Jean de Graaf

CPM-based radar waveforms for efficiently bandlimiting a transmitted spectrum

In this paper we shall demonstrate how a polyphase-coded radar waveform can be implemented using a continuous phase modulation (CPM) framework so as to achieve spectral containment while maintaining a constant envelope to maximize energy-on-target. Current modulation techniques such as derivative phase shift keying (DPSK) and minimum shift keying (MSK), which are applicable to binary-coded waveforms, are well-known implementation schemes for spectral containment. The CPM implementation is applicable to polyphase codes and can also achieve better spectral containment, though a by-product is increased range sidelobes that result due to the deviation from the idealized code (implicitly defined for squared-shaped chips). To ameliorate the increased range sidelobes, a version of Least-Squares mismatched filtering is employed that accommodates the continuous nature of the CPM structure. Also, continuous rise/fall-time transitions of the pulse are addressed as part of the holistic implementation of the CPM-based waveform. It is observed that for the CPM implementation the rise/fall-time becomes the limiting factor on spectral containment and a rather simple scheme based on Chireaux out-phasing is suggested as a means to “slow down” the pulse rise/fall.

Polyphase-coded FM (PCFM) radar waveforms, part I: implementation

Polyphase radar codes promise enhanced performance and flexibility due to greater design freedom. While the search for better codes continues, the implementation issues of transmitter bandlimiting and nonlinear distortion have precluded their widespread use in high-power systems. This paper introduces a modified continuous phase modulation implementation that converts an arbitrary polyphase code into a nonlinear frequency-modulated waveform that is constant envelope and spectrally well contained. Experimental results assess the receive sampling and pulse compression effects.

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.

Robust transmit nulling in phased array antennas

The ability to create nulls in the transmit pattern of a phased array antenna has many applications for radar systems, including interference and clutter mitigation. Most nulling techniques introduce small perturbations in amplitude and phase, or phase-only, at each element of the phased array. For practical reasons, phase-only perturbations are desired and they create acceptable rms levels of null depth. However, the phase shift at each element will vary with the frequency of the transmitted signal. As a result, the depth and pointing accuracy of the transmit null will not be uniform over the bandwidth of the transmitted signal. This paper will describe some of the inherent limitations in achieving robust transmit nulling performance over a wide bandwidth when using phase-only, open-loop control on a typical phased array antenna. Some solutions to overcome these limitations and improve the performance of transmit nulls will be proposed.

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.

Array architecture for wideband transmit nulling

The ability to create nulls in the transmit pattern of a phased array antenna has many applications for communication and radar systems, including interference and clutter mitigation. Most nulling techniques introduce small perturbations in amplitude and phase, or phase-only, at each element of the phased array. For ease of implementation, phase-only perturbations are usually desired and provide acceptable null depths. However, the phase shift at each array element will vary with the frequency of the transmitted signal. As a result, the depth and pointing accuracy of the transmit null will not be uniform over the bandwidth of the transmitted signal. A more robust transmit nulling approach is to insert a tapped delay line (TDL) behind each array element instead of a phase shift. As shown in this paper, the null depths achieved over wide signal bandwidths are far superior to conventional phase-only approaches.