A Compact Dual-Mode Bandpass Filter with High Out-of-Band Suppression Using a Stub-Loaded Resonator Based on the GaAs IPD Process

In this letter, a compact dual-mode bandpass filter (BPF) with an ultra-wide stopband that employs two folded open-loop resonators (FOLRs) and stub-loaded resonators (SLRs) is proposed. The dual-mode resonators are optimized by loading two SLRs onto the folded open-loop resonators, and this process is analyzed using the dual-mode theory. To miniaturize the device size and increase chip performance, the proposed BPF is fabricated by a III-V compound semiconductor-fabrication process using a high-performance GaAs substrate based on the integration passivation device (IPD) fabrication process. A compact dual-mode BPF with low insertion loss and high return loss is designed and fabricated. Two extra transmission zeros (TZs) located in the high-frequency range increase the wide stopband, and the two TZs near the passband result in a higher selectivity. A resonant frequency centered at 7.45 GHz with an insertion loss of -1.21 dB and a measured return loss of higher than -23.53 dB and 3 dB fractional bandwidths of 5.8% are achieved. The stopband can be suppressed up to 20 GHz owing to the two tunable TZs resulting in higher selectivity and wideband rejection. The size of the filter was drastically optimized using a simplified architecture of two FOLRs and SLRs.