A 16–34 GHz CMOS Reconfigurable Bandpass Filter Using Mode-Switching Transformer

A millimeter-wave reconfigurable CMOS bandpass filter using a mode-switching transformer (MSTF) is proposed in this article. The proposed filter consists of two MSTFs, eight sets of tunable varactors, and eight cross-coupled nMOS transistor cells which together form a fourth-order Q-enhanced bandpass filter. Each of the MSTF consists of multisection metal strips that are connected by a set of mode switches. By controlling the mode selecting switches, the MSTF can work at two different modes including the high-frequency (HF) and the low-frequency (LF) mode, significantly expanding the frequency range of the corresponding resonant tanks and thus increasing the frequency tuning range (TR) of the resulted filter. To accurately model the proposed MSTF for filter design, the exact equivalent circuit of the MSTF including the effective inductance and coupling coefficients is studied and given in detail. Meanwhile, quantitative analysis of the ON-state resistance (ON-resistance) from the switch is given and critical conditions are derived to minimize the degradation of the resonator quality factors (Q) and remove the spurious resonance due to ON-resistance from the switch. With the proposed configuration, a frequency-reconfigurable filter with wide frequency TR, low shape factor, and compact size can be easily obtained. To demonstrate the proposed technique, a filter prototype covering a wide frequency TR of 16–34 GHz is designed and manufactured using a commercial 55 nm bulk CMOS technology. The proposed results in a low shape factor of 1.6, and has a core area of only $0.30\times 0.59$ mm2. The measurement result agrees well with the simulation result, validating the proposed design techniques.