Differential Narrow Bandpass Microstrip Filter Design for Material and Liquid Purity Interrogation

In this paper, a new narrow bandpass microstrip filter is proposed with a differential structure based on an analytical approach. A new model of the differential structure, employing an LC equivalent circuit is discussed. Differential/common mode using an odd/even mode analysis is used to calculate the operating frequency of the circuit theoretically. The narrow bandwidth structure is a suitable choice in sensory applications to detect the purity of various liquid materials. The permittivity of materials was measured by using various liquid materials as upper substrates; this changed the resonance frequency depending on the material used and the results could be interpreted as a measured material permittivity. The resonance frequency of the proposed filter fabricated with Rogers RO4003 substrate was 1.75 GHz. Compact size (0.3018λ × 0.141λ), higher sensitivity, roll-off sharpness, noise reduction, and a new theoretical method are advantages of this proposed design.