Low-loss 3-bit tunable SIW filter with PIN diodes and integrated bias network

A 3-bit digitally tunable band-pass filter with up to 8 nearly equi-spaced frequency responses between 4-4.4 GHz has been designed and simulated in a low-cost single-layer Substrate Integrated Waveguide (SIW) technology. The structure is based on coupled coaxial SIW resonators embedded into the dielectric. The proposed filter achieves discrete frequency tuning by using PIN diodes as tuning elements of a switchable loading capacitance. Thus, a 2-pole filter demonstrator has been designed on a 3.175 mm-thick Rogers RT/Duroid 5880 substrate (ε
_r
= 2.2, tan δ = 9·10
^-4
). The integration of the bias network onto the SIW metal layers allows a simpler assembly without degrading the filter performance. Insertion loss ranging from 1.45 dB to 2.6 dB have been obtained, with minim return loss better about 10 dB. Additionally, the proposed filter presents a very compact layout due to the combline topology and the integration of the bias circuitry. The device shows promising application for the design of discretely tunable narrow filters with higher Q factor.