A 1.9-18-GHz Filter Bank With Improved Passband Flatness Based on Asymmetrical Low-Loss SP7T Switch

A novel on-chip single-pole seven-throw (SP7T) switch with asymmetrical paths is proposed in this article. By grouping the seven channels of the SP7T switch into different groups with appropriate matching networks, low insertion loss (IL) can be obtained. On the other hand, in order to achieve better flatness for the passband of the bandpass filter (BPF), a flatness compensation structure (FCS) consisting of series inductance, capacitance, and resistance is proposed. By adjusting the resonant frequency and unloaded Q of the FCS, the transmission peak of the BPF at the center frequency is effectively suppressed, resulting in a flat filter passband. To demonstrate the proposed SP7T switch and FCS, an SP7T switch covering dc-to-18 GHz, a single BPF covering 7.9-13.8 GHz, and a seven-channel switching filter bank (7CSFB) are implemented and fabricated for measurement, using 0.15-mu m GaAs pHEMT process. The SP7T switch achieves a satisfactory IL of less than 1.44 dB and only 1.30 dB at 18 GHz, and the measured IP1dB is greater than 26.5 dBm. The measured results of the single BPF indicate that the addition of FCS optimizes the flatness to +/- 0.5 dB at the expense of some loss; the measured results of 7CSFB show that the passband IL is lower than 8.6 dB with a favorable flatness better than +/- 1 dB, the return loss (RL) is better than - 10 dB, while the out-of-band rejection is up to 40 dB.