This chapter reviews the microwave complex ratio measuring (MCRM) circuits which are used for complex reflection coefficient measurement. This MCRM circuit is relatively simple and cost-effective. There are various structures for the MCRM circuit, such as multi-probe transmission line circuits, five-port ring circuits, six-port hybrid coupler-based circuits, switched-reflector circuits, dual-generator circuits, and Wheatstone bridge-based circuits. Each structure of the circuits has its own advantages and disadvantages. In this chapter, the MCRM circuit calibration process has been described in detail.
TopIntroduction
RF and microwave technology are rapidly expanding due to fourth industrial revolution (Industry 4.0), development of Internet of things (IoT), and 5th Generation wireless communication technology (5G) that will be implemented in the future. Recently, the commercial rollouts 5G technology is located at 3.4 to 4.2 GHz and 4.4 to 5 GHz (sub-6 GHz spectrum bands) as well as 28 GHz of high frequency bands. Therefore, microwave equipments, such as reflectometer systems are increasingly important today. For instance, two up to date commercial reflectometers which were used for the complex reflection coefficient measurements are shown in Figure 1.
Figure 1. (a) CABAN R54 Vector Reflectometer (85 MHz to 5.4 GHz). (b) Arinst VR 23-6200 (23 MHz to 6.2 GHz).
Microwave complex ratio measurement (MCRM) system is one of the important alternative tools used in the microwave measurements and components design. In fact, MCRM system is a vector system that can be used to measure the complex reflection coefficient of the device under test (DUT) over a relatively broad frequency range. The MCRM circuit has been studied and developed rapidly by some researchers since the 70's (Hoer, 1972; Engen and Hoer, 1972; Caldecott, 1973; Engen, 1976, 1977), due to its simple circuit design and it has long-term stability of operation. Several types of MCRM circuits have been proposed and configured, such as multi-probe transmission line circuits, five-port ring circuits, six-port hybrid coupler-based circuits, switched-reflector circuits, dual-generator circuits, and Wheatstone bridge-based circuits. In fact, the combination of two MCRM systems will produce a vector network analyzer which is capable of measuring reflection and transmission coefficients simultaneously of the DUT. In this book chapter, those types of MCRM circuits and its calibration process will be described in detail.
TopMicrowave Complex Ratio Measuring (Mcrm) Circuits
Microwave complex ratio measuring (MCRM) circuit is a part of passive circuit in vector measurement instruments, such as vector reflectometer. In general, the MCRM circuit is a multi-port circuit in which it is capable of measuring the magnitude, |Γ| and phase shift, θ of reflection coefficient for device under test (DUT), based on the number of n power ratios between the measured output power amplitudes, Pn (n = 1, 2, and 3) and input reference power amplitudes, Pref. One input port of MCRM circuit is applied by power source and another port is terminated with DUT. The remaining output ports (normally three ports or four ports) will be connected to power detectors in which the power ratios of the DUT are measured. For instance, a six-port complex ratio measuring system is shown in Figure 2.
Figure 2. Arbitrary six-port microwave complex ratio measuring (MCRM) system.