Novel microfluidically tunable bandpass filter with precisely-controlled passband frequency

A novel microfluidically tunable bandpass filter (BPF) is proposed and it consists of a second-order microstrip BPF with five plastic tubes placed above the coupled quarter-wavelength resonators. The tubes can be filled with high-permittivity water. By properly choosing the locations of the tubes and the water-filling sequence, the electrical length of the resonator can be tuned, and thus the passband centre frequency of the filter can be precisely adjusted. A demonstration example is designed, and the simulated and measured results of the filter are given for verifying the theoretical prediction.