**** 2011****

ELE569MICROWAVE ELECTRONICSDuration: 2 hours 30 minutes

YOU ARE NOT PERMITTED TO READ THE CONTENTS OF THIS QUESTION PAPER UNTIL INSTRUCTED TO DO SO BY AN INVIGILATOR.

Answer FOUR Questions. The Smith Chart is Attached.

calculators arE permitted in this examination. Please state on your answer book the name and type of machine used.

Complete all rough workings in the answer book and cross through any work which is not to be assessed.

important note: thE academic Regulations state that possession of unauthorised material at any time when a student is under examination conditions is an ASSESSMENT offence AND CAN LEAD TO EXPULSION FROM THE COLLEGE. Please CHECK NOW TO ENSURE YOU DO NOT HAVE ANY NOTES in your possession. IF YOU HAVE ANY THEN please RAISE YOUR hand AND GIVE them to AN invigilator IMMEDIATELY.

EXAM PAPERS CANNOT BE REMOVED FROM THE EXAM ROOM

Examiners: Prof. Y. Hao and Prof. C. G. Parini

© Queen Mary, University of London, 2010

Question 1

Answer the following questions on the Smith Chart and its applications.

(a) Starting from the definition of Reflection Coefficient, explain the construction of the Smith Chart. It is NOT necessary to derive the equations for the constant impedance and constant reactance circles.

[8 marks]

It is a polar plot of the complex reflection coefficient. It is also known as the 1-port scattering parameter s or s11, for reflections from a normalised complex load impedance z = r + jx;

[4 marks]

[2marks]

[2marks]

(b)Consider the transmission line circuit below (Figure 1). Use the Smith Chart to find SWR on the line, the return loss, the reflection coefficient at the load, the load admittance, the input impedance to the line, the distance to the first voltage minimum, the distance from the load to the first voltage maximum.

[9 marks]

0.8 Wavelength

Z0 =50 Ω ZL =70+j40 Ω

Zin

Figure 1

[pic]

1mark for each answer except for last two (2 marks)

(c)A load impedance of ZL = 100-j150 Ω is to be matched to a 50 Ω line using a single shunt-stub tuner. Find two solutions using short-circuited stubs.

[8 marks]

2 marks each for the following four answers

[pic]

Question 2

(a) Consider an arbitary microwave transistor with scattering matrix [S], connected to source and load impedances as shown in Figure 2.

[pic]

Figure 2

Derive the following equations concerning (in and (out.

[pic]

[7 Marks]

with reference to figure 1, the refelection coefficient seen looking forward the load is

[pic]

while the reflection coefficient seen looking toward the source is

[pic]

[1 Marks]

in general, the input impedance of the terminated two-port network will be mismatched with a reflection coefficient given by (in, which can be defined by the following analysis. From S parameters definition,

[pic]

Eliminating V2-, and solving for

[pic]

[4 Marks]

Similarly, (out can be obtained.

[2 Marks]

(b) In a transistor oscillator, a one-port negative-resistance is effectively created by terminating a

potential unstable transistor with an impedance designed to drive the device in an unstable

region as shown in Figure 2.

[pic]

Figure 2

Assuming that S parameters of the transistor in a common-gate configuration are

S11=(2.18, -35(), S12=(2.75, 96(), S21=(1.26, 18(), S22=(0.52, 155().

Design load and teminating networks using a combination of one-eighth...