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In this article, a novel compact planar diplexer based on the modified SR was proposed. The method of adjusting the stopband filter characteristic has also been demonstrated. The proposed SR structure provides good selectivity and high isolation between very narrow-spaced TX and RX operating channels. The isolation is greater than 30 dB. The insertion losses in the working channel are less than 1.25 dB in the RX band and less than 1.0 dB in the TX band. The return losses on the input ports are below 220 dB. The proposed diplexer has been fabricated using the standard PCB process on RO4003 laminates. Due to the very compact size of the SR embedded in transmission line the proposed diplexer, it is a good solution for applications where compact size, sharp selectivity, and high isolation are required. ABSTRACT: A new compact, low insertion loss, and wide stopband balanced bandpass filter (BPF) with two common-mode (CM) transmission zeros within the differential-mode (DM) passband is designed in this article. The DM filtering topology is formed by two quarterwavelength resonators and a short stub loaded source-load coupling structure which can generate new transmission zeros to improve the selectivity and suppress the harmonic to widen the stopband. The centerloaded step-impedance open stub can be applied to misalign two CM fundamental resonant frequencies, meanwhile adjust one CM transmission zero to the DM passband. To enhance the CM suppression within the DM passband, another CM transmission zero is created by the folded stub loaded in the 50-X input feedline which creates the first DM transmission zero located at 2f d 0 (f d 0 is the center frequency of DM passband) and has little effect on the DM in-band and lower-stopband performance. Finally, a high CM suppression balanced BPF prototype for WLAN application is designed and fabricated. The simulated and measured results show a good agreement.

A microwave microfluidic sensor for dielectric characterization of liquids in real time is presented in this paper. The sensor is implemented in microstrip technology and consists of a symmetric splitter/combiner configuration loaded with a pair of identical split ring resonators (SRRs) and microfluidic channels placed on top of them (gap region). The sensor works in differential mode and sensing is based on frequency splitting. Thus, if the structure is unloaded or if it is symmetrically loaded with regard to the axial plane, only one transmission zero (notch) in the frequency response appears. However, if the axial symmetry is disrupted (e.g., by the presence of different liquids in the channels), two transmission zeros arise, and the difference in magnitude (notch depth) and frequency between such transmission zeros is indicative of the difference in the dielectric properties (complex dielectric constant). A circuit schematic, including transmission line sections to describe the distributed components, lumped elements to account for the SRRs and their coupling to the lines and lumped elements to model the liquid properties, is presented and validated. After proper calibration, the functionality of the proposed sensor is demonstrated by measuring the complex permittivity in solutions of deionized (DI) water and ethanol as a function of the ethanol content.

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