Q4.
a. Give the two advantages and disadvantages of the AM DSB-FC and AM SSB.
[4 marks]
b. If an antenna transmits a 15kW power at 80% modulation using conventional
AM (DSB-FC). Determine the amount of power saving if single side band (SSB)
transmission is used for the same intelligibility.
[4 marks]
c. Name one application for the low-level AM transmitter and high-level AM
transmitter.
[2 marks]
Q5.
a. Discuss the advantages and disadvantages of the two methods: single conversion and
three conversion SSB AM transmitters.
[4 marks]
b. Define Superheterodyne Receiver. Construct an AM superheterodyne receiver block
diagram and explain briefly its operation for each of the 5 stages.
[6 marks]
Note: Total Marks = 50 marks, one question = 10 marks.
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