Janne P. Aikio

Detailed distortion analysis technique based on simulated large-signal voltage and current spectra

This paper presents an analysis technique implemented on top of normal harmonic-balance simulation, where the simulated nonlinear voltage and current spectra is used for fitting a polynomial device model around the large-signal bias point. Further, using the fitted model and spectra of the controlling voltages, the detailed structure of the third-order intermodulation distortion is calculated using a simplified form of Volterra analysis. This detailed information can be used to find the dominant causes of distortion and possible cancellation mechanisms, to study the bandwidth-dependent memory effects caused by up- or down-converted mixing results, or to find harmonic matching impedances that minimize the overall distortion.

A Comprehensive Analysis of AM–AM and AM–PM Conversion in an LDMOS RF Power Amplifier

In this paper, a Volterra analysis built on top of a normal harmonic balance simulation is used for a comprehensive analysis of the causes of AM-PM distortion in a LDMOS RF power amplifier (PA). The analysis shows that any nonlinear capacitors cause AM-PM. In addition, varying terminal impedances may pull the matching impedances and cause phase shift. The AM-PM is also affected by the distortion that is mixed down from the second harmonic. As a sample circuit, an internally matched 30-W LDMOS RF PA is used and the results are compared to measured AM-AM, AM-PM and large-signal S11.

Performance of an Integrated 2.1 GHz Analog Predistorter

This paper presents the structure and measured results of a RF predistorter IC fabricated in a 0.35mum SiGe BiCMOS process. The IC implements a 5th-degree polynomial predistorter which has been used to linearize a 2.1 GHz LDMOS power amplifier. The measurement setup and results are described. Up to 20 dB cancellation is achieved in 2-tone measurements and up to 8 dB with WCDMA data. The circuit includes also output for the squared envelope signal that can be used as a baseband injection signal to cancel 2nd order distortion components that typically cause memory effects

AM-PM distortion caused by transistor's signal-dependent input impedance

AM-PM distortion causes spectral regrowth in RF transmitters with modulated signals. One reason for AM-PM is the fact that signal-dependent transistor input capacitance pulls the frequency response of the input matching network and causes phase shift already at the input of the transistor. This paper studies the Miller capacitance effect in most common fixed-supply and supply modulated power amplifier topologies and typical matching networks, and shows that the signal-dependent input capacitance can cause significant AM-PM distortion.

A 5th order Volterra study of a 30W LDMOS power amplifier

A 30 W LDMOS is modeled using a 5th order polynomial model. The polynomial model is compared to the large-signal MET model using harmonic balance, and as the results agreed very well, the polynomial model was imported to a numerical Volterra simulator to find out the dominant cause of distortion for a class A biased amplifier. The characterization technique is briefly discussed.

Fitting of 2-dimensional polynomial device model based on simulated voltage and current spectra

This paper presents a fitting technique for polynomial 2-dimensional nonlinearity, based on voltage current spectra. It will be shown that despite the strongly correlating controlling voltages, the nonlinear 2-dimensional drain-source current (I/sub DS/) of the 30 W RF power transistor model can be fitted. Furthermore, a simplified Volterra presentation of the 3rd order intermodulation distortion (IM3) contributors of I-V and Q-V sources can be constructed. The results match well with the data simulated using harmonic balance. Also IM3 phasors of the input and output of the device are presented. The analysis shows that the total output IM3 current is dominated by the distortion from I/sub DS/. Also a significant portion of the IM3 current is caused by the fact that output IM3 voltage appears across linear but large drain-source capacitance (C/sub DS/).

Distortion contribution analysis of an LDMOS Doherty power amplifier

This paper presents a distortion contribution analysis of a 100 watt LDMOS RF Doherty power amplifier. The analysis is performed using recently developed distortion contribution analysis technique called as Volterra-on-top-of-harmonic-balance that is able to show the originating cause of nonlinearity and mixing mechanisms between harmonic bands. The analysis shows that the nonlinear output capacitance of the carrier amplifier is the main cause of distortion at peak power levels.

Utilization of distortion contribution analysis

This paper describes how a novel distortion contribution analysis can be used efficiently for improving the linearity of a nonlinear circuit. The distortion analysis technique called Volterra-on-Harmonic-Balance calculates a distortion contributions in each tone and node for each nonlinear elements of the nonlinear devices in the circuit. As the analysis provides large amount of data, also the flow of interpreting the results contains many steps that are described in detail in this paper.

Polynomial fitting of nonlinear sources with correlating inputs

Purpose – The purpose of this paper is to propose methods to improve the least square error polynomial fitting of multi-input nonlinear sources that suffer from strong correlating inputs. Design/methodology/approach – The polynomial fitting is improved by amplitude normalization, reducing the order of the model, utilizing Chebychev polynomials and finally perturbing the correlating controlling voltage spectra. The fitting process is estimated by the reliability figure and the condition number. Findings – It is shown in the paper that perturbing one of the controlling voltages reduces the correlation to a large extend especially in the cross-terms of the multi-input polynomials. Chebychev polynomials reduce the correlation between the higher-order spectra derived from the same input signal, but cannot break the correlation between correlating input and output voltages. Research limitations/implications – Optimal perturbations are sought in a separate optimization loop, which slows down the fitting process....

Detailed analysis of IMD in an LDMOS RF power amplifier

A detailed analysis of an LDMOS RF power amplifier is presented. Analysis shows that intermodulation distortion (IMD) sweet spot is a result of a vector sum of several cancelling mechanisms. This can make the sweet spot sensitive to center frequency and bandwidth. The analysis technique is implemented on top of harmonic-balance (HB) simulation. The simulated large-signal voltage and current spectra are used to fit polynomial models of the nonlinear I-V and Q-V sources of the device. The contributions of different nonlinearities can be calculated by using the polynomial model. The analysis relies on the accuracy of the simulation model, but the overall simulated results match well with the measurements.