arXiv subjects

Ingmar Kallfass

Publications and source records attributed to Ingmar Kallfass.

4 recordsLinked to original sources

arXiv2026

Overcoming the limitations of NMR Field Probes: A Novel Integrated Sensor Utilizing Pre-Polarization for (Ultra) Low Field MRI

Access to magnetic resonance imaging (MRI) remains severely limited in low- and middle-income countries, especially in sub-Saharan Africa, despite rising rates of non-communicable diseases. Low-field MRI presents an affordable, locally developable diagnostic solution, but its performance is constrained by magnetic field instability. We present a novel NMR field probe designed to overcome these challenges using a rapid non-adiabatic switch-off of a pre-polarization field resulting in precessing spin magnetization. Achieved by first use of high-voltage silicon carbide transistors operating in controlled avalanche breakdown, it measures the Larmor frequency without prior field knowledge, unlike conventional probes. This capability is crucial during magnet development with often unknown fields, allowing early detection of magnet issues, and offering an urgently needed tool for magnet design and image-quality improvement. Validated from 1 mT to 45 mT (up to 1,000 times stronger than similar systems) its low-cost, modular design supports replication, upgrades, and enhanced field control, helping expand global MRI access.

physics.med-ph↗

arXiv2025

Sub-THz Power Amplifiers: Measurements, Behavioral Modeling and Predistortion Algorithms

With global IMT traffic expected to grow 10-100 times from 2020 to 20301, the Terahertz (THz) spectrum offers a promising solution to satisfy such forecasts. However, occupying the THz spectrum comes with its own challenges, an important one being impairments caused by broadband RF components in THz transceivers. Nonlinearities in power amplifiers (PAs) complicate meeting link budget requirements, with amplitude and phase distortions degrading the system's performance, especially when adopting waveforms with high peak-to-average power ratios (PAPRs), such as Orthogonal Frequency Division Multiplexing (OFDM). In this paper, we present characterization results of a 300 GHz PA using small-signal and large-signal continuous-wave measurements. Models capturing Amplitude-to- Amplitude Modulation (AM-AM) and Amplitude-to-Phase Modulation (AMPM) behavior across 270-330 GHz are developed and verified with wideband measurements, confirming the compression behavior, while nonetheless showing inaccuracies for low input powers due to unaccounted frequency dependencies. Based on the derived models, a predistortion algorithm is designed and analyzed, revealing significant error performance degradation when switching between single- and multi-carrier waveforms. We finally show that an appropriate selection of pre-distorter parameters can significantly improve the performance.

eess.SP↗

arXiv2024

Agricultural On-Demand Networks for 6G enabled by THz Communication

The transforming process in the scope of agriculture towards Smart Agriculture is an essential step to fulfill growing demands in respect to nourishment. Crucial challenges include establishing robust wireless communication in rural areas, enabling collaboration among agricultural machines, and integrating artificial intelligence into farming practices. Addressing these challenges necessitates a consistent communication system, with wireless communication emerging as a key enabler. Cellular technologies, as 5G and its successor 6G, can offer a comprehensive solution here. Leveraging technologies following the ITU-R M. 2160 recommendation like THz communication, low-latency wireless AI, and embedded sensing, can provide a flexible and energy-efficient infrastructure. This paper introduces on-demand networks based on the OpenRAN approach and a 7.2 functional split. By implementing THz front-hauling between components, a flexible application of 5G or future 6G networks can be realized. Experiments demonstrate that THz communication is suitable for data transmission over the eCPRI interface, particularly in terms of data rate, thereby reducing the need for wired alternatives such as fiber optic cables. Furthermore, limitations such as limited range are discussed, and possible initial solutions are presented. The integration of the OpenRAN standard further enhances flexibility, which is crucial in dynamic agricultural environments. This research contributes to the ongoing discourse on the transformative potential of 6G-enabled wireless communication in shaping the future of smart agriculture.

cs.NI↗

arXiv2021

A Compact Model for SiC Power MOSFETs for Large Current and High Voltage Operation

This work presents a physics based compact model for SiC power MOSFETs that accurately describes the I-V characteristics up to large voltages and currents. Charge-based formulations accounting for the different physics of SiC power MOSFETs are presented. The formulations account for the effect of the large SiC/SiO2 interface traps density characteristic of SiC MOSFETs and its dependence with temperature. The modeling of interface charge density is found to be necessary to describe the electrostatics of SiC power MOSFETs when operating at simultaneous high current and high voltage regions. The proposed compact model accurately fits the measurement data extracted of a 160 milli ohms, 1200V SiC power MOSFET in the complete IV plane from drain-voltage $V_d$ = 5mV up to 800 V and current ranges from few mA to 30 A.

eess.SY↗