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Cork Institute of Technology
Bachelor of Engineering in Communications Systems - Award
(NFQ Level 7)
Summer 2006
Telecommunications Hardware
(Time: 2 Hours)
Answer four questions, including at least one question from each section [All questions are worth equal marks] Use separate answer books for each Section
Values for constants; Speed of light in a vacuum c = 2.998 x 10 8 m s- electron charge q = 1.602 x 10 -19^ C Planck constant h = 6.625x 10 -34^ J Boltzmann constant k = 1.381x 10 -23^ J K-
Examiners: Mr. J. Berry Dr. R. O’Dubhghaill Dr. O. Gough Dr. J. Barrett
Section A
Q1. (Impedance matching)
(a) Why is impedance matching necessary in RF circuits? (3 marks) (b) Plot the following impedances and reflection coefficients on the ordinary Smith Chart provided (4 marks) i. 25+j10Ω ii. 75-j40Ω iii. 0. 7 ∠ 76 ° iv. 0. 3 ∠ − 85 ° (c) Use the Z-Y Smith Chart provided to design L-C matching networks to match the following source and load impedances at 900MHz i. 20+j75Ω source to a 50Ω load, blocking DC (9 marks) ii. 40-j30Ω source to a 30-j20Ω load, passing DC (9 marks)
Q2. (Amplifiers)
(a) Define the four s-parameters of a two-port network (4 marks) (b) What does it mean when a transistor is said to be “unilateral”? (2 marks) (c) A transistor has the following s-parameters at 432MHz:
i. Assuming unilaterality, what should be the source and load reflection coefficients to obtain a maximum gain amplifier? (2 marks) ii. Again assuming unilaterality, use the Z-Y Smith Chart to design source and load matching networks (blocking DC) to use this transistor as a maximum gain amplifier with a 50Ω source and a 50Ω load. (9 marks)
(d) Define the following terms used in power amplifier design: i. 1dB compression point (2 marks) ii. Dynamic range (2 marks) iii. Third order intercept point (2 marks)
Q3. (Oscillators)
(a) What are the Berkhausen criteria for oscillation? (2 marks) (b) Draw the circuit diagram for either a Hartley or Colpitts L-C oscillator, explain its principle of operation and show how it meets the Berkhausen criteria. (8 marks) (c) Draw the circuit diagram for an R-C oscillator and show how show how it meets the Berkhausen criteria. (8 marks) (d) (i) Define phase noise and amplitude noise in an oscillator. (2 marks) (ii) Why is phase noise more important and what effects can it have on communications and digital circuits? (5 marks)
[ ]
21 22
11 12 s s S s s
Section B
Q5. (a) An optical communications link with a data rate of 10 Gbps is being constructed. The fibre is single mode with dispersion D = 20 ps/nm-km and attenuation of 0.25 dB/km. The laser source is a DFB with spectral width of 0.1nm. The receiver has sensitivity of -28 dBm. (i) If the maximum permitted pulse spreading is one quarter of a bit period determine the maximum transmission distance. (8 marks) (ii) Compute the mean optical power required from the transmitter. (7 marks) (b) Explain the origin of Intermodal Dispersion in multimode fibres. Show that for a step index fibre the dispersion can be estimated by
∆ = ^ −
2
1 1 2 n
n n c t Ln.^ (6 marks)
(i) A 10km multimode fibre has an index step of 0.1% and mean index of 1.45. Compute the maximum bit rate for this case (4 marks)
Q6. (a) Discuss the use of wavelength division multiplexing in optical communications systems design. Sketch typical system configurations and specify typical component requirements. (10 marks) (b) An optical receiver is to use a PIN photodetector at a wavelength of 1550 nm with a quantum efficiency, η of 90%. If the receiver impedance is 100 Ohms and has a noise bandwidth of 500MHz determine the optical power which will result in a thermally limited SNR of 30 dB. (10 marks) (c) A laser diode is required to emit at 1554.2 nm for a WDM system. If the average laser waveguide index is 3.42 calculate the period of the grating required for the Distributed Feedback structure. (5 marks)
