Holger Arthaber

Full-field time-encoded frequency-domain optical coherence tomography

Ultrahigh axial resolution surface profiling as well as volumetric optical imaging based on time encoded optical coherence tomography in the frequency domain without any mechanical scanning element is presented. A frequency tuned broad bandwidth titanium sapphire laser is interfaced to an optical microscope (Axioskop 2 MAT, Carl Zeiss Meditec) that is enhanced with an interferometric imaging head. The system is equipped with a 640 x 480 pixel CMOS camera, optimized for the 800 nm wavelength tuning range for transmission and reflection measurements of a microscopic sample. Sample volume information over 1.3 x 1 x 0.2 mm(3) with ~3 mum axial and ~4 mum transverse resolution in tissue is acquired by a single wavelength scan over more than 100 nm optical bandwidth from <760 to >860 nm with 128-2048 equidistant optical frequency steps with an acquisition time of 1 to 50 ms per step. Topography and tomography with a signal to noise ratio of 83 dB is demonstrated on test surfaces and biological specimen respectively. This novel OCT technique promises to enable high speed, three dimensional imaging by employing high frame rate cameras and state of the art tunable lasers in a mechanically stable environment, due to lack of moving components while reducing the intensity on the sample.

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.

Spread-Spectrum Based Ranging of Passive UHF EPC RFID Tags

This letter introduces a ranging method which can be applied to off-the-shelf backscatter RFID tags, like standardized EPC Gen 2 tags. For this purpose a low power direct sequence spread spectrum (DSSS) sequence is superimposed onto the interrogators signal during tag to reader communication. The chip rate and sequence length is chosen to fit an entire DSSS sequence into each backscattered bit. During the tag to reader communication, the bits sent by the tag are decoded and a time domain recording with the length of one DSSS period is made for each bit. By cyclic alignment of each recording and subtracting the averaged recording of a “0”-bit from the one of a “1”-bit, static echos and antenna coupling effects are perfectly canceled. In addition, noise variance is reduced, improving the SNR. Finally, by applying a cyclic correlation with the transmitted DSSS sequence and locating the correlation maximum, the distance to the RFID tag can be determined.

RF-power amplifier characteristics determination using parallel cascade Wiener models and pseudo-inverse techniques

This article presents a black-box modeling approach of a microwave power amplifier (PA) and defines the linear and nonlinear operation regions. An in-band quasi-white real-valued noise like signal is used as stimulus for the identification process to excite every possibly source of nonlinearity. A segment of the input-output measurement data is processed to generate an initial parallel cascade Wiener model (PCWM). The first order Volterra kernel is extracted in order to estimate the amplifiers memory. Then pseudo-inverse techniques are used to find a parsimonious model. This model is cross-validated with the entire measurement data. The presented modeling approach results in a model intended to be numerically robust and having a high identification percentage based on a variance figure of merit. Finally the linear and nonlinear operation regions of the amplifier are defined, as an alternative to the use of AM/AM curves.

The influence of multipath propagation on phase-based narrowband positioning principles in UHF RFID

The influence of multipath propagation effects on phase-based positioning methods for UHF RFID is discussed in this paper. Thus, a propagation channel model for typical UHF RFID scenarios is derived. Several simulations have been carried out in order to understand the influence of the channel parameters on the narrowband phase-based localization methods.

Load Pull Characterization of GaN/AlGaN HEMTs

This work presents a large signal characterization of a high power GaN/AlGaN HEMT transistor utilizing a harmonic load pull setup. Two different methods for passive automated tuners have been compared to synthesize the load impedances

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.

High-Gain Dielectric-Loaded Vivaldi Antenna for

In this letter, a novel concept of a high-gain Vivaldi antenna operating in the Ka frequency band is proposed. The antenna is fed by the substrate integrated waveguide. The concept overcomes shortcomings of conventional antipodal Vivaldi antenna using dielectric load with printed metal transition and transverse ripples. The added parts are optimized to obtain a wideband high-gain antenna with low cross polarization. The fabricated antenna shows very good performance over the entire Ka band where the measured gain is more than 10.5 dBi.

K_{a}

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.