Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

Preface. About the Authors. Acknowledgments. 1 Power Amplifier Fundamentals. 1.1 Introduction. 1.2 Definition of Power Amplifier Parameters. 1.3 Distortion Parameters. 1.4 Power Match Condition. 1.5 Class of Operation. 1.6 Overview of Semiconductors for PAs. 1.7 Devices for PA. 1.8 Appendix: Demonstration of Useful Relationships. 1.9 References. 2 Power Amplifier Design. 2.1 Introduction. 2.2 Design Flow. 2.3 Simplified Approaches. 2.4 The Tuned Load Amplifier. 2.5 Sample Design of a Tuned Load PA. 2.6 References. 3 Nonlinear Analysis for Power Amplifiers. 3.1 Introduction. 3.2 Linear vs. Nonlinear Circuits. 3.3 Time Domain Integration. 3.4 Example. 3.5 Solution by Series Expansion. 3.6 The Volterra Series. 3.7 The Fourier Series. 3.8 The Harmonic Balance. 3.9 Envelope Analysis. 3.10 Spectral Balance. 3.11 Large Signal Stability Issue. 3.12 References. 4 Load Pull. 4.1 Introduction. 4.2 Passive Source/Load Pull Measurement Systems. 4.3 Active Source/Load Pull Measurement Systems. 4.4 Measurement Test-sets. 4.5 Advanced Load Pull Measurements. 4.6 Source/Load Pull Characterization. 4.7 Determination of Optimum Load Condition. 4.8 Appendix: Construction of Simplified Load Pull Contours through Linear Simulations. 4.9 References. 5 High Efficiency PA Design Theory. 5.1 Introduction. 5.2 Power Balance in a PA. 5.3 Ideal Approaches. 5.4 High Frequency Harmonic Tuning Approaches. 5.5 High Frequency Third Harmonic Tuned (Class F). 5.6 High Frequency Second Harmonic Tuned. 5.7 High Frequency Second and Third Harmonic Tuned. 5.8 Design by Harmonic Tuning. 5.9 Final Remarks. 5.10 References. 6 Switched Amplifiers. 6.1 Introduction. 6.2 The Ideal Class E Amplifier. 6.3 Class E Behavioural Analysis. 6.4 Low Frequency Class E Amplifier Design. 6.5 Class E Amplifier Design with 50# Duty-cycle. 6.6 Examples of High Frequency Class E Amplifiers. 6.7 Class E vs. Harmonic Tuned. 6.8 Class E Final Remarks. 6.9 Appendix: Demonstration of Useful Relationships. 6.10 References. 7 High Frequency Class F Power Amplifiers. 7.1 Introduction. 7.2 Class F Description Based on Voltage Wave-shaping. 7.3 High Frequency Class F Amplifiers. 7.4 Bias Level Selection. 7.5 Class F Output Matching Network Design. 7.6 Class F Design Examples. 7.7 References. 8 High Frequency Harmonic Tuned Power Amplifiers. 8.1 Introduction. 8.2 Theory of Harmonic Tuned PA Design. 8.3 Input Device Nonlinear Phenomena: Theoretical Analysis. 8.4 Input Device Nonlinear Phenomena: Experimental Results. 8.5 Output Device Nonlinear Phenomena. 8.6 Design of a Second HT Power Amplifier. 8.7 Design of a Second and Third HT Power Amplifier. 8.8 Example of 2nd HT GaN PA. 8.9 Final Remarks. 8.10 References. 9 High Linearity in Efficient Power Amplifiers. 9.1 Introduction. 9.2 Systems Classification. 9.3 Linearity Issue. 9.4 Bias Point Influence on IMD. 9.5 Harmonic Loading Effects on IMD. 9.6 Appendix: Volterra Analysis Example. 9.7 References. 10 Power Combining. 10.1 Introduction. 10.2 Device Scaling Properties. 10.3 Power Budget. 10.4 Power Combiner Classification. 10.5 The T-junction Power Divider. 10.6 Wilkinson Combiner. 10.7 The Quadrature (90 ) Hybrid. 10.8 The 180 Hybrid (Ring Coupler or Rat-race). 10.9 Bus-bar Combiner. 10.10 Other Planar Combiners. 10.11 Corporate Combiners. 10.12 Resonating Planar Combiners. 10.13 Graceful Degradation. 10.14 Matching Properties of Combined PAs. 10.15 Unbalance Issue in Hybrid Combiners. 10.16 Appendix: Basic Properties of Three-port Networks. 10.17 References. 11 The Doherty Power Amplifier. 11.1 Introduction. 11.2 Doherty's Idea. 11.3 The Classical Doherty Configuration. 11.4 The 'AB-C' Doherty Amplifier Analysis. 11.5 Power Splitter Sizing. 11.6 Evaluation of the Gain in a Doherty Amplifier. 11.7 Design Example. 11.8 Advanced Solutions. 11.9 References. Index.

/pdf/high-efficiency-rf-and-microwave-solid-state-power-4xdx1261hq.pdf

High Efficiency RF and Microwave Solid State Power Amplifiers

R EIEW A R TCLE Abstract As an emerging topic, photonic-assisted microwave measurements with distinct features such as wide frequency coverage, large instantaneous bandwidth, low frequencydependent loss, and immunity to electromagnetic interference, have been extensively studied recently. In this article, we provide a comprehensive overview of the latest advances in photonic microwave measurements, including microwave spectrum analysis, instantaneous frequency measurement, microwave channelization, Doppler frequency-shift measurement, angle-of-arrival detection, time–frequency analysis, compressive sensing, and phase-noise measurement. A photonic microwave radar, as a functional measurement system, is also reviewed. The performance of the photonic measurement solutions is evaluated and compared with the electronic solutions. Future prospects using photonic integrated circuits and software-defined architectures to further improve the measurement performance are also discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201600019

Photonics for microwave measurements

In this paper, a novel technique to design concurrent dual-band high-efficiency harmonic tuned (HT) power amplifiers (PAs) is presented. The proposed approach is based on a methodology developed to design multifrequency passive matching networks, which allows concurrent operability. The network design criterion is heavily investigated and later generalized both from the theoretical and practical point of view. The design, realization, and the complete characterization of a concurrent dual-band high-efficiency HT PA is finally described. A 1-mm gate periphery GaN HEMT device was used for the design and realization of the PA operating concurrently at 2.45 and 3.3 GHz. The measurement results have shown 53% and 46% drain efficiency at 33- and 32.5-dBm output power in the two targeted bands if operated in continuous wave single mode. In concurrent mode, 35% average efficiency was achieved with two simultaneously applied orthogonal frequency-division multiplexing signals.

A Design Technique for Concurrent Dual-Band Harmonic Tuned Power Amplifier

We discuss the design, realization and experimental characterization of a GaN-based hybrid Doherty power amplifier for wideband operation in the 3-3.6-GHz frequency range. The design adopts a novel, simple approach based on wideband compensator networks. Second-harmonic tuning is exploited for the main amplifier at the upper limit of the frequency band, thus improving gain equalization over the amplifier bandwidth. The realized amplifier is based on a packaged GaN HEMT and shows, at 6 dB of output power back-off, a drain efficiency higher than 38% in the 3-3.6-GHz band, gain around 10 dB, and maximum power between 43 and 44 dBm, with saturated efficiency between 55% and 66%. With respect to the state of the art, we obtain, at a higher frequency, a wideband amplifier with similar performances in terms of bandwidth, output power, and efficiency, through a simpler approach. Moreover, the measured constant maximum output power of 20 W suggests that the power utilization factor of the 10-W (Class A) GaN HEMT is excellent over the amplifier band.

3–3.6-GHz Wideband GaN Doherty Power Amplifier Exploiting Output Compensation Stages

High-efficiency power-amplifier design requires numerous efforts to investigate both input and output harmonic terminations effects. A simplified theoretical approach to clarify the relevance of such terminations is presented here, and design criteria to improve efficiency for high-frequency applications are briefly discussed. An advanced active load/source-pull test-bench has been used to validate theoretical harmonic tuning techniques, characterizing an active device. The adopted optimization strategy is presented, together with measured results obtained with a medium-power 1-mm MESFET at 1 GHz. Input second harmonic impedances effects are stressed, showing a drain efficiency spread between 37%-49% for a fixed input power level, corresponding to 1-dB compression. Finally, as predicted by the presented theory, after input second harmonic tuning, further improvements are obtained, increasing fundamental output load resistive part, demonstrating an additional drain efficiency enhancement, which reaches a level of 55% at 1-dB compression.

/pdf/an-approach-to-harmonic-load-and-source-pull-measurements-xjvbasxha8.pdf

An approach to harmonic load- and source-pull measurements for high-efficiency PA design

By convention, radio frequency (RF) and microwave frequencies range between 30 MHz and 300 GHz. Conversely, this means their wavelengths range between 10 m and 1 mm. Intense research in radar development during World War II extended the RF spectrum beyond the usual applications in radio communications. The use of shorter wavelengths resulted in laboratory equipment with proportionally smaller dimensions to generate, convey, transmit, and detect higher-frequency signals. Wavelengths shorter than 1 mm require equipment too small to be realized. Voltage, current, and impedance concepts lose their conventional meanings when the operating wavelength is approximately equal to the dimensions of the structures under test. The behavior of propagating electromagnetic waves must then be analyzed in terms of electric and magnetic field. Unfortunately, no simple and direct way exists to measure these quantities, so we must resort to indirect methods. In the following sections, we briefly review the concept and need for measurements of scattering parameters. We also describe the two key instruments for microwave measurements: VNAs and spectrum analyzers.

/pdf/microwave-measurements-part-ii-non-linear-measurements-5edem19zqt.pdf

Microwave Measurements Part II Non-linear Measurements

This comprehensive, hands-on review of the most up-to-date techniques in RF and microwave measurement combines microwave circuit theory and metrology, in-depth analysis of advanced modern instrumentation, methods and systems, and practical advice for professional RF and microwave engineers and researchers. Topics covered include microwave instrumentation, such as network analysers, real-time spectrum analysis, sampling oscilloscopes and microwave synthesizers; linear measurements, such as VNA calibrations, noise figure measurements, time domain reflectometry and dielectric measurements; and non-linear measurements, such as load- and source-pull techniques, broadband signal measurements, and non-linear NVAs. Each technique is discussed in detail, and accompanied by state-of-the-art solutions to the unique technical challenges associated with its deployment. With each chapter delivered by internationally recognised experts in the field, this is an invaluable resource for researchers and professionals involved with microwave measurements--

/pdf/modern-rf-and-microwave-measurement-techniques-54ushghcbm.pdf

Modern RF and microwave measurement techniques

In this paper, some of the latest advances in real-time load-pull technologies will be described. A recently introduced ultralow-loss directional coupler, which has been designed and realized by the authors, provides a number of advantages when used in load-pull test sets. This device has been called the load-pull head. The new ultralow-loss load-pull head can transform any passive precalibrated load-pull system into an easily calibrated and accurate real-time load-pull test set, without losing high-reflection-coefficient capabilities. Moreover, if used to realize an active loop, the load-pull head reduces the risks of oscillations and the amount of the loop amplifier output power. As an example application, measurements with a passive real-time load-pull setup of a 30-W laterally diffused MOS (LDMOS) transistor are presented. Furthermore, some advice to bypass the remaining unavoidable losses due to probes and cables is given. We will show, with measurements and with very simple calculations, that the combined use of load-pull heads, a passive tuner, and an active loop not only boosts the available GammaL but also decreases the loop amplifier output power, with a sensible reduction in the overall cost of the system.

/pdf/recent-advances-in-real-time-load-pull-systems-12g2mpf0jl.pdf

Recent Advances in Real-Time Load-Pull Systems

This paper investigates the residual calibration uncertainty effects in on-wafer load-pull measurements. After the systematic error correction (based on a traditional error-box model) has been applied, the residual uncertainty on absolute-power-level measurements can dramatically affect the accuracy of typical nonlinear parameters such as gain and power-added efficiency under different load conditions. The main residual uncertainty contributions are highlighted both by a theoretical analysis and experiments. Finally, one of the possible causes for intermodulation-distortion measurement errors is shown.

/pdf/accuracy-evaluation-of-on-wafer-load-pull-measurements-45xqx2heyx.pdf

Valeria Teppati

Papers

An approach to harmonic load- and source-pull measurements for high-efficiency PA design

Microwave Measurements Part II Non-linear Measurements

Modern RF and microwave measurement techniques

Recent Advances in Real-Time Load-Pull Systems

Accuracy Evaluation of On-Wafer Load-Pull Measurements