Andrew A. Moulthrop

Time-domain envelope measurement technique with application to wideband power amplifier modeling

This paper presents a new time-domain measurement technique for repetitive microwave signals that is applied to modeling wideband power amplifiers. The measurement technique concept consists of recording the microwave signal after conversion to baseband using a calibrated downconverter, which improves measurement accuracy compared to measurements at the carrier frequency. The modeling section describes how such time-domain measurements can be used to model wideband signal effects in nonlinear power amplifiers. The commonly used memory-less envelope model is limited to use on narrowband signals. A new model is developed which includes a filter before the memory-less nonlinearity to capture the memory effects associated with wideband signals. It is demonstrated that the accuracy of wideband signal simulations can be improved by optimizing the model parameters based on time-domain measurements of wideband signals.

Power-amplifier characterization using a two-tone measurement technique

An accurate nonlinear model is necessary to optimize the tradeoff between efficiency and linearity in power amplifiers. Gain compression (AM/AM) and amplitude-phase (AM/PM) distortion are the two primary model inputs used to characterize the nonlinearity. The amplifiers AM/AM and AM/PM characteristics are typically measured statically using a vector network analyzer. Since the input is typically a modulated signal, it is desirable to characterize the amplifier dynamically. This paper describes and demonstrates a dynamic AM/AM and AM/PM measurement and modeling technique involving a spectrum analyzer and two-tone input signals. A complete analysis of the measurement technique is presented, along with the data processing needed for the identification of a new three-box model. The test configuration and procedure are presented with special precautions to minimize measurement error. Results for a solid-state amplifier are used to accurately predict intermodulation distortion, while those for a traveling-wave tube amplifier show good agreement with that obtained dynamically using a 16 quadrature-amplitude-modulation signal.

Validation of power amplifier nonlinear block models

This paper presents a method of assessing the fidelity of nonlinear amplifier models for communications applications. The simulation predictions for BPSK and 16-QAM input signals are compared to measurements for a 20 GHz solid-state amplifier. It is shown that including a filter before a memoryless nonlinearity provides significantly improved model fidelity compared to the memoryless nonlinearity alone.

Introduction to Polyspectral Modeling and Compensation Techniques for Wideband Communications Systems

The requirements/environments for broadband commercial and military communication systems have focused attention on the issues of efficiency and nonlinearity characterization of power amplifiers, and the mitigation of the distortion they produce. Common system modeling approaches, which are dominated by probing signals of at most a multi-tone nature, prove to be inadequate for representing channels with operational bandwidths reaching the multi-GHz range. This shortfall in modeling fidelity is especially the case for higher-order, non-constant envelope modulations that are often needed to meet bandwidth efficiency demands. As a natural consequence, distortion compensation designs based on these models will likewise prove to be less than optimal in their effectiveness. This paper introduces an established nonlinear system identification technique from the mechanical systems field that essentially solves the wideband modeling problem, and in addition provides very unique assessment and design tools for distortion compensation. The technique is a special case of a formal operator series representation for the given nonlinearity with memory, and its construction is based on an extremely accurate baseband time-domain measurement technique¿using pseudo-randomly modulated signals ¿ that will also be described. The basic modeling and compensation features of the method will then be provided, followed by two representative high-power amplifier (HPA) applications to illustrate and validate the modeling method. The fidelity evaluation will be performed in the time-domain using a normalized mean-square error (NMSE) waveform metric, and compared with results found for standard block model approaches.

Transmission response measurements of frequency-translating devices using a vector network analyzer

A new method for accurately determining the transmission response of frequency-translating devices (FTDs) is presented. The absolute amplitude and phase of the FTD under test is obtained using a vector network analyzer (VNA) and two test FTDs, where one FTD must have reciprocal frequency response characteristics. The characterization of single-sideband (SSB) FTDs is obtained in a straightforward manner by combining data from three VNA two-port swept measurements. The characterization of double-sideband (DSB) FTDs can be performed in the same manner as for SSB FTDs, or, more accurately, by combining data from six two-port swept baseband measurements. A complete analysis of the characterization method using low-pass equivalent (LPE) signals and systems is presented, along with the development of the appropriate data reduction procedures needed to arrive at the de-embedded LPE FTD transmission responses. The validation and accuracy of the method is demonstrated with results for both SSB and DSB FTDs operating at 20 GHz.

Optimal-filter approach for nonlinear power amplifier modeling and equalization

This paper presents an optimal-filter method for application to the equalization and modeling of nonlinear power amplifiers. The method uses auto-spectral and cross-spectral densities of measured amplifier input/output time-domain waveforms. An optimal filter response is calculated which best approximates a given nonlinearity as quantified by a new and useful linear coherence function metric. This response can in turn be used to determine optimal equalizer responses. It is also shown that by adding an optimal filter to the output of the standard two-box amplifier nonlinear model, waveform accuracy can be improved by 3.5 dB. The approach is applied to a 20-GHz traveling-wave tube amplifier using 9.6 Gbps 16-APK input signals.

A dynamic AM/AM and AM/PM measurement technique

The AM/AM and AM/PM characteristics of a power amplifier are typically measured in a static manner using a CW signal. For communications signals, the amplitude envelope is varying so the distortion is actually occurring dynamically. This paper presents a technique to measure dynamic AM/AM and AM/PM. Results are presented for two examples of power amplification: a traveling-wave tube amplifier and a solid-state amplifier.

Application of polyspectral techniques to nonlinear modeling and compensation

This paper introduces polyspectral techniques to the microwave community by describing their application to nonlinear component and system modeling, as well as compensation design. Basic features of the method are provided, and a specific application is given to the blackbox modeling of a commercial 20-GHz traveling-wave tube amplifier using 9.6 Gbps, 16-APK input signals.

A new time-domain measurement technique for microwave devices

A new time-domain measurement technique for repetitive microwave signals is described. The microwave signal is converted to baseband before measurement, improving the accuracy compared to measurements at the carrier frequency. Applications include nonlinear device and modulator measurements, for the purpose of model generation and verification.

A nonlinear ARMA model for simulating power amplifiers

This paper presents an improved model for wideband simulation of nonlinear power amplifiers. The commonly used memoryless envelope model is limited to use on narrowband signals. The new model includes an auto-regressive moving average (ARMA) filter to improve performance predictions for wideband signals. Optimization of the model is performed using measurements of time-domain pulse envelopes. The new model is constructed for a 20 GHz helix traveling-wave tube amplifier (TWTA) and compared to the memoryless envelope model for predicting distortion with wideband signals.