Fabian Lurz

Six-Port Based Interferometry for Precise Radar and Sensing Applications.

Microwave technology plays a more important role in modern industrial sensing applications. Pushed by the significant progress in monolithic microwave integrated circuit technology over the past decades, complex sensing systems operating in the microwave and even millimeter-wave range are available for reasonable costs combined with exquisite performance. In the context of industrial sensing, this stimulates new approaches for metrology based on microwave technology. An old measurement principle nearly forgotten over the years has recently gained more and more attention in both academia and industry: the six-port interferometer. This paper reviews the basic concept, investigates promising applications in remote, as well as contact-based sensing and compares the system with state-of-the-art metrology. The significant advantages will be discussed just as the limitations of the six-port architecture. Particular attention will be paid to impairment effects and non-ideal behavior, as well as compensation and linearization concepts. It will be shown that in application fields, like remote distance sensing, precise alignment measurements, as well as interferometrically-evaluated mechanical strain analysis, the six-port architecture delivers extraordinary measurement results combined with high measurement data update rates for reasonable system costs. This makes the six-port architecture a promising candidate for industrial metrology.

A microwave interferometer based contactless quasi-TEM waveguide position encoder with micrometer accuracy

Position measurement plays an important role in industrial process technology. Hereby, a waveguide based position sensor system for industrial applications is described. The proposed system features a Phase Locked Loop controlled 24 GHz signal source, an interferometric Six-Port receiver structure, a radar coupler, and a terminated quasi-TEM parallel plate waveguide structure. Furthermore, the system concept comprises a target, which can be applied both touching and contactless in the waveguides fringing field, reflecting a part of the transmitted wave. To obtain the targets position information, the reflected wave is compared to a reference signal with respect to the relative phase difference. Hence, this phase shift is directly related to the position information.

Precise and fast frequency determination of resonant SAW sensors by a low-cost Six-Port interferometer

Surface acoustic wave (SAW) resonators are used for a broad range of wireless sensing applications, some of them having high demands on the resolution as well as the measurement update rate. This paper presents a new interrogation method for precise and fast frequency determination of passive SAW resonators based on instantaneous frequency measurement (IFM) by a low-cost Six-Port interferometer. By using a delay line, the frequency measurement is reduced to a phase measurement that can be instantaneously evaluated by the Six-Port network. No complex signal processing is necessary providing high update rates and low hardware costs. A demonstrator in the 2.4 GHz frequency band shows the feasibility and the precision of the concept with measurement times of only a few microseconds.

Ultra-short-range, precise displacement measurement setup with a near field slot-line antenna and a dedicated spiral calibration

This work introduces a precise measurement setup for displacement analysis in the near field of a tapered slot-line antenna from 0mm up to 20mm distance. Enhanced accuracy is achieved by a dedicated spiral reconstruction algorithm accounting for the near field effects. Moreover, the precision of the system in the micrometer range is guaranteed by four synchronous 24 bit analog-to-digital converters and a decimation factor of 10, leading to an overall update rate of 1 kHz.

Improved calibration procedure for six-port based precise displacement measurements

In this paper an improved calibration procedure for six-port based displacement measurement radar is proposed. The entire system characterization is done only using several measurements of the target at known positions and no additional calibration standards are required. The proposed calibration has been experimentally verified for a six-port based displacement measurement radar operating at 24 GHz. The performed measurements within the target position range 0 - 18.75 mm prove a significant improvement of the accuracy with respect to the other reported algorithms.

ADC depending limitations for Six-Port based distance measurement systems

This publication will show some considerations for selecting an appropriate analog-digital-converter (ADC) in Six-Port based radar systems for distance measurements. It will be discussed how the ADC resolution is limiting the overall system range resolution. Furthermore, configurations with non-synchronous sampling of the four baseband signals and their influences on the system performance will be investigated.

High-Speed Resonant Surface Acoustic Wave Instrumentation Based on Instantaneous Frequency Measurement

Surface acoustic wave (SAW) resonators are used for a broad range of wireless sensing applications, some of them having high demands on the resolution as well as the measurement update rate. This paper presents a new interrogation method for precise and fast frequency determination of passive SAW resonators based on instantaneous frequency measurement by a low-cost six-port interferometer. Using a delay line, the frequency measurement is reduced to a phase measurement that can be instantaneously evaluated by the six-port network. No complex signal processing is necessary providing high update rates and low hardware costs. An in-depth analysis of the system concept and its building blocks is presented, and the advantages as well as the limitations are discussed and compared with the current state of the art. Finally, a demonstrator in the 2.4-GHz frequency band shows the feasibility and the precision of the concept with measurement times of only a few microseconds.

Substrate integrated waveguide fed antenna for 61 GHz ultra-short-range interferometric radar systems

Radar sensors play a key role in todays industrial automation. Being an alternative to frequency modulated continuous wave radar, Six-Port based radar sensors recently attracted the interest of the community as well as of the industry. This paper presents a 61 GHz Six-Port radar sensor for ultra-short range applications and a dedicated substrate integrate waveguide fed tapered slot antenna design. Due to an optimized design, the antenna allows for measurements in the direct near-zone of 0mm to 10mm without ambiguity effects. Making use of a spiral fitting operation, the presented measurement results show an absolute distance error of less than 100 μm.

Diode detector design for 61 GHz substrate integrated waveguide Six-Port radar systems

Radar sensors play a key role in todays industrial automation. Besides high accuracy, modern radar sensors have to feature high precision as well as challenging update rates. Six-Port wave correlators enable high phase resolution at high frequencies and therefore a high accuracy in radar applications. A low-cost Schottky diode detector applied to a substrate integrated waveguide (SIW) Six-Port is presented and measured. Furthermore, a displacement measurement using the SIW structure is performed.

A High-Sensitivity Radar System Featuring Low Weight and Power Consumption

In this article, a lightweight, low-power radar system for vibration monitoring has been presented. Based on a theoretical analysis of the competition scenario, an optimized system concept has been developed. The radar front end is able to sequentially evaluate the in-phase and quadrature component of the received signal with only a single hybrid coupler and power detector, reducing the overall weight to 5 g including the power-supply cable. A low duty cycle of 1% allowed the power to average 1.5 mW. As the minimum required sensitivity was limited to 0.5-mm oscillation amplitude, the most challenging parts in this student competition were, in our opinion, the tight power and weight optimizations. As the competition required only vibration detection butno ranging, a minimalistic system concept optimized for this application was proposed. The system features a free-running VCO sourced directly from the power supply without any further stabilization. For ranging applications, temperature changes as well as supply and tuning voltage variations at the VCO could have a severe impact on the systems accuracy. However, for pure vibration detection of a well-defined target, this is not a problem. Further improvements could be achieved when designing a customized 24-GHz oscillator, optimized for high efficiency [9] and fast turn-on time. In addition, the necessary number of measurements per second could be further investigated. Currently, with ten measurements per second, there is a convenient safety margin against false detections; however, fewer measurements would directly reduce the duty cycle and thus the average power consumption of the system.