Juan-Mari Collantes

Stability verification of microwave circuits through Floquet multiplier analysis

This work presents a procedure to verify the stability of periodic regimes of microwave circuits that are simulated with harmonic-balance algorithms. The technique is based on computing the Floquet multipliers associated to the system linearization around the periodic regime. The proposed procedure has been applied to a C-band GaAs prescaler. Stability analysis results provide an estimation of the circuit input sensitivity curve that is in good agreement with measurement results.

Effects of DUT mismatch on the noise figure characterization: a comparative analysis of two Y-factor techniques

Device mismatch seriously degrades accuracy in noise figure characterization. The suitability of corrections to the gain definitions for a more precise noise figure evaluation for mismatched devices is investigated and compared to classical techniques. The effects of device mismatch on the noise figure of the noise-meter receiver and its impact on the final accuracy are analyzed.

Analysis and elimination of parametric oscillations in monolithic power amplifiers

This work describes an analysis and design methodology for eliminating parametric oscillations in microwave power amplifiers. Large-signal stability analyses based on system pole-zero identification techniques are proposed to guide the design process towards a stable circuit. In order to demonstrate the proposed approach, parametric oscillations of a Ku-band MMIC power amplifier have been eliminated, while maintaining the original performances of the circuit.

Systematic Approach to the Stabilization of Multitransistor Circuits

This paper proposes a systematic approach for the elimination of spurious oscillations in circuits with multiple active elements. The procedure is based on detecting the sensitive parts (nodes or branches) of the complex circuit at which we can have a strong control of the dynamics responsible for the instability. To do so, stability analyses based on pole-zero identification are performed at multiple observation ports. Once sensitive locations are detected, stabilization is achieved by adding series or shunt stabilization networks at the suitable node or branch. Standard techniques from linear control theory (pole placement strategies) are used to automatically calculate the values of the stabilization elements ensuring circuit stability. Here, the methodology is applied to the stabilization of a two-stage Ku-band high-power amplifier for active antenna space applications.

A multifrequency eletromagnetic applicator with an integrated AC magnetometer for magnetic hyperthermia experiments

In the present paper, a lab-made electromagnetic applicator for magnetic hyperthermia experiments is described, fabricated and tested. The proposed device is able to measure the specific absorption rate (SAR) of nanoparticle samples at different magnetic field intensities and frequencies. Based on a variable parallel LCC resonant circuit fed by a linear power amplifier, the electromagnetic applicator is optimized to generate a controllable and homogeneous AC magnetic field in a 3.5 cm3 cylindrical volume, in a wide frequency range of 149–1030 kHz with high field intensities (up to 35 kA m−1 at low frequencies and up to 22 kA m−1 at high frequencies). In addition, a lab-made AC magnetometer is integrated in the electromagnetic applicator. The AC magnetometer is fully compensated to provide accurate measurements of the dynamic hysteresis cycle for nanoparticle powders or dispersions. From these dynamic hysteresis loops the SAR of the nanoparticle samples can be directly obtained. To show the capabilities of the proposed set-up, the AC hysteresis loops of two different magnetite nanoparticle samples with different sizes have been measured for various field intensities and frequencies. To our knowledge, no other work reports an electromagnetic applicator system with integrated AC magnetometer providing such characteristics in terms of frequency and intensity.

An accurate neural network model of FET for intermodulation and power analysis

An accurate nonlinear FET model based on a Neural Network representation of the drain current source has been developed to predict intermodulation distortion as well as output power performance. This new neural network model has been implemented in a commercial harmonic balance simulator and its efficiency is evidenced by a comparison with an empirical model (Tajima). The accuracy of the proposed model is verified by active load-pull measurements on a Texas HFET at 10 GHz.

Efficient nonlinear Stability Analysis of Microwave Circuits using Commercially Available Tools

In this paper, two techniques for checking stability of multi-transistor MMIC circuits are analyzed. The two approaches allow both linear and nonlinear stability analysis of any complex circuit fed by small or large signals. The agreement found gives a unified way to propose a fast and efficient tool to ensure stability of a design during the design step using commercially available software. An example is given to evidence the suitability of the approach. It concerns a MMIC HBT power amplifier, which exhibited a frequency division phenomena that has been detected by simulations and confirmed by measurements.

Period-doubling analysis and chaos detection using commercial harmonic balance simulators

Two of the most common phenomena leading to chaos are the period-doubling cascade and the formation of transverse homoclinic orbits. In this paper, a bifurcation analysis technique is presented for the prediction of both phenomena in microwave circuits. The fact that the technique is based on the use of commercial harmonic balance software constitutes a major advantage for the circuit designer. The accuracy of the method relies on the capability to detect and calculate the successive period doubling, which, in period-doubling cascades, provides a good estimation of the parameter values for the onset of chaos. Another important aspect of the new method is the equilibrium point determination, necessary for the prediction of the homoclinic chaos. The accuracy in the calculation of the limit cycle, taking into account the most influential period doublings, ensures a good estimation of the parameter values for the formation of possible homoclinic orbits. In order to validate the method, it is initially applied to an RL-diode circuit, with a period-doubling route to chaos. A practical microwave frequency doubler is then analyzed, determining its parameter ranges for stable operation. Excellent results are obtained in comparison with the time-domain simulations. As an example of the methods capabilities for the prediction of homoclinic chaos, the bifurcation loci of Chuas circuit, with a cubic nonlinearity, are obtained and they agree closely with time-domain simulations.

Stability analysis of microwave circuits

It is common knowledge that stability analysis is a critical step of RF design flow. Nowadays powerful CAD tools allow accurate simulation of the performances of microwave and RF circuits. But the prediction of instabilities giving rise to frequency divisions or spurious oscillations is still a challenge, especially for MMIC designs whose adjustment after fabrication is impossible. This paper presents a new tool for stability analysis of microwave circuits, valid for small-signal and large-signal regimes. The technique used in this tool allows to detect and determine the nature of oscillations, such as parametric oscillations in power amplifiers that can be for example function of the input drive signal. Knowledge of the type of oscillation mode facilitates the insertion of stabilization networks, with a better balance between the required oscillation avoidance and maintaining the original circuit performances. The integration of this tool in the design flow with commercial CAD tools will be presented.

Stability Analysis of Nonlinear Circuits Driven With Modulated Signals

Existing methods for the large-signal stability analysis of microwave circuits assume a periodic excitation, usually consisting of a single tone. However, practical circuits such as power amplifiers will generally be driven by modulated signals or multiple input tones. This paper presents a procedure for the stability analysis of linear time-varying systems, applicable to circuits excited with nonperiodic input signals. The procedure is based on the determination of the time-varying poles associated to an input-output representation of the system. Under modulated signals, this representation is obtained by linearizing the envelope-transient system that rules the circuit behavior. The formulation and methodology have been applied to a simple nonlinear circuit with arbitrary excitation and a practical power-combined amplifier at 3 GHz with very good agreement in comparison with independent simulations and experimental results.