Bandwidth and Center Frequency Reconfigurable Waveguide Filter Based on Liquid Crystal Technology

Abstract

This paper presents for the first time a fully electronically reconfigurable waveguide filter tunable in bandwidth and center frequency based on liquid crystal (LC) technology. A continuously reconfigurable two pole bandpass filter is designed and characterized in the Ka-band at 30 GHz. To be able to tune both center frequency and bandwidth independently, the resonators and coupling structures are filled with LC as tunable material. Hence, the filter's center frequency and coupling strengths can be tuned and, furthermore, tuning with constant filter characteristic is possible. To tune the LC, a novel two-layer electrode design for waveguide structures is presented, which is simple to integrate and provides a high tuning efficiency with low insertion loss. By applying different bias configurations, the LC's effective permittivity can be varied, and therefore, also the resonators' electrical lengths. The presented two pole filter can adapt its center frequency from 29.8 GHz to 30.7 GHz with a maximum 3 dB bandwidth variation from 660 MHz to 870 MHz. The measurements are carried out with bias voltages up to ±250 V.INDEX TERMS Microwave filter, liquid crystals, millimeter wave communication, tunable circuits and devices, K-band.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

Novel Hybrid Electric/Magnetic Bias Concept for Tunable Liquid Crystal Based Filter

“…In this section, a center frequency tunable second order bandpass filter with adjustable coupling strengths is designed in GGW technology, which is operating at 30 GHz with an intermediate equal ripple bandwidth (BW ) of 300 MHz and a return loss (RL) of 20 dB. Similar to our previous work [6], two main resonators (MR) and three coupling resonators (CR), which resonate at frequencies far below the passband, are used to design a reconfigurable filter. By inserting LC into the MR and CR and controlling the LCs' permittivity, the center frequency of each resonator can be tuned independently.…”

“…Hence, the resonators only differ in their length l and coupling pins height d c . More information regarding the design process can be found in previous work [6].…”

“…Moreover, it has no mechanical moving parts, which can cause wear-out failures. In a previous work [6], a bandwidth and center frequency reconfigurable LC filter has been reported in waveguide technology. However, the integration of the electrodes into the waveguide is very challenging and, furthermore, deformation of the metal, fabrication flaws and imperfect assembly lead to degraded RF performance.…”

“…By simultaneously applied magnetic and electric bias fields, the LC's effective permittivity can be controlled with a reduced number of electrodes, lower bias voltage and higher tuning efficiency. Furthermore, a groove gap waveguide (GGW) technology is chosen for to implement the LC filter instead of a rectangular waveguide as in [6], since it offers the unique advantage of DC-decoupled metal line sections at the top and bottom according to Fig. 1(a), while maintaining similar low insertion losses than rectangular waveguides [7].…”

“…DC decoupling is achieved by the small gap between the bed of nails structure at the bottom and the top metal plate, in which no propagation is possible, due to an electromagnetic stopband. This simplifies the electrode system in particular with the hybrid bias concept, and hence, the assembly of the filter significantly compared to the one in [6], since the bed of nails structure at the bottom is used at the same time as ground plane for the bias voltages to tune the resonators. This GGW technology has been already used and proven for fixed high-Q filters [8] and tunable LC phase shifters [9], but not yet for tunable filters.…”