Matthew Moldovan

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.

LINC power amplifiers for reducing out-of-band spectral re-growth: A comparative study

The Linear Amplification with Nonlinear Components (LINC) power amplifier structure combiner implementation has an effect on the linearity and efficiency of the design. Understanding and using this tradeoff is helpful to radar and communication designers and can be used to assist in operating with optimum efficiency while meeting spectral mask requirements instituted by regulatory agencies. In this paper, a simulation-based investigation is performed into implementations with a 180-degree coupler and with a Chireix outphasing network. In general, the overall efficiency in both designs is a function of the amplitude modulation of the message signal in the LINC configuration; however, the 180-degree coupler has a potential to obtain higher linearity in many cases due to the isolation of its two input ports. In certain cases of amplitude modulation, however, much of the output power in the 180-degree coupler design is delivered to an unused termination rather than to the network output.

A test platform for real-time waveform and impedance optimization in microwave radar systems

A test platform has been constructed at Baylor University to develop methods of simultaneous waveform and circuit optimization for cognitive radar. The ultimate goal of this work is to allow on-chip, simultaneous optimization of the waveform and the load impedance of the transmitter power amplifier from an FPGA cognitive-radio platform. The test bed includes a vector signal generator, load-pull tuners, a DC power supply, a power meter, and a spectrum analyzer, all controllable by MATLAB. The power meter and DC power supply are used to measure the power efficiency of the device under test, while the spectrum analyzer can be used to assess the spectral spreading, and hence linearity, of the device, through measurement of adjacent-channel power ratio or other means. Computationally intelligent routines for both load-impedance and waveform optimization will be created and evaluated using the test bed.

Chirp optimization using piecewise linear approach

A chirp optimization for in-band flatness under specific spectral constraints has been simulated in MATLAB using a search through piecewise linear characterizations of the chirps time-frequency profile. In general, the piecewise linear chirp consists of both up- and down-chirps. An exhaustive search seeks the chirp best satisfying in-band energy and flatness criteria.

Use of a step-response approximation for thermal transient modeling in power MOSFETs

In a previous ARFTG paper, we presented the measurement of the thermally-induced transient drain voltage response of a power Si MOSFET to a step excitation in gate voltage. The data was fit by assuming the drain voltage takes an exponential shape. This relies on the assumption that the drain current is a step function, which is actually only approximate due to the dependence of the drain current on temperature. In this work, we show that the exponential approximation actually possesses the same asymptotic behavior of a more accurate, model-based solution we have obtained that incorporates thermal feedback. We show that the more theoretically accurate solution and exponential approximation of the solution both provide excellent fits to measured data for the Si MOSFET.

Spectrum Analysis Considerations for Radar Chirp Waveform Spectral Compliance Measurements

The measurement of a radar chirp waveform is critical to assessing its spectral compliance. The Fourier transform for a linear frequency-modulated chirp is a sequence of frequency-domain impulse functions. Because a spectrum analyzer measures the waveform with a finite-bandwidth intermediate-frequency (IF) filter, the bandwidth of this filter is critical to the power level and shape of the reported spectrum. Measurement results are presented that show the effects of resolution bandwidth and frequency sampling interval on the measured spectrum and its reported shape. The objective of the measurement is to align the shape of the measured spectrum with the true shape of the signal spectrum. This paper demonstrates an approach for choosing resolution bandwidth and frequency sampling interval settings using the example of a linear frequency-modulation (FM) chirp waveform.

Understanding transistor channel temperature in nonlinear microwave measurements and device operation

With the widespread use of high-voltage GaN devices and other high-power transistors, understanding the heating of a device during large-signal excitation and measurement is critical, both to ensure efficient operation and to prevent destruction of devices during measurement. Device self-heating during RF excitation is directly dependent upon the power-added efficiency of operation. All DC and RF power that is not converted to output RF power is dissipated in the device as heat. When properly applied, the traditional electrothermal model used in circuit simulators adequately calculates this self heating and yields some interesting results. In this paper, it is demonstrated that the load impedance providing maximum power-added efficiency also results in the lowest average dissipated power, and hence the lowest channel temperature, of the device. The variation in average dissipated power (and hence channel temperature) is also examined for different loading conditions; it is shown that some loading conditions produce a dissipated power that is actually higher than the DC power, a situation which often can lead to device failure during load-pull measurements.