Mid Term Examination Communication Systems EKT 358

UNIVERSITI MALAYSIA PERLIS

Peperiksaan Pertengahan Semester 1

Semester Pertama

Sidang Akademik 2016/2017

18 Oktober 2016

EKT 358 –Communication Systems

[Sistem Perhubungan]

Masa: 45 minit

NAME :_____________________________________________

MATRIC NO. :_____________________________________________

PROGRAM :_____________________________________________

MARKS

Q1 (10)

Q2 (10)

TOTAL (20)

TOTAL (100%)

SULIT -1-

QUESTION 1

[C4, CO1, PO1, PO2]

(a) Figure 1 shows a communication system that comprises of an amplifier with a gain of

45000 cascaded with an unknown component.

(i) If the voltage input is 20μV, determine the voltage ratio of the unknown component to

keep the output voltage to be 100 mV.

[2 Marks]

(ii) Calculate the voltage ratio of Av1 and Av2 in dB

[2 Marks]

dBdBA

dBdBA

v

v

191111.0log20)(

9345000log20)(

2

1

(iii) The possible component that you think constitutes in the Av2 and justifies your answer.

[1 Marks]

Attenuator because Av2 <1 and Av2 (dB)= -Ve dB

Vin = 20μV Vout = 100mV

Av1 = 45000 Av2

Figure 1: Communication system

1111.0000,45

000,5

000,51020

10100

1

221

6

3

A

AAthereforeAAA

V

VA

TT

in

outT

SULIT -2-

[C4, CO1, PO1, PO2]

(b) A radar receiver consists of an antenna with cable loss L1 = 1 dB and NF1 = 1 dB, an RF

amplifier with NF2 = 6 dB, and gain G2 = 20 dB, followed by a mixer whose noise figure is NF3

= 10 dB and conversion loss L3 = 8 dB, and finally, an integrated circuit IF amplifier with NF4 =

6 dB and gain G4 = 60 dB.

(i) Calculate the overall noise figure.

[3 marks]

2598.110)( 10

1

1

unitlessF , 7943.010)( 10

1

1

unitlessG

9811.310)( 10

6

2

unitlessF , 10010)( 10

20

2

unitlessG

1010)( 10

10

3

unitlessF , 1585.010)( 10

8

3

unitlessG

9811.310)( 10

6

4

unitlessF

3628.5)1585.0)(100(7943.0

19811.3

)100(7943.0

110

7943.0

19811.32598.1

111

321

4

21

3

1

2

1

GGG

F

GG

F

G

FFFT

dBNF 294.73628.5log10

(ii) An input signal of 50 dB higher than the noise level was fed at the input of the network.

Calculate SNR at the output.

[2 marks]

Given SNR in = 50 dB

F =

F (dB) = SNR in (dB) – SNR out (dB)

SNR out (dB) = 50 dB– 7.294 dB = 42.706 dB

SULIT -3-

QUESTION 2

[C4, CO2, PO1, PO2]

(a) AM double-sideband full carrier (AM DSBFC) is the most commonly used and the oldest and

simplest form of AM modulation. Sometimes is also called conventional AM or simply AM.

Explain what will happen to the modulated signal at the output of the AM modulator when the

modulating signal amplitude is larger than the carrier signal amplitude?

[2 marks]

Answer: overmodulation and distortion of the modulated wave-Clipping of the negative peak will occur -The

envelope will not be the shape of the original modulating signal -If the distortion is great enough, the original

signal cannot be reconstructed.

[C4, CO2, PO1, PO2]

(b) Study the given AM modulated wave below and determine the following parameters;

πt)(+πt)(πt)(=(t)vam 980.25cos310203.25cos1000sin12

(i) Carrier signal frequency and modulating signal frequency [2 marks]

Hzt

tf

ttft

c

c

5002

1000

,10002

Hzft

tf

ttfft

cm

mc

105005102

1020

,1020)(2

(ii) Maximum and minimum amplitudes of the AM envelope [1 marks]

5.5)25.3(212min VmVcV

5.18)25.3(312max VmVcV

SULIT -4-

(iii) Upper and lower side band frequency [2 marks]

Hzfff

Hzfff

mclsf

mcusf

49010500

51010500

(iv) Modulation index and percentage modulation [1 marks]

%2.54%100*

542.012

)2(25.3

25.32

mM

m

VmEc

(v) Show the frequency spectrum with amplitude [1 marks]

(vi) Illustrate and label the output envelope of the modulated signal [1 marks]

Ec = 12 V, Emax= 18.5 Vp, Emin= 5.5 Vp

12Vp

3.25Vp 3.25 Vp

510 490