Norbert Leder

Broadband terahertz amplification in a heterogeneous quantum cascade laser

We demonstrate a broadband terahertz amplifier based on ultrafast gain switching in a quantum cascade laser. A heterogeneous active region is processed into a coupled cavity metal-metal waveguide device and provides broadband terahertz gain that allows achieving an amplification bandwidth of more than 500 GHz. The temporal and spectral evolution of a terahertz seed pulse, which is generated in an integrated emitter section, is presented and an amplification factor of 21 dB is reached. Furthermore, the quantum cascade amplifier emission spectrum of the emerging sub-nanosecond terahertz pulse train is measured by time-domain spectroscopy and reveals discrete modes between 2.14 and 2.68 THz.

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.

Efficiency enhanced switched mode power amplifier for digital RF transmitters

This paper outlines a design concept of a PWM driven switched mode power amplifier (SMPA) with direct filter connect, making up all circuitry necessary for an energy efficient digital RF transmitter. For the output matching network design the simplified real frequency technique (SRFT) has been extended in a way to achieve high output power in signal band while reducing power consumption by controlling the termination for out of band components simultaneously. For concept verification the PA was designed and realized using a commercially available GaN 10W HEMT (Cree CGH40010F). The proposed PA shows a significantly increased DC to RF efficiency for PWM excitation.

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.

Calibration method for coupler based time domain waveform measurements

This paper addresses the issues related probing radio frequency (RF) waveforms in nonlinear circuits. Having access to time domain waveforms is essential when developing nonlinear circuits. However, in many cases these signals are not easy accessible which urge the need for some kind of coupler integrated onto the printed circuit board (PCB). Defined mostly by spatial circuit constrains these couplers usually perform suboptimal and in addition also the measuring instrument may add distortion. This paper describes a methodology that tackles all these issues by a compact two stage calibration approach. It allows to pre-characterize a coupler in conjunctions with the oscilloscope in a way that time domain waveforms can be measured directly in a circuit at a well defined reference plane.

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).

Design of a PWM driven continuous mode class F amplifier using harmonic load pull measurements

This work outlines the design and construction of a continuous mode Class F amplifier using harmonic load pull measurements. To overcome the limits implied by the load pull setup a custom prematch was designed. With this design methodology an amplifier can be build without prior knowledge of the used devices behavior. To design the prematch and the matching networks the Simplified Real Frequency Technique (SRFT) has been applied. By using a commercially available GaN 10W HEMT transistor (Cree CGH40010F), an amplifier was designed and constructed to achieve 12.5% relative bandwidth at a center frequency of 2.5 GHz. Over this band the amplifier achieves an efficiency (PAE) greater than 65% with an average gain of 11.8 dB.

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.

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.