ECE 5320 – Microwave Engineering I – Labs
Project: Magnetic Levitation Control System
This is a five week long experiment about frequency and amplitude modulation. Using electronic components, students build a frequency shift keying (FSK) circuit. Both modulator and demodulator modules are constructed and tested. The carrier frequencies used here are 1 kHz and 5 kHz, for representation of logical 0 and 1, respectively. We have purposefully chosen these low frequencies to allow straightforward prototyping in the laboratory using bread boards. The concepts from this experiment are expanded in the EM class where students build the FSK system in the MHz range.
Lab 1 – Using ADS and the Network Analyzer
Introduction: We will learn how to use a portion of the software tools that analyze linear, passive microstrip circuits, and make swept plots of the reflection and transmission versus frequency. You will also learn how to use the graphics tools which output masks for microstrip photographic processing. Next semester, and beyond, you will use additional tools to analyze active circuits which contain transistors and diodes. Models of all these elements are contained in ADS, and circuits composed of these elements can be assembled together in this new software by simple “drag and drop” operations on the GUI interface.
Documents: For equipment and procedures, see:Lab 1 Handout
Lab 2 – Load Matching by Shunt Capacitive Elements
Introduction: Load matching is a very important subject in microwave engineering because reflected power is usually wasted power. Complicated and extended microwave circuits should be matched component by component to get the best results over some desired operating bandwidth. If there are several mismatched components in a circuit, the overall signals will exhibit rapid variations with frequency and there is no effective way to further reduce the SWR by overall matching. Matching needs to be done between every pair of components, if possible.
Documents: For equipment and procedures, see: Lab 2 Handout
Lab 3 – Design and Testing of Microstrip Couplers
Introduction: Couplers are used in microwave circuits for the purpose of dividing and combining signals in transmission lines or sampling the signals. You should always remember that power conservation in a lossless multi-port requires a “unitary” scattering matrix (S*TS = 1) and that one result of this is that all lossless, perfectly matched 4-ports are directional couplers. In this case, it is always possible to identify the ports as (1) “input”, (2) “through”, (3) “coupled”, and (4) “isolated”. There are only two possible types of lossless, perfectly matched 4-ports. Both have the same general matrix structure of zeros on both diagonals so that the input signals always combine in pairs to form the outputs (See p. 312). In one case, the combinations are 90Âº out of phase (related by “j”) and, in the other, they are just signed “Â±” (0 or 180Âº phase relation). When the coupling is 3 dB (i.e., a power divider), these two type couplers are called “90Âº hybrids” or “180Âº hybrids”.
Documents: For equipment and procedures, see: Lab 3 Handout
Lab 4 – Design and Testing of Low Pass Microstrip Filters
Introduction: Filters are used in microwave circuits for the purpose of selecting and controlling transmission of signals that differ in frequency. Filters come in four basic varieties called LP (Low Pass), HP (High Pass), BP (Band Pass), and BS (Band Stop) where the names are descriptive of the functions performed. In reality, however, any circuit, which performs a frequency selective function, can be called a filter; e.g., an impedance matching circuit (or filter) suppresses reflections over a given band. Also, combinations of filter types exist which can legitimately be thought of as a single filter such as a filter designed to operate over two adjacent bands to pass one and stop the other. Such a filter would be designed as a single circuit having the fastest possible transition between the pass and stop functions so that the two bands of interest can be close together.
Documents: For equipment and procedures, see: Lab 4 Handout
Lab 5 – Design and Testing of a Coupled-Line Band-Pass Microstrip Filter
Introduction: Band Pass (BP) Filters are especially useful in microwave communications because they allow only a selected band of frequencies to pass through. For microwave receivers, this helps to eliminate the effects of extraneous signals and especially noise so that the receiver signal-to-noise ratio is maximized. For multiplexed system carriers that combine many sub-carrier signals into a single wide band system, BP filters play a pivotal role in combining and separating the multiple sub-carriers.
To reach inductance values that can be approximated with microstriplines requires much wider bandwidths of nearly 100%. At this time, we do not know a way to make narrow band BP filters with connected microstrip elements although we suspect that it may be possible using the composite filter approach with a tandem pair of LP and HP filters which overlap in frequency to create the pass band. Because of this, a commonly-used BP filter is the so-called “coupled line filter” that is based on 4-port coupled line elements as described in Pozar, Section 8.7 (pp. 416-426).
Documents: For equipment and procedures, see: Lab 5 Handout