Integrated circuit techniques for thz wave rotational spectroscopy

Mansha, Muhammad Waleed
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Shur, Michael
Zhang, Tong
Wilke, Ingrid
Hella, Mona
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Electrical engineering
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This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute (RPI), Troy, NY. Copyright of original work retained by author.
Full Citation
The frequency bands beyond 100 GHz promise large bandwidth that can be utilized forseveral applications ranging from high-data-rate wireless backhaul for 5G communication systems, to high-resolution radars for precise position measurement, and chemical sensing using rotational spectroscopy. The detection and generation of millimeter-wave/THz signals is critical for enabling such applications. THz wave rotational spectroscopy is a characterizing technique which utilizes the characteristic absorption of THz wave photons by polar molecules, to identify them. It offers distinct advantages for the detection of polar volatile molecules compared to other analytical techniques. For the implementation of THz wave detectors, several technology options are available that vary in terms of their sensitivity, specificity, size, cost and required operating temperatures. THz wave detectors using field effect transistors (FETs) fabricated in standard silicon processes provide a viable option for cost-effective, room temperature detection. A THz detector comprised of a single FET is fabricated and shown to provide the sensitivity needed for rotational spectroscopy. To reduce the loading effect of the readout circuitry on the FET detector, an N-FET detector architecture is proposed and a special case with two FET elements is presented. The detector with two FETs fed from a single dual feed antenna showed improved performance and was employed in a rotational spectroscopy setup for the demonstration of the detection of more than one gas in a mixture. Efficient signal generation in the mm-wave frequency range continues to be a challenge for silicon-based processes (CMOS/SiGe) despite recent advances producing transistors with ft/fmax beyond 300 GHz. To generate higher power, the output from multiple signal sources is combined spatially in air or physically on chip. A signal source architecture comprised of a loop of four coupled fundamental oscillators which feed a single quad-feed octagonal slot antenna is presented. The coupled oscillator loop is designed to operate with a phase difference that allows the quad-feed antenna to combine as well as radiate the output power from each oscillator without the need for a dedicated power combining network. The fabricated signal source at 148 GHz achieves good DC-EIRP efficiency. However, the tuning range of the fundamental signal source is small due to the poor-quality factor of varactors at THz frequencies. One method of increasing the tuning range is through the utilization of phase shifters between the oscillators in the coupled loop. Two integrated phase shifter topologies are described that offer the fine phase resolution needed for frequency tuning in coupled oscillator loops. A coplanar waveguide (CPW) based transmission line phase shifter is a passive and scalable approach that offers bidirectional, true time delay characteristics. However, the signal loss, chip area, and aspect ratio are some of its limiting factors. A 5-bit digitally controlled CPW-based phase shifter is designed and fabricated for operation in the W-band from 75 GHz to 110 GHz. Simulated results of the phase shifter shows that over 90 degree phase control can be achieved at 100 GHz with a phase resolution of less than 4 degree. In a vector modulator phase shifter, full 360 degree phase control is possible at the expense of additional dc power consumption and loss of true time delay characteristics. An active vector modulator phase shifter is also designed and fabricated for operation at the center of the E-band from 65 GHz to 85 GHz. The phase shifter offers signal gain as well as continuous control over the output phase. To overcome the limited tuning range of existing signal generators operating at mmWave frequencies, a configurable harmonic generation architecture is proposed which utilizes controlled integrated phase shifters and can be configured to generate the fundamental, second or third harmonic signals, resulting in an overall wider fractional bandwidth than Nth harmonic oscillators. The signal source architecture consists of a loop of six coupled oscillators with a controlled phase shifter following each oscillator in the loop. A proof-of-concept signal source based on the proposed architecture is designed and fabricated for second harmonic signal generation beyond 150 GHz. An improvement to the proposed signal source architecture is proposed which can increase the harmonic output power and provide amplitude modulation to the THz wave signal source output.
School of Engineering
Dept. of Electrical, Computer, and Systems Engineering
Rensselaer Polytechnic Institute, Troy, NY
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