An MSCL-Based Attenuator With Ultralow Insertion Loss and Intrinsic ESD-Protection for Millimeter-Wave and Terahertz Applications

This article presents a new magnetically switchable coupled-lines (MSCLs) attenuator topology. The attenuator is passive and features compact size, ultralow insertion loss (IL), high operation frequencies, and intrinsic electrostatic discharge (ESD)-protection. Thus, it is suitable for millimeter-wave (mmW) and terahertz integrated systems. In the conventional topologies, the ON-resistance $R_{\mathrm{\scriptscriptstyle ON}}$ of switch transistor exists in the signal path at reference state, which increases the IL of attenuators. In contrast, the proposed topology avoids $R_{\mathrm{\scriptscriptstyle ON}}$ at the reference state by utilizing $R_{\mathrm{\scriptscriptstyle ON}}$ as part of the attenuation network at the attenuation state instead; thus, it presents negligible attribution to IL of the attenuator. In addition, the MSCL is miniaturized for easy integration and reduced ohmic/radiation/substrate losses. The attenuation range is customized by sizing its switch transistors, while the total attenuation range is extendable by cascading multiple attenuator cells. To validate the design theory, a $G$ -band prototype is designed using 0.13- $\mu \text{m}$ CMOS technology. The fabricated attenuator operates from 190 to 220 GHz. It features low IL of 1.4–2.0 dB and rms amplitude error of 0.12–0.34 dB across the operation frequencies. In addition, the attenuator achieves 3-kV ESD-protection at the radio frequency (RF) ports without using ESD-protection circuits due to the structural isolation between transistor terminals and RF ports. The circuit occupies a core area of 0.026 mm2 and consumes no dc power.

I. Introduction

The amplitude tuning circuits are indispensably adopted in analog and radio frequency (RF) integrated circuits and systems, especially in the phased arrays for their significant role in channel modulation and calibration for beam steering and sidelobe suppression [1], [2]. Typical specifications for an amplitude tuning circuit are amplitude resolution, insertion losses (ILs), amplitude tuning range, operation frequency/bandwidth, amplitude/phase errors, power consumption, and compactness. In addition, the linearity of attenuators is demanded for systems with high dynamic range requirements. In the aspects of circuit realization, it is often divided into active-type and passive-type categories, which are selected based on system architectures and specifications but are limited by the transistor characteristics of technology platforms.