Matthew Fellows

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.

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.

Optimization of power-amplifier load impedance and waveform bandwidth for real-time reconfigurable radar

To create a reconfigurable radar transmitter, the power amplifier circuitry and waveform must be able to adjust in real time to changing operating frequencies and spectral requirements. For range radars, better range resolution can be accomplished by using waveforms of higher bandwidth. In this case it is desirable to maximize the chirp waveform bandwidth while maintaining spectral compliance and meeting power-added efficiency requirements. Based on the concept of the Smith Tube (presented in a recent conference article), this article describes a new intelligent, vector-based search for the load impedance and waveform bandwidth. The search provides the largest possible bandwidth while maintaining the power-added efficiency and adjacent-channel power ratio within specified requirements. The algorithm is demonstrated in measurement with multiple starting impedances and bandwidth search ranges, and consistently accurate and useful results are obtained. Multiple iterations of the two-part search can be performed for increased precision if the time needed to take additional measurements can be tolerated.

Real-time load impedance optimization for radar spectral mask compliance and power efficiency

This paper presents a fast optimization algorithm for power amplifiers in radar transmitters based upon a metric designed to assess spectral mask compliance. The search finds the load impedance maximizing the power-added efficiency (PAE) while providing compliance with the assigned spectral mask. Measurement results illustrate consistency in the chosen optimum values of efficiency, while spectral mask requirements are consistently met at the optimum load impedances chosen by the search. This algorithm will allow adaptive radar transmitter amplifiers to quickly adjust their load impedances to change frequency bands of operation, change spectral output properties based on nearby spectrum users, and meet dynamically varying spectral mask requirements.

The Power Smith Tube: Joint optimization of power amplifier input power and load impedance for power-added efficiency and adjacent-channel power ratio

A new visualization tool for design is introduced: the Power Smith Tube. It provides aid in designing for optimum power-added efficiency (PAE) while meeting adjacent-channel power ratio (ACPR) requirements. The Power Smith Tube is a three-dimensional cylindrical extension of the Smith Chart with input power as the vertical axis. As such, this Smith Tube allows the visualization of design metrics, such as PAE and ACPR, as a function of both input power and load impedance. Performance of load-pull measurements or simulations at multiple values of input power provides data for the design. Design examples using the Power Smith Tube based on both simulation and measurement data are presented to demonstrate selection of load impedance and input power providing the highest PAE under ACPR constraints.

Radar power amplifier circuit and waveform optimization for spectrally confined, reconfigurable radar systems

Increasingly stringent spectral spreading constraints are motivating a paradigm shift in the spectrum engineering of radar systems. Future spectrum requirements will likely dictate narrower spectral masks that will change based on geography. The desire to operate radar systems efficiently with waveforms that will provide desired detection capabilities must mesh with the need to meet spectral mask criteria. To this end, a collaborative research effort between Baylor University and the U.S. Naval Research Laboratory has resulted in the design of reconfigurable circuit and waveform approaches to optimize spectral confinement, power efficiency, and detection capabilities. This paper surveys the joint optimization approach and describes innovations by the authors in circuit and waveform reconfiguration that will be useful in future flexible radar transmitters.

Real-time amplifier optimization algorithm for adaptive radio using a tunable-varactor matching network

Fast load impedance tuning of a varactor diode matching network to maximize amplifier gain in real-time reconfigurable circuitry is demonstrated. A published tunable varactor-diode matching topology is designed for operation at 1.3 GHz to provide significant Smith Chart coverage. A steepest-ascent algorithm is applied for fast optimization, and measurement results indicate excellent convergence from multiple starting points within the Smith Chart. Algorithm data compares well with traditional loadpull results measured with both a commercially available tuner and the tunable-varactor network, and searching with the varactor tuner search is much faster than a traditional mechanical tuner.

The bias smith tube: Simultaneous optimization of bias voltage and load impedance in power amplifier design

Multiple factors must be considered in power-amplifier design for wireless communications and radar, including bias voltage, input power, and load impedance. The Bias Smith Tube is presented as a three-dimensional extension of the Smith Chart with bias voltage as the vertical axis. It allows simultaneous visualization of nonlinear output characteristic behaviors over transistor bias voltage and load reflection coefficient. Simulated and measured three-dimensional surfaces of constant power-added efficiency (PAE), adjacent channel power ratio (ACPR), and delivered power are shown in the Bias Smith Tube, and a design approach is illustrated that finds the combination of load impedance and bias voltage providing maximum PAE under ACPR and/or delivered power constraints.

Circuit optimization algorithms for real-time spectrum sharing between radar and communications

The ability of radar and communication applications to share the radio spectrum will require the use of innovative agile circuit techniques for radar and communications. Reconfigurable circuits can provide real-time adjustment of operating frequency and spectral output, while maintaining system performance and maximizing power efficiency. This paper discusses recent developments in circuit optimization techniques for power efficiency and spectral performance. Optimization of a single parameter (load reflection coefficient) for multiple criteria is first addressed, followed by multiple-parameter, multiple-criteria optimizations. The use of the recently innovated Smith Tube to optimize additional parameters, such as input power and bias voltage, simultaneously with the load impedance is discussed. Optimization examples and a forward look to fast, emerging multidimensional circuit optimization techniques are provided.