WiSNet: Six-Port Technology for Millimeter-Wave Radar and Imaging ApplicationsSix-port Technology for Millimeter-wave Radar and Imaging Applications
Kamel Haddadi and Tuami Lasri Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN-DHS), Université Lille 1
Avenue Poincaré CS 60069 – 59652 Villeneuve d’Ascq Cedex – France
Abstract —This paper presents an overview of recent developments of millimeter-wave six-port technology dedicated to wireless sensing applications. In particular, complete solutions with features such as low cost, low-power consumption, compactness, robustness and ease of use are described. Radar and imaging applications are experimentally demonstrated to validate the approach proposed.
Index Terms — Six-port, multi-port, millimeter-wave, sensor, radar, imaging, non-destructive testing.
I . INTRODUCTION
Interest in the six-port technology has been growing steadily owing to its outstanding potential [1]. Indeed, the six-port concept takes advantage of the capability of easily and precisely retrieving the magnitude and phase of a signal in a variety of applications. In addition, this technology benefits from a great flexibility in the design and realization of low complexity multi-port structures and the possibility to correct the hardware imperfections by suitable calibration software. All these assets together have given rise to many applications such as automotive radar sensors [2], high data rate wireless communication [3] or angle-of-arrival detection of a received wave [4].
Another emerging and promising application field is millimeter-wave wireless sensing [5]. Actually, in this frequency band, attractive characteristics such as the possibility to design small and light systems exist. It has to be mentioned that most of the existing millimeter-wave sensors are built on the basis of heterodyne architectures. The major drawbacks of these systems based on active I Q demodulators are the need of high frequency pre- amplification, the difficulty to design mixers with low unbalance and a limited frequency bandwidth [1]. Consequently, in the millimeter-wave frequency regime, the six-port technology is expected to play an important role. Despite limited relatively noise performance and sensitivity (active mixers are replaced by power measurements via diodes), the six-port architecture is a very competitive solution because it is fully passive (except the source), leading to simple design and circuit fabrication. The possibility to correct the hardware imperfections by a suitable calibration procedure offers
even more a great flexibility in the realization of six-port systems that results in relaxing the design constraints.
In this paper, the development of six-port reflectometers (SPR) for V-band sensing applications is presented. In section II , the design and fabrication of a 60 GHz S P R are reported briefly. Radar and imaging applications with high measurement accuracy are described respectively in Sections III and IV .
I I . SIX-PORT DESIGN AND REALIZATION
Generally speaking, the S P R is a passive linear circuit with one port connected to the signal source, one port to a device under test (DUT), and the remaining four ports to power detectors. The four power readings together with a mathematical treatment permit the measurement of the complex reflection coefficient Γ of the D U T . Fig. 1 gives an illustration of the 60 GHz SPR developed for sensing applications [6].
Analog outputs Vi (i=1,..,4)
Fig. 1. Photograph of the 60 GHz six-port reflectometer [6].
The SPR has been implemented in microstrip technology with hybrid couplers, a 90° phase-shifter and 50Ω resistors. The detection circuit is based on an Agilent Technology HSCH-9161 zero-bias Schottky diode. A signal conditioning block (SCB) consisting of DC high speed instrumentation amplifiers with gain adjustment to match the input requirements of the analog-to-digital converters (ADC) is used. The system is completed by a 12-bits ADC peripheral component interconnect (PCI)
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card for the sampling of the detected voltages V 1 to V4
and a computer with dedicated calibration software implemented in C++. As already mentioned, the versatility, applicability and possibilities offered by the multi-port technology allow envisaging a wide range of applications. There after two of the most promising are addressed.
I I I . RADAR APPLICATION
A compact six-port distance measurement system is described in this section. The radar operates at the single frequency of 60 GHz and makes use of both the magnitude and the phase of the measured free-space reflection coefficient to overcome the ambiguity of phase [7]. The block diagram of the proposed distance measurement sensor is presented in Fig. 2. It consists of a voltage-controlled oscillator phase-locked loop (VCO- PLL), the millimeter-wave SPR (Fig. 1) and a waveguide horn antenna (aperture = 5X7.5 mm2).
reference F = 60 GHz, P0
architectures for industrial applications (ex: vibration analysis, roughness control).
plane if
target
Fig. 2. Block diagram of the 60 GHz six-port distance measurement sensor.
The VCO-PLL generates a continuous wave signal at 60 GHz at the input 1 of the SPR. The horn antenna (working as transmitter and receiver) is connected to the output 2 of the SPR. The powers measured by the detectors associated to a calibration operation allow the determination of the complex reflection coefficient Γ. Experimental investigations on a metallic plate target have been carried out. The target (10X8 cm2) is moved from the contact between the target and the antenna aperture (d = 0) to the distance dmax = 27.5 mm with an increment of dstep = 50 µm. To retrieve the distance d from the measured data, a free-space calibration procedure depicted in [7] is applied. In Fig. 3, we present the remaining distance error calculated from the data collected. According to this graph, the maximum relative error observed is about 2.5 % for all the distances considered. These results demonstrate that this system presents a viable alternative to conventional heterodyne
2.5
2.0 [
1.5
1.0
0.5
0.0 '^&ιν&Μ 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5
Nominal distance [mm ]
Fig. 3. Remaining error of distance measurement. F = 60 GHz.
I V . MILLIMETER-WAVE IMAGING
In traditional free-space imaging techniques, the spatial resolution achievable, set by the diffraction limit, is in the order of half the wavelength of operation. To bypass the diffraction limit, methods based on evanescent waves can be used [8] . Fig. 4 shows the schematic of a near-field scanning millimeter-wave microscope.
F = 60 GHz, P0
& stage control
Fig. 4. Schematic of the 60 GHz scanning near-field millimeter- wave microscope.
To improve the measurement sensitivity, a waveguide E/H tuner matches the high impedance of the probe to the 50 Ω input impedance of the measurement system. The object to be imaged is mounted on a stage xyz. To complete the test bench, a data processing unit is used to control the position of the sample, to record the output voltages measured by the detectors, and to calculate the corresponding complex reflection coefficient Γ (Fig. 4). Two probes are considered in this study. They are made of microstrip gold lines tapered to a tip radius of respectively 7 µm and 25 µm and fabricated on a 127 μπι- thick ceramic substrate.
d
2
The first example is dedicated to sub-surface profile measurement. In the experiment, the probe-to-sample separation is kept in the near-zone of the probe (25 µm tip) at a constant value of 10 μm. The test sample consists of 200-μm width rectangular SiO2 boxes processed into a 450 μm-thick single crystal silicon wafer at a 20-μm depth. The SiO2 boxes are separated from each other by 300 µm (inset of Fig. 5). Then, a 1.5 μm-thick photoresist layer AZ1518 is deposited on the whole wafer. The sample is linearly scanned along 0x axis with a step size of 5 μm. In Fig. 5, we present the phase-shift of the reflection coefficient Γ for this one-dimensional scan.
90
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60
50
40
30
20
O x
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Ox (mm)
Fig. 5 Phase-shift of the reflection coefficient Γ for a one-dimensional scan along 0x axis at F=60 GHz.
(c) Fig. 6 (a) Side view of the measurement configuration. (b) Microphotograph of the pattern. (c) 60 GHz image of ||.
This graph shows good measurement sensitivity for the phase-shift of Γ (about 30°) and demonstrates that the SiO2 boxes are easily detected and located. In the following, we demonstrate that the millimeter-wave microscope is suitable for the imaging of solid media with sub-wavelength spatial resolution. A crystal silicon wafer is etched to produce a patterned inclusion at 20-μm depth [Fig. 6(a)]. The pattern consists of four triangle-shaped pockets [see Fig. 6(b)]. The silicon wafer is then covered with a 1-µm thick gold layer. Fig. 6(c) presents a 60 GHz image of the magnitude of obtained by scanning the sample surface (7 µm tip). The measured data return the image of the subsurface pattern with good fidelity, without particular signal processing (raw data).
V . CONCLUSION
In this paper, sensing systems based on the six-port technology have been proposed in the millimeter-wave frequency range. Measurement results show that the six-port technology is a viable and attractive solution for radar applications, spatially resolved dielectric characterization and imaging. However, a lot of other nondestructive testing and evaluation applications requiring features such as low-cost, low-power consumption, compactness and ease of use can benefit from millimeter-wave six-port instrumentation.
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