Charles Baylis

Modeling GaN: Powerful but Challenging

As GaN technology has developed, first in research laboratories and more recently in multiple commercial device manufacturers, the demand for improved nonlinear models has grown alongside the device process improvements. The need for improved models for GaN is twofold: first, GaN devices have unique nuances in behavior to be addressed; second, there is a desire for improved accuracy to take full advantage of the performance wins to be gained by GaN HEMT performance in the areas of high efficiency and high-power operation.

Direct measurement of thermal circuit parameters using pulsed IV and the normalized difference unit

New methods are presented for direct extraction of thermal resistance and capacitance from pulsed IV measurements. Because thermal effects are frequency dependent, a thermal circuit is used in many models to characterize the device channel temperature as a function of frequency. Thermal resistance is determined using pulsed IV data sets taken at varied ambient temperature, while the thermal capacitance is found through use of a new normalized difference unit (NDU) for IV data. Determination of thermal subcircuit parameters from readily available pulsed IV measurements reduces the complexity of electrothermal model development for microwave transistors.

Radar chirp waveform selection and circuit optimization using ACPR load-pull measurements

Due to tightening spectral criteria, joint optimization of radar transmitter waveform and circuit is considered to minimize the nonlinearity and waveform induced spectral spreading of radar transmitters. This work demonstrates the ACPR load-pull measurement of a power amplifier under excitation from two chirp waveforms. The results are compared to allow selection of the optimum chirp and transmitter load impedance. This work represents a beginning effort toward the real-time, computationally intelligent transmitter optimization in future reconfigurable radar systems.

Designing for spectral conformity: Issues in power amplifier design

Spectral constraints placed upon radar systems by regulatory agencies require the design of highly linear amplifiers. Spectral spreading in power amplifiers is a result of transistors operated in the nonlinear regime to optimize efficiency. Several different methods are employed by power amplifier designers to maximize both linearity and efficiency. The methods of predistortion, feedforward, envelope tracking, Doherty, and “Linear Amplification Using Nonlinear Components” (LINC) are discussed in this paper. Tradeoffs and challenges inherent in these design approaches are surveyed.

A Peak-Search Algorithm for Load–Pull Optimization of Power-Added Efficiency and Adjacent-Channel Power Ratio

Power amplifiers in modern wireless systems must meet increasingly stringent spectral constraints while still operating with high power efficiency. A new peak-search algorithm is demonstrated to find the Pareto optimum of power-added efficiency (PAE) while producing an adjacent channel power ratio (ACPR) value under a specified maximum. A peak-search is first performed for the maximum PAE, and then a small-step steepest descent walk in the ACPR is taken from the PAE maximum toward the ACPR minimum until ACPR requirements are met. This approach reasonably approximates the Pareto optimum for the device. Simulation and measurement data are presented for searches from multiple starting points, and good correspondence is obtained for the Pareto optimum reflection coefficient, as well as the optimum PAE and ACPR values. This measurement-based algorithm is expected to find useful application in real-time adaptive radio and radar transmitters, as well as in bench-top measurements for amplifier design.

Calculation of the radar ambiguity function from time-domain measurement data for real-time, amplifier-in-the-loop waveform optimization

The ambiguity function is a measure of a radars range and Doppler detection capability. Cognitive radar systems require the capability to adjust the waveform in realtime to obtain desired range/Doppler detection capability while meeting stringent spectral requirements. While the ambiguity function of the waveform input to the transmitter can be simulated, it is the ambiguity function of the transmitter power amplifiers output waveform that will be used for the detection. As such, it is very helpful to be able to measure the ambiguity function output from a power amplifier in the optimization process. This paper describes a technique that can be used to quickly calculate the ambiguity function for the output waveform from the radar amplifier as measured on an oscilloscope. Brief examination is also given to the effect of amplifier nonlinearity on the ambiguity function.

Efficient Optimization Using Experimental Queries: A Peak-Search Algorithm for Efficient Load-Pull Measurements

In the process of hardware optimization, physical queries requiring laboratory experiments are often necessary. This is similar to optimization using software where queries are made to a computer model. In both the laboratory optimization and optimization using computer models, queries come at a cost: laboratory time or computer time. Finding efficient searches using a small number of queries on average is therefore motivated. In this paper, techniques used in computer search are shown to be transparently applicable to certain instances of hardware optimization. The hardware example presented is a load-pull peaksearch algorithm for power amplifier load-impedance optimization. The successful search shown in this paper allows high-resolution measurement of the maximum power with a significant reduction in the number of measured reflection-coefficient states. The use of computationally intelligent procedures for reducing time costs in design optimization using hardware has significant potential applications in a number of iterative experimental procedures performed in the laboratory.

The Smith tube: Selection of radar chirp waveform bandwidth and power amplifier load impedance using multiple-bandwidth load-pull measurements

The operation of a radar system requires a trade-off between detection capabilities, power efficiency, and adjacent channel power minimization. Specifically, wide signal bandwidth is important for range detection. This paper presents how load-pull data taken for multiple linear frequency-modulated chirp waveforms with different bandwidths can be used to select the chirp waveform with the largest bandwidth possible, while meeting adjacent-channel power ratio and power-added efficiency requirements. This approach utilizes plots of adjacent-channel power ratio and power-added efficiency surfaces within a Smith tube, a three-dimensional, cylindrical extrapolation of the traditional Smith chart. A measurement example is given to illustrate the design approach. This approach will be useful in the design of range radars, and also is likely to find use in enabling real-time reconfiguration of future radars for varying spectral environments and efficiency requirements.

Fast, simultaneous optimization of power amplifier input power and load impedance for power-added efficiency and adjacent-channel power ratio using the power smith tube

Reconfigurable adaptive amplifiers are expected to be a critical component of future adaptive and cognitive radar transmitters. This paper details an algorithm to simultaneously optimize input power and load reflection coefficient of a power amplifier device to obtain the largest power-added efficiency (PAE) possible under a predefined constraint on adjacent-channel power ratio (ACPR). The vector-based search relies on estimation of the PAE and ACPR gradients in the three-dimensional power Smith tube. Accurate convergence to the optimum impedance in the Smith chart is demonstrated in simulation and measurement search experiments requiring between 20 and 60 experimental queries. This paper presents a fast search to jointly optimize the input power level and load impedance. This method is feasible for future implementation in real-time reconfigurable power amplifiers.

Understanding pulsed IV measurement waveforms

Pulsed IV analysis allows the development of more accurate nonlinear models for RF and microwave device operation. An analysis of pulsed IV waveform allows better exploitation of measurement capabilities to produce accurate results. Static and dynamic IV measurement waveforms produced by a commercially available pulsed IV analyzer are examined. Because transistors can become unstable during any type of IV measurement, the use of bias tees allows a frequency-dependent impedance to be presented. However, it is shown that care must be used when using bias tees in pulsed IV measurement to choose a bias tee with an inductor time constant significantly higher than the pulsing frequency but lower than the frequency at which oscillations develop.