Linus Boehm

A number of millimeter-wave (mm-wave) integrated radar sensors above 100 GHz were proposed in the past five years. The comparability between these radar systems is limited due to different semiconductor processes, synthesizer topologies, and external periphery. Especially, the issue of using a monostatic or bistatic radar monolithic microwave integrated circuit (MMIC) for single-chip sensors is not discussed in detail. This paper provides a comparison between the different properties of bistatic and monostatic MMICs based on two realized almost identical silicon germanium (SiGe)-MMICs at 154 GHz. In the monostatic case, the influence of the transmit-receive coupler on the performance of the system is explained. In the bistatic case, the illumination of a focusing lens and the leakage from transmitter to receiver are investigated. All properties are verified by measurements. The system performance of the monostatic and bistatic MMIC are compared by the calculated link and noise budget. The detection performance and the signal-to-noise ratio performance are evaluated in two radar measurements. Limitations and suggestions for improvement are given for monostatic and bistatic mm-wave frequency-modulated continuous-wave radar MMICs.

On-chip antennas are frequently used as radiating elements for MMICs operating above 100 GHz. Their radiation pattern is wide since the radiator extension is typically about half a wavelength. In a package the wide radiation pattern is easily disturbed by interference from parasitic radiation of the package. In this paper an approach to minimize and adjust the interference of the package is proposed and applied to a QFN housing. A structured metal sheet positioned between chip and exposed pad is used to generate enough degrees of freedom for the optimization of the 3D-radiation pattern in open-cavity and molded QFN packages. For the open-cavity QFN package a U-shaped slot is introduced, which reduces the propagation of surface waves to the sides and creates a uniform radiation pattern. For the molded QFN package with opening around the on-chip antenna a corrugated metal sheet is introduced. This leads to a robust packaged radar MMIC with on-chip antenna at 160 GHz, which provides a directed radiation pattern to boresight. Both optimized radiation patterns are well-suited for the illumination of dielectric lenses, which results in a higher lens gain and a lower side lobe level of the lens pattern compared to the standard QFN packaging. The proposed concepts are verified by measurements for the open-cavity and molded QFN package at 160 GHz.

The increasing demand for radar sensors and the wide distribution of handheld communication devices push the development of low-cost components with a small form factor. One approach to achieve smaller, low-cost devices is the integration of the required components on a monolithic microwave-integrated circuit (MMIC). At frequencies above 100 GHz, passive components are small enough to be integrated onto a chip, and the advances in semiconductor technology make it possible to build active components that can operate at millimeter (mm)-wave frequencies [1], [2]. The available bandwidths of several gigahertz at mm-wave frequencies offer high data rates for communication devices or high resolution for remote-sensing applications. By integrating the radio-frequency (RF) components and the antennas, lossy off-chip transitions can be avoided, thus limiting the required connections to the power supply and baseband signals.

Feeding integrated antennas during a measurement requires special feeding structures. Due to the small antenna dimensions, these feeding structures are often much bigger than the antenna under test (AUT) itself and with chip sizes of around 1 mm ² , the achievable separation between antenna and feed is limited. Wafer probes have to be used to feed the AUT during passive antenna measurements and present a large reflective surface in close proximity to the AUT. Reflections from the wafer probe cause interference on the measurement surface and distort the results. The same is true for active antenna measurements, where bondwires and the package can have a significant effect on the radiated fields. The fragility and size of the components do not allow to reduce reflections with absorbers, which is why modal filtering was used in this paper to mitigate undesired reflections and improve the measurement result through postprocessing. Two issues that limit the performance of the algorithm are discussed, namely, phase center inaccuracies of the AUT and a limited measurement surface. It is shown that modal filtering is applicable to integrated antenna measurements at frequencies over 100 GHz and that a significant improvement in the measured radiation pattern can be achieved. Furthermore, it is shown that the postprocessed results make it possible to measure the directivity of integrated antennas, despite strong probe reflections.