In this contribution, we describe a reconfigurable surface, designed in 130um SiGe technology, to realize a fully-electronic, planar chopper aiming at passive terahertz imaging applications. This surface consists of subwavelength metallic patches that are interconnected using FET-based varactors. By switching the bias voltage of the varactors between 0V and 1.2V, the reconfigurable surface can switch between a transmissive and opaque state, respectively. The expected transmissivity is simulated to be between 0.2 and 0.9 over a wide frequency band, from 250 GHz to 550 GHz. This chopping solution would enable a high degree of system integration for future passive terahertz cameras.

This article presents the development of a focal plane array (FPA) for terahertz imaging applications with a near diffraction-limited resolution achieved through a very tight sampling of the focal plane. The antenna array is integrated with direct detectors in a 22-nm CMOS technology and operates from 200 to 600 GHz. The tight sampling of the focal plane is realized by using a combination of leaky-wave radiation and a dual-polarized connected array configuration that closely resembles a chessboard. By utilizing both the polarizations in the chessboard design, the number of array elements per unit area is effectively doubled. The geometry of the chessboard array was co-optimized together with that of a silicon elliptical lens to achieve both high aperture efficiency and beam overlap. Measurements in the WR2.2 band of a fabricated demonstrator showed that an aperture efficiency of −4.1 dB was realized at 400 GHz. The average gain roll-off between two diagonally adjacent array elements was measured to be −1.5 dB at 400 GHz. Compared to the reference configuration of an idealized, equivalently sampled hexagonal FPA, the improvement in gain at the edge of coverage yields 1.2 dB, which includes 1.9 dB of ohmic losses in the chessboard array. The agreement between measurements and simulations proved to be within 1 dB from 325 to 475 GHz.

The continuous advancements in the coverage and sensitivity of terahertz direct-detector imagers places increasing constraints on the test benches needed for characterization. Already, relative differences on the order of a few decibels are observed between simulated and measured performance in the state-of-the-art literature. This contribution elaborates on our experimental strategies to maximize the characterization accuracy and precision of terahertz direct-detection imagers using a broadband, over-the-air measurement procedure. As a demonstration, four elements within a dense detector array were characterized in the WR2.2 band. By optimizing the modulation frequency in the setup for minimal impact of low-frequency noise and interference, a measurement dynamic range up to 30 dB was achieved, including the path loss over 20 cm. The hardware-to-model agreement of the characterized array is below 1 dB between 325 and 450 GHz.

The design of a focal plane array (FPA) for imaging at sub-mm wavelengths generally is a trade-off between resolution and sensitivity. For maximum angular resolution, minimal spacing between FPA elements is desired, which leads to increased losses due to spillover and mutual coupling and therefore deteriorates the imaging sensitivity. This work presents the characterization of an ultra-wideband (200 GHz 600 GHz) FPA with integrated direct-detectors, achieving a tight sampling of the focal plane by implementing overlapping of the feed elements, hence alleviating the penalty in aperture efficiency. The overlapping of the feed elements in implemented using a combination of a dual-polarized connected array configuration resembling a chessboard, and leaky-wave propagation in the CMOS stratification. The measured radiation patterns and aperture efficiency show

A current-mode direct-digital RF modulator (DDRM)-based transmitter (TX) architecture is proposed in this article for energy-efficient wireless applications. To demonstrate its key principles, a 2×13 bit demonstrator is implemented in a 40-nm CMOS technology. This DDRM can operate standalone or as a driver for a common-gate (CG)/common-base (CB) power amplifier (PA). The proposed DDRM is based on current-steering radio frequency digital-to-analog converters (RFDACs) that feature an extra current division path to allow the generation of the optimum current-mode class-B drive profile for the final CG/CB PA, facilitating energy-efficient TX operation without compromising linearity. For this purpose, the DDRM uses signed-IQ mapping combined with a class-B harmonic rejection (HR) technique. In addition, an advanced dynamic biasing technique is introduced to further enhance the TX line-up efficiency in deep power back-off (PBO) region. The DDRM driver standalone can provide 19.6-dBm RF peak output power. It supports a '160-MHz 256-QAM' signal at 2.4 GHz with an adjacent channel leakage ratio (ACLR) of -40.3 dBc and an error vector magnitude (EVM) of -33 dB, without using any digital pre-distortion (DPD). When connected to a CB SiGe PA, the overall TX line-up achieves an output power of 27 dBm and an overall TX system efficiency of 20%. This DPD-free TX line-up achieves an ACLR of -37.7 dBc and an EVM of -30 dB, respectively, when operating with an '80-MHz 64-QAM' signal at 2.2 GHz.

In this contribution we will present the diffraction-limited imaging capabilities of a focal plane array (FPA) of antenna-coupled direct-detectors at submillimeter wavelengths. The FPA prototype is a tightly sampled, 12-pixel array that was developed in a 22 nm CMOS technology and it covers a band from 200 GHz to 600 GHz. A quasi-optical (QO) setup was developed to actively illuminate this FPA in order to perform imaging with > 40 dB SNR. The resulting images will be the first that have diffraction-limited angular resolution at these wavelengths, which demonstrates that this FPA design can be very attractive for future passive THz imaging applications.