Electrical biasing substrate integrated waveguide tunable band-pass filter with liquid crystal technology

In this paper, a continuously tunable substrate integrated waveguide (SIW) bandpass filters is developed with liquid crystal, which has an electrical biasing scheme. The electrical biasing circuit is implemented by square copper patches embedded in SIW resonator cavities. To validate the proposed design concept, a three-pole tunable SIW filter with a centre frequency of 7.6 GHz and a fractional bandwidth of 200 MHz is fabricated and measured. Due to the electro-optics effect of liquid crystal, the permittivity of liquid crystal changes with biasing voltage and the central frequency tunability of the filter is realized. The experimental results show that a continuous tuning range of 262 MHz is obtained. In addition, the insertion loss is 3.5-5.9 dB with a return loss of 12.5 dB over the whole tuning range. Therefore, the proposed filter provides good potential for modern multifunction communication systems.