Abstract
ABSTRACTWe present here a compact liquid crystal (LC) based tunable notch filter with a narrow 3 dB bandwidth and a high quality factor. The structure of the proposed filter consists of two embedded resonators in serial, with an impedance adaptor and linear taper at each port. The notch filter combines tunable dielectric material LC with inverted microstrip technology, and was designed, fabricated and characterized to deliver a continuously tuned rejection frequency range from 3.545 GHz to 3.731 GHz, together with a narrow 3 dB bandwidth of ∼ 0.8 GHz and a high quality factor of ∼ 4.6. Such a notch filter is both compact (41 mm × 13 mm) and light weight (18.08 grams, including SMA connectors). It is suitable for working in the S band to suppress WiMAX signals that may cause interference in UWB communication systems. The narrow stopband and high quality factor of the device can be further improved at the expense of a certain extent of tuning efficiency, by using a thicker LC layer, and this is verified in experiment. The measured results of the proposed devices confirm the feasibility of using the current LC technology to manufacture tunable notch filters with a narrow bandwidth and a high quality factor.
Highlights
Liquid crystal (LC) materials have been attracting attentions as excellent electro-optic materials for the low driving voltage, low power consumption, and a high degree of anisotropy
This work demonstrates an innovatively designed tunable notch Bandstop filters (BSFs) based on tunable LC dielectric material
Experiments have shown that good device performance can be achieved through optimization of the filter structure using two serially embedded spurline resonators, and by selecting an LC material with low dielectric loss
Summary
Liquid crystal (LC) materials have been attracting attentions as excellent electro-optic materials for the low driving voltage, low power consumption, and a high degree of anisotropy (in shape, dielectric and optical). The high Q factor of this embedded spurline BSF structure is useful for filtering out spurious frequencies and noise signals without too much influence on the required signals, due to it producing less radiation than other conventional shunt-stubs and coupled structures [13,17]. Another benefit of this embedded spurline structure based BSF is the ease of its manufacture and integration. The fact that UWB transmissions cross the boundaries of some currently licensed frequency bands implies that UWB transmission may cause interference This raises the need for BSFs with extremely narrow stopbands to attenuate unwanted inference signals. The BSF reported here was designed to attenuate the interference of Worldwide interoperability for Microwave Access (WiMAX, 3.3–3.6 GHz) in the UWB range of applications
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