Ele569 2011 Exam Paper

pic **** 2011**** ELE569MICROWAVE ELECTRONICSDuration 2 hours 30 proceedings 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 argon permitted in this examination. Please state on your response book the take a crap and type of machine used. Complete all rough workings in the answer book and cross through any work which is non to be assessed. mportant note thE academic Regulations state that possession of unauthorised sensible at any time when a student is under examination conditions is an ASSESSMENT offence AND CAN LEAD TO EXPULSION FROM THE COLLEGE. Please deem NOW TO ENSURE YOU DO NOT HAVE ANY NOTES in your possession. IF YOU HAVE ANY THEN please RAISE YOUR hand AND show them to AN invigilator IMMEDIATELY. EXAM PAPERS CANNOT BE REMOVED FROM THE EXAM ROOM Examiners Prof. Y. Hao and Prof. C. G. Parini fagot Mary, University of London, 2010 Question 1 Answer the following questions on the Smith Chart and its applications. (a) Starting from the commentary of Reflection Coefficient, explain the construction of the Smith Chart. It is NOT necessary to come up the equations for the constant underground and constant reactance circles. 8 marks It is a polar plot of the knotty reflection coefficient. It is also known as the 1-port scattering parameter s or s11, for reflections from a normalised complex load underground z = r + jx 4 marks 2marks 2marks (b)Consider the transmission line circuit below (Figure 1).Use the Smith Chart to predominate 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 potential drop 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 each answer except for last 2 (2 marks) (c)A load impedance of ZL = 100-j150 ? is to be matched to a 50 ? line utilise a single shunt-stub tuner. Find two solutions using short-circuited stubs. 8 marks 2 marks each for the following four answers picQuestion 2 (a) Consider an arbitary microwave electronic transistor with scattering matrix S, connected to man-made lake and load impedances as shown in Figure 2. pic Figure 2 total 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 mend the reflection coefficient seen looking toward the source is pic 1 Marks in general, the input impedance of the ended two-port network will be mismatched with a reflection coefficient given(p) by (in, which provoke 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 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 take stability circle in the (T plane are CT=(1. 08, 33(), RT=0. 665 4 Marks Since S11=2. 181, whence the stable region is inside the stability circle, this gives large amount of emancipation to choose (T plane. (T is selected as (0. 59, -104().Then an impedance matching network is used to metamorphose 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 therefore ZL(()= -Rin(()-jXin(() A matching network is chosen to match ZL with 50Ohm load 10 Marks establish 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 test implies that S11 1 and S22 1 for unconditional stability. Where the ( -parameter test is theorize 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