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The University of the West Indies

Department of Physics

Examination December 2005

(Full Time Students Only)

Course Code and Name: P 24H COMMUNICATION SYSTEMS

Date and Time: WEDNESDAY DECEMBER 15, 2005: 9:00 A.M 11:00 A.M

Duration: 2 HOURS

INSTRUCTION TO CANDIDATES

This paper consists of 5 pages and 4 questions. Students must answer THREE (3)questions.

The use of non-programmable calculators is permitted.

Assume where necessary: Boltzmanns Constant k 1 38 x 10-23 J/K

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1.

Consider the circuit diagram in figure 1. The signal from figure 2 is applied to itsinput and an oscilloscope was used to observe the waveform at different point in

the circuit.

a.

What would be the waveform observed at point A [4]

b. What would be the waveform observed at the output. [4]

The output of this circuit is then passed through another circuit which contains a 1Mresistor and operating at 17C over a 1 MHz bandwidth.

c. Calculate the noise voltage produced by this resistor. [2]

Finally the signal is passed through a three stage amplifier. The first stage has apower gain of 30 dB and a noise figure of 3 dB. The second and third stages are

identical, with power gains of 20 dB and noise figure of 6 dB.

Figure 1Figure2

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4. A FM signal Vfm= 4000sin[4x 1010

t + 2sin(3x 105t)], is applied to a 1 K

antenna.

a. Define Frequency Modulation. [2]b.

Determine the carrier frequency fcand the intelligent frequency fi. [2]

c.

Determine the modulation index m. [1]

d. Determine the band width using the TWOmethods. [4]e.

Calculate the total power of the FM signal. [2]

f.

Calculate the power of the carrier and each of the side band. [6]

g.

Draw the power spectrum of the FM signal. [3]

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The University of the West Indies P 24H-Finals December 20055