: The six-port reflectometer is a high-frequency measurement device which allows to determine the reflection coefficient of a device under test (which depends directly on its input impedance) or alternatively to find the ratio in terms of amplitude and phase between two different signals. This type of measurement is often used in the high-frequency domain, on the one hand in laboratories in order to characterize components, and on the other hand in applications like security distance radar for automobiles, digital demodulators or control of adaptive antennas.
One of the advantages of the six-port reflectometer with respect to other systems which measure the same quantities is its very simple structure: it is essentially a linear circuit with six ports, four of which are connected to power detectors. In consequence, it can be realized much more easily and with lower costs than these other types of instruments which generally require more sophisticated components as for example high quality mixers.
After calibrating the six-port reflectometer, it is possible to calculate the reflection coefficient of the device under test or the ratio in terms of amplitude and phase between two different signals, as well as information about the precision of these values, from the electrical powers measured by the four detectors. One can say that numerical calculations using a computer replace the need for a very sophisticated measurement circuit which permits to obtain the results in a more direct way.
In spite of the considerable advantage presented by the simplicity of the circuit, six-port reflectometers are to this day mostly used in metrology laboratories because of the high measurement precision they allow to achieve; however, they are very little used in commercial products, except very specialized applications. There are several reasons for this, among others the rather narrow frequency span of most six-port reflectometers, the large dimensions of certain structures which often contain several directional couplers, as well as problems which may appear in certain situations during the calibration procedure.
In this thesis, we present several developments to solve these problems in order to make possible the implementation of six-port reflectometers in high-volume industrial products. First, we introduce a very robust calibration algorithm which eliminates the problems which the algorithms used before might present in certain situations. Next, we give details about our newly developed six-port reflectometer in low-cost hybrid technology with a surface of 20 cm² including the power detectors, which has a very large frequency span, from 1.5 MHz to 2200 MHz. Finally, we present a six-port reflectometer we have realized in monolithic microwave integrated circuit (MMIC) technology. This circuit occupies a surface of 2.2 mm² including the detectors and functions between 1.3 GHz and 3.0 GHz. It is most accurate around the frequency of 1.8 GHz, the frequency of the new mobile phone series DCS 1800.
All these developments should greatly facilitate the use of six-port reflectometers in industrial applications aimed at the general public such as security distance radars for automobiles or digital demodulators for mobile phones. We hope that the results of our work will help to convince a larger number of companies of the advantages of this still relatively new device.