Ele569 2011 Exam Paper

pic **** 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 body politic on your set book the name and type of machine used. Complete all rough workings in the solving book and cross through any work which is not to be assessed. mportant disgrace 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 go along 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 1Answ er the side follow 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 factor coefficient. It is also known as the 1-port scattering parameter s or s11, for reflections from a normalised complex encumbrance impedance z = r + jx 4 marks 2marks 2marks (b)Consider the transmission line circuit below (Figure 1).Use the Smith Chart to bring forth SWR on the line, the return loss, the reflection coefficient at the weighting, the load admittance, the input impedance to the line, the distance to the first electric potential minimum, the distance from the load to the first voltage maximum. 9 marks 0. 8 Wavelength Z0 =50 ? ZL =70+j40 ? Zin Figure 1 pic 1mark for all(prenominal) answer except for last two (2 marks) (c)A load imped ance of ZL = 100-j150 ? is to be daystared to a 50 ? line using a single shunt-stub tuner. Find two solutions using short-circuited stubs. 8 marks 2 marks each for the chase four answers picQuestion 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 facial expression 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 all over 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 intentional to take up the device in an unstable region as shown in Figure 2. pic Figure 2 take for granted that S parameters of the transistor in a parkland-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 nd a quarter wavelength impedance transformers for a microwave oscillator. Please note that the stability circle can be calculated using the following equations pic 18 Marks S11 is quite big, which means that the common gate transistor configuration is unstable The centre and radius of the output stability circle in the (T plane are CT=(1. 08, 33(), RT=0. 665 4 Marks Since S11=2. 181, then the stable region is inside the stability circle, this gives large make out of freedom to choose (T plane. (T is selected as (0. 59, -104().Then an impedance matching network is used to convert ZT into 50Ohm match load From (T , (in=(3. 96, -2. 4(). It is obtained from pic Zin can be found to be (-84-j1. 9)Ohm, and then ZL(()= -Rin(()-jXin(() A matching network is chosen to match ZL with 50Ohm load 10 Marks Based on the following equations, an impedance matching network can be designed (T is selected as (0. 59, -104(), ? T can be found as Zin can be found to be (-84-j1. 9)Ohm, and then ZL(()= -Rin(()-jXin(()=84+j1. 9 Question 3 Answer the following questions about microwave amplifiers a)Show that for a unilateral device, where S12=0, the ( -parameter examination implies that S11 1 and S22 1 for unconditional stability. Where the ( -parameter test is formulated as pic 8 marks pic pic (b) Use the ( -parameter test to determine which of the following devices are unconditionally stable, and of those, which has greatest stability. 6 Marks Device S11 S21 S12 S22 A 0. 80