Measurement techniques of scattering parameters based on six-port technology for microwave applications

Abstract

Vector network analyzers have been used to provide accurate measurements of the scattering parameters of a microwave network and are essential instruments that have gained more and more attention in biomedical industry. In this context, low-power electromagnetic waves that are transmitted into the cells under test can be successful in cancer detection due to the dielectric contrast between malignant tumors and normal cells at microwave frequencies. However, these instruments are often bulky and expensive which is not suitable for clinical and Point-of-care rapid tests. Therefore, many research activities have been made to replace them by low cost and portable alternative six-port measurement systems. The aim of this thesis is to present a wideband microwave six-port system that is able to measure the reflection and the transmission coefficients of a device under test (DUT). The proposed six-port design is required to give the accuracy needed for the measurements to be done in a wide band of operation in the S-band. While many studies have been carried out to develop six-port reflectometers, there is still a need to optimize the six-port circuit performance by keeping the design complexity low and by reducing the computational effort for six-port calibration. To do this, hybrid couplers and power dividers need to be optimized as they are building blocks in the proposed six-port architecture. To this end, the design of a modified ring power divider, built by using a conventional ring power divider and an additional Stepped-Impedance Resonator (SIR) is introduced to improve its operational bandwidth and performance compared to the reported ring structure. A comparative study between the designed device and the previously reported ring power divider is conducted. The electromagnetic (EM) simulated results show that the proposed divider has a fractional bandwidth of 90.2 % at the center frequency of 3.17 GHz. The measurement results of the fabricated prototype demonstrate high performance over the considered operational bandwidth from 1.7 GHz to 4.64 GHz with a return loss lower than -10 dB while maintaining a good insertion loss and a good isolation between the output ports. The maximum amplitude imbalance is better than 0.21 dB and the phase imbalance is better than 2.5 degrees between the output signals. In addition, a compact two-section 90-degree coupler with a rounded structure is designed to operate at 3 GHz. The input reflection coefficient S11 is under -10 dB between 2.3 GHz and 3.96 GHz which means a wide operational bandwidth of the proposed coupler. In addition, the insertion loss is -3.09 dB at the center frequency with magnitude im alance less than ±1 dB between 2.6 GHz and 3.7 GHz. The six-port junction made up of a modified ring power divider and three identical 90-degree hybrid couplers is then designed and fabricated to operate at the center frequency of 3 GHz. The six-port system's performance is demonstrated by measurements of the return loss S11 which is lower than -17 dB in the frequency band of 2.2 ˗ 3.67 GHz. Besides, the transmission coefficients with respect to port 1 are - 6.3 dB ± 1 dB from 2.2 to 3.55 GHz. These results show the excellent performance of the six-port junction. In order to build the six-port reflectometer, the microwave source and the device under test (DUT) are connected to the six-port junction unit through an additional circuit formed by a 90- degree hybrid and a ring power divider. The performance of the six-port reflectometer is then evaluated for real-time measurements by measuring the reflection coefficient of different devices under test. The measured results are then corrected by a proper calibration method to remove hardware imperfection. Compared to conventional six-port calibration methods, the proposed calibration process is easy to use and allows accurate reflection coefficient measurements while reducing the complexity and computational efforts of traditional six-port calibration techniques. Finally, a low-cost and portable six-port network analyzer (SPNA) for measuring the complex reflection and transmission coefficients is designed to operate at the center frequency of 3 GHz. The proposed network analyzer is built by two six-port junctions and 8 power detectors and an additional circuit for power distribution which is formed by one coupler and one power divider. The proposed measurement system combines the advantages of simplicity and real-time operation as well as low-cost fabrication since it is built on a single layer microstrip technology. A simplified approach for calibration based on two-port error correction with only four standards is implemented. After calibration, the measured complex reflection and transmission coefficients obtained from the different tests performed is evaluated which demonstrate the validity of calibration method. This thesis opens a new promising route for two-port Sparameter measurements based on low-cost six-port technique.

Date	6 Apr 2022
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Vahé Nerguizian (Supervisor) & Serioja Tatu (Co-supervisor)