Novel digital and analogue tunable lowpass filters

The paper presents a new concept for implementing tunable lowpass filters by employing slot resonators etched in the ground plane. When RF MEMS switches are used to short-circuit the slots in the ground plane, the effective length of the slots can be varied to achieve tunability at discrete frequencies. The concept is demonstrated by considering 4-slot lowpass filters. Continuous tuning is achieved by replacing switches with varactors as tuning elements. A varactor tuned lowpass filter was built and tested. Simple transmission line models for the proposed structure are also presented. The measured results are in good agreement with simulations confirming the validity of the proposed model. The experimental lowpass filters exhibit superior RF performance which consists of a low insertion loss and a large tuning range. The insertion loss is 0.6 dB for both the digital and the analogue tunable filters, while the achievable tuning range is 44% for the digital tunable filter and 22% for the analogue tunable filter. A much wider tuning range is obtained by combining digital and analogue tuning in one circuit.