Integration of Planar Antennas with MMIC Active Frontends for THz Imaging Applications

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

140-220 GHz Imaging Front-end Based on 250 nm InP/InGaAs/InP DHBT Process

This paper presents a pre-amplified detector receiver based on a 250 nm InP/InGaAs/InP double heterojunction bipolar transistor (DHBT) process available from the Teledyne scientific. The front end consists of a double slot antenna followed by a five stage low noise amplifier and a detector, all integrated onto the same circuit. Results of measured responsivity and noise are presented. The receiver is characterized through measuring its response to hot (293) and cold (78) K terminations. Measurements of the voltage noise spectrum at the video output of the receiver are presented and can be used to derive the temperature resolution of the receiver for a specific video bandwidth

Factorization of Gaussian Coupling Efficiency and algorithm to compute it

Gaussian Coupling Efficiency (η G) quantifies the amount of radiated power getting coupled to fundamental gaussian beam mode. This paper focus on the computations of η G for antennas from their far-field data. The mathematical analysis presented in the paper shows that the term η G can be factorized into sub-efficiencies namely, BOR 1, spill over, polarization and BOR 1 to Gaussian Coupling. This analysis is based on the definition of Body of Revolution type of antennas (BOR). Based on the analysis, an algorithm is developed to compute η G of Antennas. This algorithm accepts the far-field data of antenna as input and computes the optimum values of Beam Waist ω( o) and it's location (z o) for maximized η G. The algorithm is verified using Smooth Conical Horn. The beam parameters (ω o & z o) of Conical Horn obtained from near field measurements agree well with the beam parameters obtained from the simulated far-field. The 44 Microstrip patch Array is studied using this analysis and found that η G 70% is feasible for 320-350 GHz bandwidth

Imaging front-end for thermal detection using an InP DHBT process

This paper presents a 153-162 GHz pre-amplified power detector receiver based on a 250 nm InP/InGaAs/InP double heterojunction bipolar transistor (DHBT) process. The front end consists of a double slot antenna followed by a five stage low noise amplifier and a detector. The receiver is characterized through measurements of its response to broadband hot and cold terminations. A simplified method is presented for calculation of the temperature resolution of the receiver from measurements of the detected DC voltage and the noise spectrum at the video output of the receiver. The calculated temperature resolution for the receiver is 2.7 K at 1 ms integration time. This work addresses the need for low cost compact solutions suitable for multi pixel thermal imaging systems

A compact 340 GHz 2x4 patch array with integrated subharmonic gilber core mixer as a building block for multi-pixel imaging frontends

For linear multi-pixel imaging systems, a linear stack of pixels comprising of an antenna and a heterodyne receiver are needed. Such pixels can be realized using MMIC processes. The main constraint for such multi-pixel system is a compact array of pixels giving high coupling to quasi-optics used for focusing. This paper addresses this trade-off and presents a novel solution based on beam synthesis of two consecutive subarrays. One such sub-array along with heterodyne receiver is described as half-pixel in this paper and it is realized using 2x4 patch array and Gilbert core sub-harmonic mixer using a 250nm DHBT process. The patch array has ohmic loss better than 8 dB and mixer conversion loss is 6-8 dB over 320-350 GHz RF band. The chip size is 1mm x 2mm and therefore for 7 simultaneous beams a MMIC of 8 half-pixels is foreseen

A 340 GHz High Gaussicity Smooth Spline Horn Antenna for the STEAMR Instrument

We present the design, fabrication and measure-ments of a smooth walled spline feed horn antenna for the satellite borne climate research instrument STEAMR operating at 340GHz. A method has been developed which, for a certain desired beam waist, can be used to optimize the horn profile for high Gaussicity and ultra-low sidelobes. The simulated performance of the horn achieves a beam waist of 1.9 mm over the band 323-357 GHz with Gaussian coupling efficiency exceeding 98%. The peak cross-polar sidelobes are below -28 dB over the required frequency band. For cost effective manufacturing with high repeatability, the smooth wall spline profile is drilled in out from a metal block using a custom made broach. To validate the design and fabrication, planar measurements of the phase and amplitude have been performed and from measured E-field vital horn parameters have been extracted

A 874 GHz Mixer Block Integrated Spline Horn and Lens Antenna for the ISMAR Instrument

In this paper, we present the design of a 874 GHz mixer block-integrated antenna for the ISMAR Instrument. The spline horn antenna with a dielectric lens (Teflon) is used to fulfill the requirement of an on-axis Directivity exceeding 32 dBi. The well designed spline horn offers very high Gaussian Coupling, exceeding 97%. This property of spline horn further helps to achieve very rotationally symmetric beam even after the lens with very low cross-polarization (< -30 dB) as well as low side-lobe levels (< -28 dB). The integrated spline horn with lens has far-field Full Width at Half Maximum of 4.2±0.2° over the frequency range of 860 GHz to 890 GHz. The simulated antenna losses are less than 0.5 dB and the antenna input reflection coefficient including the circular to rectangular waveguide (WR1.2) transition is better than -20 dB

340 GHz Integrated Receiver in 250 nm InP DHBT Technology

A 340 GHz integrated receiver based on a 250 nm InP DHBT technology is presented in this paper. It consists of a 2X2 differential patch array antenna, a sub-harmonically pumped Gilbert mixer and an IF buffer amplifier. Performance of the integrated receiver is evaluated by setting up a RF link in the frequency band of 302-338 GHz. At 338 GHz RF and 170 GHz LO, the peak conversion gain of 11.8 and 14.0 dB is achieved with and without antenna, respectively. A double-sideband noise figure of 17 dB at room temperature is obtained from direct noise figure measurement