This invited paper reviews the progress of silicon-germanium (SiGe) bipolar-complementary metal-oxide-semiconductor (BiCMOS) technology-based integrated circuits (ICs) during the last two decades. Focus is set on various transceiver (TRX) realizations in the millimeter-wave range from 60 GHz and at terahertz (THz) frequencies above 300 GHz. This article discusses the development of SiGe technologies and ICs with the latter focusing on the commercially most important applications of radar and beyond 5G wireless communications. A variety of examples ranging from 77-GHz automotive radar to THz sensing as well as the beginnings of 60-GHz wireless communication up to THz chipsets for 100-Gb/s data transmission are recapitulated. This article closes with an outlook on emerging fields of research for future advancement of SiGe TRX performance.

This article demonstrates a W-band local-oscillator generation technique in 120-nm SiGe BiCMOS technology with high output power and high efficiency. The circuit employs a frequency quadrupler that is driven with differential quadrature inputs that are provided by either a quadrature voltage-controlled oscillator (QVCO) or a tunable active polyphase filter (PPF) circuit. The quadrupler employs a phase-controlled quadrature-push (PCQP) topology using stacked devices with a lower class-C common-emitter (CE) amplifier generating a current that is then modulated by an upper common-base (CB) amplifier driven out-of-phase with the lower devices. Such a structure generates a strong fourth-order harmonic. Four such stacks driven at their input using accurate differential quadrature signals increase the fourth-harmonic output power while suppressing other harmonics. The differential quadrature signals for the quadrupler are provided using either a PPF circuit or a capacitive injection-locking QVCO, which achieves wide tuning range and low phase noise. Both approaches are evaluated through the measurement of separate test circuits. The LO circuit using the QVCO provides 8-11.5-dBm output power over 75.2-83 GHz, power efficiency of 2.2-4.1%, including QVCO and buffer power, >20-dB harmonic rejection in the lower frequency range, and >14.4-dB harmonic rejection in the upper frequency range. The LO circuit using the active PPF provides 8.4-11.2-dBm output power over 75.6-82.8 GHz, power efficiency of 2.4-4.8%, including PPF and buffer power, and >23-dB harmonic rejection.

Flexible phased arrays potentially enable diverse applications not permitted by rigid systems; however, they introduce ambiguity in antenna element positions. If this position ambiguity can be overcome, flexible arrays can perform the full suite of array functions: beam steering, wavefront engineering, and beam focusing. Furthermore, shape reconstructions of arrays can be used for applications beyond beamforming. We propose a framework to reconstruct the shape of a flexible array that only uses mutual coupling measurements and does not require additional sensors or functionalities in the system. We discuss the approach, a two-step algorithm, which is highly modular and can be implemented in a variety of phased array systems. To demonstrate the accuracy of the approach, we present results from two passive 2.5-GHz phased array setups using dipole and patch antennas, as well as a 10-GHz (active) integrated circuit flexible phased array, and demonstrate the accuracy of the approach in this system. In all cases, the algorithm reconstructs the antenna shape accurately, with average position errors of approximately 6% of the wavelength. This article can serve as the beginning of the broad study of shape reconstruction algorithms and their applications.

This article presents a rigorous design method for a one-port-reflectionless filter with a Chebyshev response. In particular, we, for the first time, present inverter-coupled resonator filter topologies capable of having a Chebyshev response with zero reflection at all frequencies in theory. This work demonstrates a systematic approach for filter synthesis to find the normalized coupling (inverter) values without employing optimization methods or heuristic approaches. In addition, it provides closed-form design formulas in terms of a target frequency for the design of a one-port-reflectionless filter of coupled-line structure. We also present an exact approach to increase the reflectionless range of a distributed-element filter. Distributed-element bandpass filter structures formulated by this work are capable of producing a predefined canonical transmission response and a reflectionless feature over the entire frequency range at the same time, which has never been reported up to date. The presented topologies, synthesis results, and design methods have been applied to the design of second- and third-order filters for demonstration.

To reduce the demand for frequently adjusting phase shifts in the fifth-generation (5G) multiple-input multiple-output (MIMO) beamforming transmitters, in this article, a novel digital predistortion (DPD) technique is proposed, which employs multiple predistortion boxes to shape the pattern of nonlinear distortions in space in order to linearize multiple targets simultaneously. By using this approach, the linearization angle is widened and thus the communication quality can be kept when the user moves off the main beam direction. As a proof of concept, two DPD boxes, namely main and auxiliary boxes, are used to realize the 2-target linearization. A new model extraction method is also proposed with a dedicated over-the-air (OTA) feedback post-processing stage. Simulation and experiment are demonstrated on a 4-path beamforming transmitter. The results show that the proposed DPD method can effectively widen the linearization angle with reasonable additional complexity.

This article presents a low-loss correction technique for a self-healing load-insensitive power amplifier (PA) using a modified two-tap six-port network, wherein the varying (complex) load is first compensated for its unwanted susceptance part, followed by adjustment of the transistor output stage to the ohmic load variation, by modifying its supply voltage and drive level. This two-step approach avoids the high-Q conditions that occur in tunable matching network solutions, which aims to correct for both the real and imaginary load deviations, such as providing lower insertion loss and voltage stress. Next, to facilitate a fully automated load mismatch correction without the need for calibration, a modified two-tap six-port network for impedance detection and control loop approach is proposed. As proof of principle, a prototype 900-MHz class-AB PA featuring the proposed correction technique, as well as, the six-port reflectometer and the control loop, has been implemented as a PCB demonstrator. Measurement results show that the self-healing load-insensitive PA in the events of load mismatch significantly improves the performance and approaches the 50-Ω performance. On a 2:1 VSWR 360° mismatch trajectory and driven by a 64-QAM 3.86-MHz signal, the PA achieves a linear output power of 22 dBm with only ±0.1-dB variation and better than -45-dBc adjacent channel power ratio (ACPR).

This article presents a 16x 16 dual-polarized Ku-band (10.7-12.7 GHz) satellite communications (SATCOM) phased-array receiver with simultaneous dual-beam reception capability. The array incorporates 64 16-channel beamformer chips and 256 dual-polarized antennas. A dual-channel low noise amplifier (LNA) is employed on every antenna to lower the system noise figure (NF) and increase the antenna gain-to-noise temperature (G/T). The phased-array is built on a low-cost printed circuit board (PCB), with an antenna spacing of λ/2 at 12.2 GHz in an equilateral triangular grid to result in ±70° scan volume, and is capable of receiving two concurrent data-streams with a distinct direction of arrival (DOA) since it employs two 64:1 Wilkinson combiner networks. A transmit band (14-14.5 GHz) filter is also implemented between the LNA and the beamformer chip. The 256-element array has a directivity of 28 dB at mid-band with a G/T of 5 dB/K ( $T_{ ant}=20$ K), which results in a G/T of 11 dB/K for a 1024-element array. This array is a feasible solution for make-before-break connections and for multi-satellite reception systems, such as simultaneous television (TV) reception and connectivity.

This work presents new frequency-domain methods for the analysis and simulation of circuits based on a nonlinear resonator, with operation ranges delimited by turning points. Insightful analytical conditions fulfilled at these turning points are derived, which will enable an identification of the effect of each circuit element on their location in the solution curve. The cusp points, or co-dimension two bifurcations at which two turning points merge into one, thus delimiting the multivalued intervals, are directly calculated for the first time to our knowledge. In addition, a new numerical method, compatible with the use of commercial harmonic balance, is presented for the straightforward tracing of the multivalued solution curves, together with a new procedure to determine the locus of turning points in terms of any two analysis parameters. This relies on the use of a new mathematical condition to obtain a surface of turning points in the space defined by the two parameters and the input power. The methods have been applied to a wireless power-transfer system based on a recently proposed configuration, obtaining very good agreement with the experimental results.

Thermal modeling of different transmission lines (TLs) based on resonant scatterers is presented. The coplanar strip (CPS) and microstrip (MS) TLs are used to model resonators and to introduce the thermal dependence. A reflectometry approach is employed to validate the model by detecting the scatterers' resonance frequency and comparing it with analytical expressions. The observable shift in the resonance frequency of the scatterers with temperature variations is due to the thermal expansion of metals and temperature dependence of the substrate permittivity. Since all the measurements are done remotely with no direct line of sight, it is shown how such a reflectometry approach can be used for remote temperature sensing using a passive label composed of resonators. Unlike previous works in this domain where the thermal dependence is considered empirically, the introduced model is used to take into account all thermal effects affecting the resonant scatterers allowing to link rigorously the variations of the measured resonance frequency with the temperature without any lookup table. Temperature sensing using very simple TLs based on resonant scatterers was demonstrated in a real environment. A temperature error of less than 3 °C is obtained. Once the temperature has been determined, it is possible to go back to the TL parameters, such as the effective permittivity and the physical length.

There has been significant research devoted to the development of distributed microwave wireless systems in recent years. The progression from large, single-platform wireless systems to collections of smaller, coordinated systems on separate platforms enables significant benefits for radar, remote sensing, communications, and other applications. The ultimate level of coordination between platforms is at the wavelength level, where separate platforms operate as a coherent distributed system. Wireless coherent distributed systems operate in essence as distributed phased arrays, and the signal gains that can be achieved scale proportionally to the number of transmitters squared multiplied by the number of receivers, providing potentially dramatic increases in wireless system capabilities enabled by increasing the number of nodes in the array. Coordinating the operations of nodes in a distributed array requires accurate control of the relative electrical states of the nodes. The basic challenge is the synchronization and stability of the relative phases of the signals transmitted or received. Generally, such control requires wireless frequency synchronization, phase calibration, and time alignment. For radar operations, phase control also requires high-accuracy knowledge of the relative positions of the nodes in the array to support beamforming. Various technologies have been developed in recent years to address the coordination challenges for closed-loop applications, such as distributed communications, and more recently, there has been growing interest in new technologies for open-loop applications, such as radar and remote sensing. This article presents an overview of distributed phased arrays, the principal challenges involved in their coordination, and recent research progress addressing these challenges.