D. Barataud

The use of artificial neural networks in nonlinear microwave devices and circuits modeling: An application to telecommunication system design (invited article)

The design of telecommunication systems is a hierarchical process involving use of a large set of simulation tools and relying on appropriate modeling of system elements to obtain get-it-right-the-first-time fabrication. This paper investigates in detail possible application of neural networks to modeling of large-signal hard-nonlinear behavior of power transistors for circuit design purpose, and modeling of nonlinear circuits such as power amplifiers for system design purpose. The problem of finding a good model is discussed through solutions offered by neural networks, with particular interest in wavelet networks trained by BFGS algorithms. Finally, experimental results are reported.xa0©1999 John Wiley & Sons, Inc. Int J RF and Microwave CAE 9: 198–215, 1999.

Accurate phase measurements of broadband Multitone signals using a specific configuration of a Large Signal Network Analyzer

Accurate measurements of time domain waveforms at microwave frequencies are performed using the harmonic sampling principle. This principle used in the large signal network analyzer (LSNA) enables to transform a measured RF spectrum into an IF spectrum which is then digitized by ADCs. Unfortunately in this principle IF spectrum bandwidth is limited to 10MHz due to the need of high dynamic range ADCs (14bits). In this paper, we propose a new method that enables to accurately measure the phase relationships and the amplitudes of the spectral components of broadband microwave signals. Measurements of a 62MHz bandwidth multitone signal at L band have been performed and are shown. The extension of the proposed method to wider bandwidth is absolutely possible. Furthermore the application of the proposed technique to other harmonic samplers than those used in the LSNA is also possible

A Smart Load-Pull Method to Safely Reach Optimal Matching Impedances of Power Transistors

This paper presents a new method to find optimal load impedances of power transistors with a VNA based Load-Pull measurement setup. Most of load pull setups find the optimal load impedance of a device under test (DUT) for a given available input power. If the optimal impedance must satisfy a trade off between several parameters, such as gain compression or power added efficiency, the measurement procedure may become very time consuming. Our method automatically generates a behavioral model of the DUT. Crossing-informations from this model and measurements lead us to the good impedance optimum with a limited number of iterations.

A novel behavorial model of power amplifier based on a dynamic envelope gain approach for the system level simulation and design

This paper presents a novel and non quasi static behavioral ( black box) model of power amplifiers. It takes into account nonlinear memory effects for an accurate prediction of signal distortions and power budget in communication systems. The model topology and the model extraction procedure are described. The model proposed in this paper can be derived either from envelope simulation results of a PA circuit design or from time domain envelope measurements of a PA driven by modulated stimuli. Examples of new measurements (i.e. : dynamic AM/AM conversion characteristics) highlight nonlinear memory of SSPAs. The behavioral model proposed is very well suited for an efficient implementation in system level software packages because it consists of an extension of the complex envelope gain function. The proposed complex gain which depends on the input envelope and its first time derivatives provides a great enhancement as compared to the memoryless static complex gain derived from classical AM/AM and AM/PM conversion characteristics.

A new characterization technique of "Four hot S parameters" for the study of nonlinear parametric behaviors of microwave devices

This paper presents a new characterization system which enables calibrated Hot S parameter measurements of power transistors in a load pull environment. The device under test (DUT) is driven by a large signal at a frequency f/sub 0/ while a small signal at a frequency f is injected as a perturbation signal. A frequency sweep of the perturbation tone is performed (basically from 300MHz up to f/sub 0/ (ie lower sideband)). Upper sideband, from f/sub 0/ up to 2f/sub 0/, can be extended in a same manner. The four Hot S parameters measured at f are dependent on the nonlinear regime of the DUT forced by the large signal at f/sub 0/. The aim of this experimental purpose is to investigate nonlinear parametric behaviors like nonlinear stability. A description of the proposed measurement set-up is done. Calibration and measurement procedures are described and significant S band measurement results of HBTs are reported and discussed.

A fully calibrated four channels time domain RF envelope measurement system for the envelope characterization of nonlinear devices in a load-pull environment

This paper presents a fully calibrated four channels time domain radio-frequency (RF) envelope characterization system. This measurement system enables the acquisition and the vector-correction of the 4 input and output envelope voltage/current waves, including both amplitude and phase, of RF nonlinear microwave devices. The capabilities of the system, when it is associated with passive load pull techniques, are presented to investigate the effects of RF impedance termination on the envelope linearity of mismatched RF nonlinear devices. The proposed measurement set-up is described. Calibration and measurement procedures are detailed. Examples of the 4 fully error corrected time domain envelopes at the input/output RF ports of an optimised gain power MESFET are reported and discussed.

High Efficiency and Linear Power Amplification for OFDM Signal by Combining Dynamic Bias and Digital Baseband Predistortion

This paper presents a technique that enables both efficiency and linearity enhancements for power amplifiers (PA) used in communication systems. It is here applied to a 100 W wide-band LDMOS push-pull amplifier (50-500 MHz) driven by OFDM signals. When the amplifier is driven by a CW signal and operates at a constant 28 V drain bias voltage, it exhibits 100 W output power with 60% power added efficiency (PAE). When it is driven by an OFDM signal, a 10 dB output power back-off is necessary to have a -25 dBc ACPR and PAE decreases down to 10%. By properly implementing an envelope tracking bias system 40 W output power along with 38% PAE and -27 dBc ACPR have been reached. Applying baseband digital predistortion provides additive improvements at high PAE (for only 1 point PAE lost the ACPR increases Pof 5 dB).

The locus of points of constant output VSWR around the load optimal impedance: Evaluation of power transistors robustness

This paper determines the locus of points of constant output voltage standing wave ratio (VSWR) around the load optimal impedance at the fundamental frequency. This resolution is very helpful to determine the power transistors robustness. To illustrate this, some measurements results are presented on a Si/SiGe HBT of STMicroelectronics technology. The measurements are realized on two multi-harmonic load-pull test-benches: one passive and one active.

Comparison of calculation techniques ofconstant VSWR impedance circle: evaluation of power transistors robustness

This paper determines the locus of points of constant output voltage standing wave ratio (VSWR) around the load optimal impedance at the fundamental frequency. The proposed resolution has no hypothesis and there is a solution for whatever impedance. This resolution is very helpful to determine the power transistors robustness. Some measurements results are presented on a Si/SiGe HBT of STMicroelectronics technology. The measurements are realized on two multi-harmonic load-pull test-benches: one passive and one active

Overview on System Level Simulation Environment for Characterization, Modeling and Simulation of RF and Microwave devices

This paper presents an overview of research activities on system level design characterization and analysis for RF and microwave devices. The strategy consists on the use of a development platform for both simulation and instrumentation. The kernel of this platform was developed under Scilab/Scicos environment. This open-source development tool drives both instrumentation and simulation algorithms. The measurement setup, interfaced within Scilab and dedicated to the system level analysis will be described. Some measurement based models, extracted from this characterization setup, and recent advanced in system level simulation with Scicos will be also presented. The use of Scilab/Scicos for both characterization and simulation enables an hardware in the loop in system level simulations in order to allow an high flexibility for device characterization and modeling processes