Microwave multiport measurement systems for sensor applications

General description

Microwave measurements significantly differ from electrical measurements performed at lower frequencies. An electrical signal having the frequency of several GHz can be radiated and received by antennas, it can also penetrate objects. These phenomena are widely used in sensor systems utilizing an influence of the measured physical parameter, such as properties of liquid being penetrated by a microwave signal, or displacement of object from which the signal is reflected, on the electrical signal measured by an appropriate apparatus. As fundamental devices serving for such measurements vector network analyzers are commonly used, which nowadays can be found in a majority of microwave laboratories. Such analyzers, however, feature considerable size, significant complexity and high price. Although they work well in laboratory conditions, due to the above drawbacks the cannot be applied in sensor systems operating at microwave frequencies, where circuitry simplicity, possibility of miniaturization, and low costs of the measurement system production are crucial issues. The listed advantages characterize multiport measurement systems, which are suitable for sensor applications as these mentioned above. In general multiport systems are composed of a passive multiport power division network, signal source and power detectors. With the use of at least three power detectors and an appropriately designed power division network one can obtain the functionality of a vector network analyzer utilizing, however, a significantly simpler measurement circuitry. Thus, nowadays the multiport systems are commonly used in sensor systems, e.g., for detection of the angle of signal arrival, remote measurement of displacement and vibration, solution concentration or fat content in milk.

Research project objectives

The goal of the project is the development of a new class of multiport measurement systems intended for sensor applications, featuring the minimum measurement uncertainty in the range of the measured signal variation. The currently used multiport systems are designed to provide the minimum measurement uncertainty for a wide range the measured scattering parameters, which results from their primary application as vector network analyzer. However, in sensor applications, the measured physical parameter (e.g., permittivity change, object displacement) results in small variation range of the corresponding scattering parameter, i.e., reflection or transmission coefficient, which is measured by a multiport system. Hence, the measurement becomes crucial only for a section of the complex plane, for which the measurement uncertainty should be as low as possible. It should be noted that until now the multiport systems featuring such narrow measurement range have not been investigated. Thus, the main research hypothesis of the project can be formulated: it is possible to significantly reduce the uncertainty of measurement with the use of a multiport system with respect to the currently reported solutions by narrowing the measurement range of this system to the variation area of the scattering parameter corresponding to the measured physical parameter. The research goals to realize in the project can be summarized as follows:

  • development of methods for the analysis of multiport measurement systems for sensor applications allowing for determination of the optimum parameters due to the measurement uncertainty,
  • development of design methods and new circuits for multiport measurement systems which will provide an increased sensitivity to the complex signal varying within a constrained section of the complex plane resulting from a given application (e.g., object displacement, moisture variation, fat content change in food products, etc.)
  • development of calibration procedures for the multiport systems in which the calibration standards are not precisely defined,
  • realization of the measurement systems involving the developed multiports
  • conducting experimental verification of the developed multiport measurement system for physical parameters measurements (e.g., solution concentration, fat content in milk and others)