Design of an Ultracompact On-Chip Bandpass Filter Using Mutual Coupling Technique

In this paper, design of an ultracompact bandpass filter (BPF) in GaAs technology without compromising its electrical performance is investigated by means of both theoretical analysis and electromagnetic simulation. In particular, the relationship between the external quality factor and the coupling coefficient of the second-order BPF is formulized to better understand the principle of the mutual coupling effect. To prove the concept, the designed filter is implemented in a commercial 0.1-
$\mu \text{m}$
GaAs technology. A step-by-step design guideline is elaborated. The BPF has not only the merits of ultracompactness, but also remarkable insertion loss (IL) compared with other state-of-the-art on-chip designs. The measurement results show that the 1-dB bandwidth of the BPF is from 28 to 36 GHz, while the IL is less than 1 dB at 29.5 GHz. In addition, more than 40-dB rejection is achieved from 56 to 69 GHz. The size of the filter is only
$230 \times 280\,\,\mu \text{m}^{\mathrm {\mathbf {2}}}$
, excluding the pads, which is equivalent to
$0.074 \times 0.09\,\,\lambda \text{g}^{\mathrm {\mathbf {2}}}$
at 28 GHz. To the best of our knowledge, the proposed design is known to be the most compact one in the open literature using GaAs technologies.