Integration of Planar Antennas with MMIC Active Frontends for THz Imaging Applications
Abstract: In recent years, there has been constant growth in using THz frequencies or mm, sub- mm wavelengths for various applications such as: Astronomy, Atmospheric studies, security, bio-medical imaging. All these applications are now seen more feasible due to rapid enhancements of semi-conductor processing technologies. The state of the art MMIC processing techniques offering increased cut-off frequencies (> 500 GHz) of HEMT/HBT transistors open up new opportunities for integrating systems on chip along with an antenna for either Transmit/Receive architecture. The work carried out in this thesis mainly deals with the development of antenna structures which are compatible to available MMIC processes and have well defined interface with the active circuit components for microwave as well as mm/sub-mm wave applications. The thesis briefly reviews the THz applications and modern MMIC process techniques. There- after the emphasis is on various possible antenna structures which are feasible to fabricate with MMIC layer topologies. Such antenna structures are further compared in terms of their Gain, Bandwidth, Directivity, Gaussian Coupling Efficiency and Compactness. The main focus of the thesis is towards the development of multi-pixel front ends for THz imaging of concealed weapons for security applications. The requirement in this type of application is the heterodyne detection of reflected THz signals from the distant objects (> 20 m) with tightly integrated pixels constituting of antenna integrated receiver (Antenna + Mixer + LO-Multiplier chain) giving real-time video imaging. Thus the work is focused towards Co-design of Antenna + Mixer aiming towards compactness and minimizing physical area of pixel for tighter integration. One of the important results obtained in this work, is the integration of a Double Slot Antenna with a sub-harmonically pumped resistive mixer. The novelty in this work is the new geometrical placements of slots and microstrip feed network. This new topology has differential excitation of two parallel slots for broadside beam. With this new arrangement, the need of conventional power combining network from two slots is eliminated and the transistors can directly be placed between the two slots, thus minimizing the physical area. Such arrangement is fabricated and tested at frequency of 200 GHz using 50 nm HEMT process. Encouraging results are obtained with mixer conversion loss of ~15 dB with +3 dBm LO power at sub-harmonic of 100 GHz. The next key result of this thesis is the integration of a differential 2 x 2 array of microstrip patch antennas with Gilbert Cell type sub-harmonically pumped mixer. This integration is achieved using 250 nm DHBT process. Considering the antenna ohmic efficiency, mixer conversion loss and gain of IF amplifier; the overall receiver front end features a conversion gain of ~ 14±1 dB at frequency of 320 GHz when pumped with sub-harmonic LO of 160 GHz with ~4 dBm on chip power. This receiver was also tested close to 340 GHz, which is a target frequency for security imaging applications. Another important aspect of this work is to quantify the ability of a planar antenna to couple radiated power in to the THz quasi-optical system. This is often evaluated as Gaussian Coupling Efficiency or Gaussicity. Therefore MMIC integrated antennas are needed to be characterized in terms of their Gaussicity as well. For this, a new algorithm has been developed which accepts the far-field of the antenna as input and computes the optimum beam parameters (waist and its position) which maximize the Gaussicity. Furthermore this algorithm is applied to different antenna array configurations to quantify their radiation pattern for Gaussian Coupling Efficiency.
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