INSTITUTE OF SPACE TECHNOLOGY
Department of Electrical Engineering
308201 – Communication Systems
Batch & Section:
Assessment Final/OHT: Final
Prepared by: Dr. Adnan Zafar
Student Registration No.:
Max Time Allowed: 3 hours
Approved by: Dr. Khurram Khurshid
Student Full Name:
Max Marks: 100
Instructions for Students
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This is a CLOSE BOOK exam. All the required formulas are given in the appendix at the end of the paper.
Sit in a quiet room during the exam time. Prepare answer sheets in advance.
Write your particulars including Name, Registration Number, Batch, Subject Name, Date on the first sheet.
Write your registration number on top of all answer sheets that you use.
Place your Student ID Card with you during exam for invigilation purposes
Your video/mic should be turned on during the duration of the exam
Ask any questions from the teacher during the first 10 minutes only. After that, no query will be entertained at all.
You are not allowed to leave the examination room before submitting/uploading the answer sheets.
Solutions will not be accepted through any other medium except MS Teams. No solution will be acceptable after due time.
The preparation of solution is an individual effort and is assumed to be completed with all academic honesty.
Cheating by any means is prohibited and any such attempt may lead to serious consequences.
Convert your answer sheet into PDF and save it with file name format (Student Reg. No – Subject – Section - Department)
30 min will be given to upload the file after the exam time
All questions are to be attempted
Question # 1
a) Illustrate the terms “synchronous detection” and “envelope detection”.
b) Illustrate how pre-emphasis and de-emphasis filters can improve the output SNR in a FM system?
c) For an input signal 𝑥(𝑡), The output 𝑦(𝑡) is defined as
𝑦(𝑡) = 𝑥(𝑡) + 2𝑥 2 (𝑡)
Where, 𝑥(𝑡) = 𝐴 cos 𝜔1 𝑡 + 𝐵 sin 𝜔2 𝑡 and 𝑓1 = 4𝐾𝐻𝑧 and 𝑓2 = 3𝐾𝐻𝑧
i. Predict the kind of distortion that will be introduced in the above system?
ii. Illustrate the effect of distortion on the input signal?
iii. Compute the frequency components in the output signal?
d) Compute the Fourier transform of the signal using FT table. Identify each property when applied.
i. 𝑦(𝑡) = ∏(𝑡 + 5) + 2∆(𝑡/5)
ii. 𝑦(𝑡) = 𝑒 −2|𝑡| cos(1000𝑡)
[4]
[5]
[3]
[3]
[5]
[5]
[5]
[CLO-01; PLO-02; Q Marks 30]
Question # 2
a) Suppose the signal 𝑥 (𝑡) = 𝑚 (𝑡) + 𝑐𝑜𝑠2𝜋𝑓𝑐 𝑡 is applied to a nonlinear system whose output is defined as
𝑦 (𝑡) = 𝑥 (𝑡) +
1 2
𝑥 (𝑡).
2
Compute the spectrum 𝑌(𝑓) using the Fourier transform table. (Hint: You can define
the Fourier transform of 𝑚(𝑡) as 𝑀(𝑓)).
[5]
b) A lowpass signal 𝑥(𝑡) has a Fourier transform shown in Figure (a). This signal is applied to the system shown in
Figure (b). The blocks marked by ℋ represent Hilbert transform blocks and it is assumed that 𝑊 ≪ 𝑓𝑜 . Compute
the expression of signal 𝑥𝑖 (𝑡) 𝑓𝑜𝑟 1 ≤ 𝑖 ≤ 6, and can you predict what kind of system is in Figure (b)? [10]
11 January 2021
Fall-2020
INSTITUTE OF SPACE TECHNOLOGY
Department of Electrical Engineering
308201 – Communication Systems
[CLO-02; PLO-02; Q Marks 15]
Question # 3
a) An angle modulated signal with carrier frequency 𝑓𝑐 = 10𝑀𝐻𝑧 is described by the equation [CLO-02; PLO-02]
𝜑𝐸𝑀 (𝑡) = 100 cos(𝜔𝑐 𝑡 + 4 sin 2000𝜋𝑡)
i.
ii.
iii.
iv.
v.
Compute the power of the modulated signal.
Compute the maximum frequency deviation ∆𝑓𝑚𝑎𝑥 .
Compute the maximum phase deviation ∆𝜙𝑚𝑎𝑥 .
Compute the bandwidth of 𝜑𝐸𝑀 (𝑡).
Analyze the signal 𝜑𝐸𝑀 (𝑡) and predict if it is a FM or a PM signal?
[3]
[3]
[3]
[3]
[3]
b) Design an Armstrong indirect FM modulator to generate an FM carrier with carrier frequency of 98.1𝑀𝐻𝑧 and
∆𝑓 = 75𝐾𝐻𝑧. A narrowband FM generator is available at a carrier frequency of 100𝐾𝐻𝑧 and a frequency
deviation ∆𝑓 = 10𝐻𝑧. The stock room also has an oscillator with an adjustable frequency in the range of 10 to 11
MHz There are also plenty of frequency doublers, triplers, and quintuplers. [CLO-03; PLO-03]
[10]
[Q Marks 25]
11 January 2021
Fall-2020
INSTITUTE OF SPACE TECHNOLOGY
Department of Electrical Engineering
308201 – Communication Systems
Question # 4
a) Consider a digital communication system where an analogue signal with bandwidth of W Hz is sampled and
converted into pulse-code-modulation (PCM) words. Each PCM word has 𝑚 bits. Let 𝑉𝑝𝑝 and 𝐿 be the peak-topeak voltage of the analogue signal and the number of quantization levels, respectively. Further, we
use ∆ and 𝑒 to denote the uniform interval between two adjacent quantization levels and the quantization error,
respectively. The system is designed such that the maximum quantization error will not exceed a fraction, 𝑝 i.e.,
10% of the 𝑉𝑝𝑝 voltage. Formulate the number of bits per PCM word. (Hint: Find out the number of bits per PCM
word in terms of the fraction 𝑝).
[10]
𝑛
b) A uniform quantizer for PCM has 2 levels. The input signal is 𝑚(𝑡) = 𝐴𝑚 [cos 𝜔𝑚 𝑡 + sin 𝜔𝑚 𝑡]. Assume the
dynamic range of the quantizer matches that of the input signal.
i.
Construct the expressions for the signal power, quantization noise power, and the SNR at the output of the
quantizer.
[6]
ii.
Explain which value of 𝑛 will ensure that the output SNR is about 62dB.
[2]
c) Construct the following line code waveform for the 8 bits input sequence i.e., 01100110. Clearly highlight the bit
durations and the waveform crossings.
i.
Polar RZ & NRZ
[4]
ii.
Bipolar RZ & NRZ
[4]
iii. Manchester
[2]
iv.
Explain which ones of the above line codes are transparent?
[2]
[CLO-03; PLO-03; Q Marks 30]
APPENDIX
cos 𝐴 sin 𝐵 =
1
[sin(𝐴 + 𝐵) + sin(𝐴 − 𝐵)]
2
1
cos 𝐴 cos 𝐵 = [cos(𝐴 + 𝐵) + cos(𝐴 − 𝐵)]
2
cos2 𝜃 =
1 + cos 2𝜃
2
sin2 𝜃 =
1 − cos 2𝜃
2
∞
1
𝑥(𝛼)
1
ℋ{𝑥(𝑡)} = 𝑥ℎ (𝑡) = ∫
𝑑𝛼 = 𝑥(𝑡) ∗
𝜋
𝑡−𝛼
𝜋𝑡
−∞
𝑥ℎ (𝑡) ⇔ 𝑋ℎ (𝑓)
𝑋ℎ (𝑓) = −𝑗 sgn(𝑓) 𝑋(𝑓)
11 January 2021
Fall-2020
INSTITUTE OF SPACE TECHNOLOGY
Department of Electrical Engineering
308201 – Communication Systems
Carson’s Rule
𝐵𝐸𝑀 = 2(∆𝑓 + 𝐵𝑚 )
𝐴 cos 𝛼 + 𝐵 sin 𝛼 = 𝐶 cos(𝛼 + 𝜃) = 𝐶 cos(𝛼 − 𝜑)
Where:
𝜃 = tan−1
𝐶 = √𝐴2 + 𝐵 2
−𝐵
𝐴
𝐵
𝜑 = tan−1 𝐴
𝐴 = 𝐶 cos 𝜃 = 𝐶 cos 𝜑
𝐵 = −𝐶 sin 𝜃 = 𝐶 sin 𝜑
𝑁𝑞 =
11 January 2021
(∆𝑣)2
12
=
𝑚2 𝑝
3𝐿2
where
Fall-2020
∆𝑣 =
2𝑚𝑝
⁄
𝐿
INSTITUTE OF SPACE TECHNOLOGY
Department of Electrical Engineering
308201 – Communication Systems
11 January 2021
Fall-2020