A 2∼18GHz compact microwave band-pass filter suitable for planar and three-dimension flexible integration

In this paper, a ultra wide-band microwave band pass filter suitable for planar and three-dimension integration is presented. Taking the advantage of mechanical flexibility, ultra thin Liquid Crystal Polymer (LCP) with 50 μm thickness is used as the substrate. Firstly, to estimate the bending effects of microwave devices based on LCP substrate, a set of 50 Ohm microstrip lines is fabricated and tested. And the performances of mcirostrip lines at the range of bending radius from 1.5 mm to 0.75 mm are demonstrated. The microstrip lines show little deterioration under the bending radii of 1.5mm and 1mm, but when the radius was reduced to 0.75 mm, the insertion loss sharply increased to 30 dB, showing the complete reflection phenomenon; Then, a 2-18GHz ultra wide-band filter are designed and tested with overall LCP dimensions of 7.52 × 6.31 mm2. The planar measurement result show that the maximum insertion loss is 2.6dB at 18GHz and maximum return loss is 11.2dB at 6.7GHz, respectively; Finally, the performance of the proposed filter is measured at twisted circumstance with the maximum insertion loss is 2.8dB at 18GHz and maximum return loss is 12.1dB at 6.6GHz, respectively. And the comparison results show a very low susceptibility due to twisted behavior. These results show that the ultra thin LCP substrate is a promising candidate for future planar and 3D integration applications in wearable and portable devices.