Henri Ruotsalainen

A New Quadrature PWM Modulator With Tunable Center Frequency for Digital RF Transmitters

State-of-the-art quadrature-type modulators for all-digital radio-frequency (RF) transmitters operate optimally at an RF center frequency (fc) that is a fraction of the modulator switching rate. This is a major limitation if the modulators of this type are to be used in multichannel transmitter systems. In this brief, a novel quadrature modulator based on pulsewidth modulation with controllable in-band distortion is presented, which permits continuous adjustment of fc in digital domain. In addition, distortion generation related to digital upconversion of pulsed baseband signals is studied, and suitable compensation methods are given. The performance of the proposed modulator is compared with those of previously published encoding methods in terms of reconstructed RF signal quality and computational complexity. The evaluation results suggest that significant improvements in signal band distortion can be expected with a computational load suitable for real-time reconfigurable hardware realizations.

Behavioral modeling of digital transmitters with time delay neural networks

In this work a novel time delay neural network based complex baseband model is proposed for fully digital pulse driven power amplifier concepts. It will be shown that the given technique is able to capture the non-linear effects related to the analog components of the transmitter. Results based on simulations and measurements indicate an accurate modeling performance over a wide frequency range.

Equivalent Complex Baseband Model for Digital Transmitters Based on 1-bit Quadrature Pulse Encoding

In this paper an equivalent complex baseband representation of the analog component related non-linearity of digital transmitters relying on 1-bit complex baseband encoding is derived. By exploiting the properties of the pulsed RF encoding the novel behavioral modeling technique is able to represent accurately the non-linear memory effects of the power amplification stage. Furthermore, a band-limited kernel technique leads to more efficient modeling of the complete digital transmitter, and to relaxed sampling rate. For the parameter estimation the linear regression, common to Volterra model identification, can be employed. According to the simulation and measurement based verification results, the novel modeling technique excels the state-of-the-art in terms of modeling accuracy. It can be assumed that the given methodology serves both as a basis for future behavioral models and for the development of advanced encoding techniques for linearization purposes.

Characterization and optimization of pulse drivers for switched mode power amplifier measurements

This paper focuses on the evaluation of pulse drivers for the use in class-S power amplifier setups with respect to linearity. Nonlinear effects are likely to degrade the overall system performance, though they are hard to characterize especially in an early stage of design. Based on measurements conducted with a configurable pulse driver, namely Triquints TGA4954-SL, a simple gauging method was developed. It allows the identification of favorable bias-parameter settings or the comparison of different devices while requiring only very little knowledge of the overall system.

Quantization Noise Reduction Techniques for Digital Pulsed RF Signal Generation Based on Quadrature Noise Shaped Encoding

The digital generation of pulsed radio frequency (RF) signals based on noise shaped encoding in complex baseband is a popular concept in digital RF transmitters. However, as the pulsed complex-baseband signals are up-converted to an RF frequency, a conjugate image overlaps with the signal band, which complicates the center frequency tuning for the transmitters in question. In this paper novel ways related to equalization techniques used for imbalanced digital complex valued systems are presented to mitigate conjugate image noise. Firstly, the distortion generation mechanism of the digital up-conversion is explained. Based on the analysis two cancelation techniques are derived. Secondly, novel quadrature encoder topologies implementing the given compensation techniques are developed. The analytical and the simulation based results show conjugate quantization noise suppression of more than 50 dB, which extends the potential center frequency tuning range considerably. Finally, a comparison with previously given encoders reveals nearly 10 dB improvement in dynamic range.

Computationally efficient table based modeling for digital RF transmitters

In this paper a lookup table (LUT) based model, suitable for describing the nonlinear behavior of digital radio frequency (RF) power amplifiers (PA), is introduced. Futhermore, the LUT model is compared to nonlinear auto-regressive (ARX) models with respect to performance and computational complexity. This is accomplished by modeling a digital RF transmitter with both techniques and comparing the results to measurements. Both models are identified in time domain using short and generic excitation sequences. Having access to such models is essential when designing digital RF transmitters in order to cope with undesired nonlinear distortion. Only if sufficiently accurate models are available, linearization methods like predistortion can be developed without complicated and costly hardware in the loop setups. This paper addresses the advantages and weakpoints of those two methods based on measurements of Triquints TGA4954-SL as the device under test (DUT).

Nonlinear modeling and model-validation for digital switched mode RF power amplifiers

In this paper a methodology for identifying broadband nonlinear models for digital switched mode RF power amplifiers is presented. This can be accomplished by training a black box model by observing an amplifiers output waveform in time domain. It is demonstrated that the presented method is feasible for using a simple and relatively short excitation sequence. The required model-complexity for a given amplifier is unknown in general, therefore, a testing method is presented in order to select a model of sufficient complexity. This is necessary since selecting a model solely on how well it reproduces the training sequence has been proven not to be sufficient. A digital power amplifier lab-setup serves as test platform in order to verify the validity of the so derived model.

Quantization noise cancelation scheme for digital quadrature RF pulse encoding

Quadrature type pulsed radio frequency (RF) signal encoding combined with class-S switched-mode power amplification (SMPA) constitute a potential solution for high efficiency RF transmitters. However, the digital center frequency adjustment for such encoding methods is limited by quantization noise folding. In this paper, a novel quantization noise cancelation scheme is proposed that in an ideal case suppresses the folding distortion components completely. In addition, a low complexity practical solution easily embedable in an existing modulator architecture is given. According to the measurement results, up to 7 dB improvement in signal-to-noise-and-distortion ratio was achieved over a total center frequency tuning range of 160 MHz using a 20 MHz LTE signal excitation.

Hierarchical-Table-Based Model for All-Digital RF Transmitters

Transmitters based on nonlinear radio-frequency (RF) modulators and switched-mode power amplifiers are systems that, at least theoretically, can provide high linearity and energy-efficient generation of RF signals at the same time. However, the nonlinear memory-affected behavior remains an issue hindering practical applications. This paper shows that it is possible to model the nonlinear memory of such circuits based on time-domain observations and how to use this information in computationally efficient time-domain RF circuit models. In contrast to other works, this model covers the full bandwidth of the active device (dc–10 GHz) and it uses hierarchical data structuring to adaptively find a compact model without prior knowledge of the circuit’s memory depth. The data for this work were gained from a laboratory setup, designed to be used as an LTE transmitter for a signal bandwidth of 20 MHz at a fixed center frequency of 2.5 GHz.

Nonlinear simulation of digital RF transmitters under modulated excitation

A technology push towards digital radio frequency (RF) transmitters (DRFTx) is all apparent due to the many beneficial properties they offer. However, their distortion mechanisms pose a challenging problem for practical realizations. To a significant extend this is caused by the nature of the excitation signal. DRFTx are driven by a binary signal with rates in the Gbit/s domain and only a fraction of this signal constitutes the desired modulated RF output. All other signal components are required to achieve an energy efficient operation of the DRFTxs PA stage. Such significant cross dependencies call for nonlinear simulations that predict the behavior of a circuit in question. This paper addresses this issue by demonstrating a method that allows to predict a DRFTxs output time-domain waveform in a computational efficient manner. The underlying nonlinear model is based on readily available nonlinear transistor models within RF computer aided design (CAD) tools. Additionally this simulations are compared to measurements of the same amplifier in a lab setup.