Multifunctional Bandpass Filter/Displacement Sensor Component

This paper presents the design and realization of a multifunctional bandpass-filter/ displacement-sensor using an edge-coupled microstrip bandpass filter loaded by a pair of split ring resonators (SRRs). It is shown that while the structure acts as a bandpass filter at its operating frequency, the phase of the reflection coefficient from a movable loading resonator at the resonance frequency of the resonator can be used for displacement sensing. With this aim, and to avoid any interference with the filter functionality, the resonance frequency of the SRRs is chosen within the stopband of the bandpass filter, where all the input signal is reflected back to the input port. Therefore, moving the loading resonators does not have any adverse effect on the filtering performance. To validate the concept, a fifth-order Chebyshev bandpass filter with a feed line loaded with a pair of SRRs is designed and the numerical evaluation of the proposed multifunctional bandpass-filter/displacement-sensor is provided. The proposed multifunctional component is also validated through fabrication and measurement. It is shown that the proposed component has a sensitivity of 8°/mm at the designed frequency of SRRs. However, the sensitivity can be increased by scaling the SRRs to resonate at a higher frequency within the upper stopband of the filter.