A 24 GHz CMOS miniaturized phase-invertible variable attenuator incorporating edge-coupled synthetic transmission lines

This paper presents a CMOS miniaturized phase-invertible variable attenuator (PIVA) at K-band. The PIVA only consists of one 3-dB transmission line (TL) based directional coupler, two NMOS transistors, and simple matching network. The technique of complex impedance matching is proposed to improve the precision of the phase reversal, and reduce the phase shifting of the PIVA. Furthermore, the 3-dB coupler, which is realized by the edge-coupled synthetic transmission lines, so-called the complementary-conducting strip coupled lines (CCS CLs), is applied to PIVA design for the circuit miniaturization. The presented attenuator design is analyzed and fabricated by using standard 0.18 mum 1P6M CMOS technology. The prototype, which has only a chip size of 242 mum times 246 mum excluding the contacting pads, are experimentally characterized through the on-wafer measurements. The measured phase reversal is 181.4 degree, nearly approaching to the ideal state. From 20 to 28 GHz, the measured attenuation dynamic range is more than 20 dB with a phase shifting less than 8.8 degree. The input and output return losses are lower than -10.2 and -11 dB, respectively.