Telmo R. Cunha

The Linearity-Efficiency Compromise

This article attempts to improve the average efficiency and linearize the work-horse of RF power amplification- the class-B (or class AB) PA-relying, for example, on predistortion, dynamic load modulation or ET. Nevertheless, whenever higher efficiencies are desired, the current-mode class-B PA must be replaced by switchedmode PAs (for example, class-E or class-F), completely modifying the paradigm of RF amplification. In fact, as these are saturated circuits, they cannot respond to excitations of variable amplitude; and thus the original signal envelope must be restored by operating the PA as an AM modulator via dynamic supply adaptation. A good example of this is the EER arrangement.

Monitoring Sandy Shores Morphologies by DGPS—A Practical Tool to Generate Digital Elevation Models

Abstract The highly dynamic nature of some sandy shores with continuous morphological changes require the development of efficient and accurate methodological strategies for coastal hazard assessment and morphodynamic characterisation. During the past decades, the general methodological approach for the establishment of coastal monitoring programmes was based on photogrammetry or classical geodetic techniques. With the advent of new geodetic techniques, space-based and airborne-based, new methodologies were introduced in coastal monitoring programmes. This paper describes the development of a monitoring prototype that is based on the use of global positioning system (GPS). The prototype has a GPS multiantenna mounted on a fast surveying platform, a land vehicle appropriate for driving in the sand (four-wheel quad). This system was conceived to perform a network of shore profiles in sandy shores stretches (subaerial beach) that extend for several kilometres from which high-precision digital elevation models can be generated. An analysis of the accuracy and precision of some differential GPS kinematic methodologies is presented. The development of an adequate survey methodology is the first step in morphodynamic shore characterisation or in coastal hazard assessment. The sample method and the computational interpolation procedures are important steps for producing reliable three-dimensional surface maps that are real as possible. The quality of several interpolation methods used to generate grids was tested in areas where there were data gaps. The results obtained allow us to conclude that with the developed survey methodology, it is possible to survey sandy shores stretches, under spatial scales of kilometres, with a vertical accuracy of greater than 0.10 m in the final digital elevation models.

Validation and Physical Interpretation of the Power-Amplifier Polar Volterra Model

Although the Cartesian signal decomposition has been the preferred representation in baseband polynomial power-amplifier (PA) behavioral models, this is not the only 2-D reference frame that could be considered for representing the input complex envelope signal. Indeed, in this paper, we demonstrate that, if the alternative polar representation is considered, the resulting Volterra series model is much more adequate to model the physical behavior of PA devices. This is the feature that supports the design of an innovative PA model, denominated the Polar Volterra model, which is more flexible and general than the traditional Volterra series commonly used in PA baseband modeling. The closeness of the new model formulation with the PA physical operation enabled, for the first time in PA low-pass equivalent behavioral modeling, the theoretical derivation of a Volterra series model directly from the PA circuit analysis. In fact, as the proposed model directly isolates such PA physical characteristics, a significant reduction of the number of model coefficients is achieved when compared with the traditional Cartesian Volterra model. Finally, validation results that highlight the advantages of the Polar Volterra model are presented. These were based on the laboratory measurements performed on two PAs with distinct architectures: a conventional class-AB amplifier and a polar transmitter.

Base-band derived volterra series for power amplifier modeling

This paper proves that the traditional way of deriving power amplifier low-pass equivalent complex-signal Volterra models from their original band-pass RF real-signal Volterra models is too restrictive, and so does not lead to an optimal model. Then, it proposes a much richer alternative approach. Instead of deriving the base-band Volterra model from the RF Volterra model, we started by a general Volterra series expansion of a complex-signal to only then impose the restrictions of odd parity required by the low-pass equivalent polynomial approximation. This way, not only we prove that the theoretical reticence that was raised to similar approaches previously proposed for the memoryless polynomial and the memory polynomial were unfounded, as experimental results fully justified this novel approach.

A Multiple Time-Scale Power Amplifier Behavioral Model for Linearity and Efficiency Calculations

This work describes a new power amplifier, PA, behavioral model intended to predict both the PA amplitude and phase input-output signal relationship and the average or instantaneous dc power consumption and thus average or instantaneous signal-dependent efficiency. Hence, contrary to other previously published PA behavioral models, such a model is not only capable of describing the complex low-pass equivalent fundamental zone output, but also the real low-pass current consumption output. In addition, a special attention is paid to accurately account for the multiple time-scale dynamics shown by modern GaN HEMT based high power amplifiers.

Reduced-Order Parametric Behavioral Model for Digital Buffers/Drivers With Physical Support

In this paper, we present a new behavioral model for high-speed digital output buffers/drivers. In the conceived model, the output currents relationship with the output voltage is expressed as a summation of a static nonlinearity plus linear dynamics. This separation in the model format is supported by the measurements as well as the physical structure of a general driver circuit. This approach merges the features of equivalent circuit and parametric approaches to build a reduced-order parametric behavioral model which, compared to other published models, is more adequate to describe the devices electrical behavior from transient input-output data. A simple single-step identification procedure is conceived to extract a model that proved to be stable and capable of significantly improving the simulation speed and accuracy of prediction. Finally, the resulting model is validated in a realistic signal integrity simulation setup, and is compared to transistor-level models and to the state-of-the-art input-output buffer information specification model.

Design of a Power-Amplifier Feed-Forward RF Model With Physical Knowledge Considerations

In this paper, the design of a general feed-forward topology for modeling RF power amplifiers (PAs) is presented. It consists of producing the best nonlinear nonrecursive approximator of a feedback model which traduces the physical characteristics of general PAs. Following the procedure presented in this paper, the feed-forward structure, which has a polynomial form with memory (Volterra series), can be exactly determined up to the desired order of approximation. Evidently, increasing the order of the feed-forward topology results in a more complex structure, with more laborious model extraction procedures. This paper presents in detail the structures of the general nonrecursive model up to the fifth-order kernel. A systematic extraction procedure, based on standard RF laboratory continuous wave and two-tone tests, is proposed under the assumption that the PA linear output does not vary within the considered fundamental frequency band (flat behavior). Model validation is demonstrated by means of ADS simulation data and from real data measurements.

A Precise and Efficient Methodology to Analyse the Shoreline Displacement Rate

Abstract The shoreline change rate is one of the most significant parameters in analysing sandy shore behaviour with time. This parameter can be monitored by means of low- and high-resolution survey methods, depending on the objectives of the monitoring programme. Survey efficiency is also very important for achieving high resolution in both space and time. Another important aspect is the precision and significance of the obtained results, not only from the survey method itself but also from the comparative analysis used to process the data from several surveys. The survey method and the processing algorithms are the basis of shoreline analysis. This paper presents an evaluation of two proposed high-resolution methods that are simultaneously highly accurate and very efficient. These methods are based on a global positioning system (GPS) in differential mode for surveying and on novel algorithms for assessing the spatial change rate of the shoreline. The most significant difference of the two presented survey systems is the physical support: whereas one uses a land vehicle (motor-quad) to delineate the shoreline in wide straight coastal stretches, the other considers an on-foot simplified version to survey small, more irregular stretches. The analysis of the error associated with the proposed methodologies is thoroughly described in this paper. In both modes of operation, system-inherent errors are within the centimetre level—in general lower than 0.05 m. Operation-specific errors can remain within the centimetre level, but if instrument handling is careless in the on-foot survey method, they can reach decimetre levels. If successive monitoring surveys are not carried out under similar field morphological conditions, when the frontal dune baseline is adopted as a shoreline indicator, rough errors can be introduced. Two case studies of the application of these methods, evaluating the shoreline evolution of two distinct coastal regions in Portugal, are presented.

A Physically Meaningful Neural Network Behavioral Model for Wireless Transmitters Exhibiting PM–AM/PM–PM Distortions

Artificial neural networks (ANNs) have been widely used to model wireless transmitter low-pass equivalent behavior. However, previously proposed ANNs either do not account for PM-AM and PM-PM distortions or do not satisfy a fundamental constraint imposed by the bandpass nature of wireless transmitters. The purpose of this work is twofold. First, it is shown that PM-AM and PM-PM distortions observed in wireless transmitters excited by wideband signals can have a significant impact on the performance of their behavioral models. Second, a novel ANN topology for wireless transmitter behavioral modeling is proposed. Contrary to previously published ANNs, this one only generates physically meaningful contributions as well as retaining the ability of accounting for PM-AM and PM-PM distortions. The accuracy of the proposed ANN is then compared with two commonly used ANNs of the same computational complexity and for fitting experimental data measured on a GaN-based class-AB amplifier chain. Improvements of up to 7 dB in NMSE and ACEPR results are achieved if the proposed ANN is used instead of a commonly used ANN that neglects the PM-AM and PM-PM distortions. Furthermore, improvements as high as 16 dB in NMSE and ACEPR are achieved by the proposed ANN in comparison with a traditional ANN that also accounts for the PM-AM and PM-PM distortions but does not satisfy the fundamental odd-parity constraint imposed by the bandpass nature of wireless transmitters.

A Novel Two-Port Behavioral Model for I/O Buffer Overclocking Simulation

This paper presents the first behavioral model solution to the computationally efficient simulation of digital I/O buffers under overclocking operation. Our physics-based two-port approach relies on predicting the timing signals that control the activation of the drivers output stage. The identified nonlinear dynamic model operators of the input port replace the concatenated fixed step-input describing functions of the previous table-based I/O buffer information specification and of other parametric approaches. The implemented gray-box model produces more accurate results than the previous methodologies when assessing the signal integrity performance of high-speed digital links in normal and overclocking conditions under various input excitations. However, it still preserves the computational efficiency recognized for its behavioral model predecessors.