Quasi-Lumped-Element Filter Based on Substrate-Integrated Suspended Line Technology

Quasi-lumped-element low-pass filter (LPF) and high-pass filter (HPF) based on substrate-integrated suspended line (SISL) technology are presented in this paper. Double interdigital capacitor, which has advantages of high capacitance density and high quality factor, is analyzed and designed to minimize the filter size. Based on the theoretical analysis, quasi-lumped-element SISL LPF and HPF prototypes are designed. The experimental results show that the SISL LPF achieves ultra-wide stopband up to 24 f
_c
(f
_c
is the cut-off frequency) with compact size of 0.180 λ
_g
×0.082 λ
_g
(λ
_g
is the guide wavelength at f
_c
). The SISL HPF features the low insertion loss of only 0.3 dB, while the size is only 0.108 λ
_g
× 0.085 λ
_g
. Moreover, the methods of substrate excavation and packaging optimization are proposed to further enhance the performance of SISL LPF and SISL HPF. By doing so, the stopband of SISL LPF is further extended to 32 fc, and the insertion loss of SISL HPF is significantly reduced to 0.1 dB.