Gas sensor

Transmitter Array Helps Detect Gas at 245 GHz

June 14, 2016
A four-transmitter SiGe chip with integrated antennas forms part of a spectrometer for detection of gas at 245 GHz.

Terahertz frequencies are becoming more achievable with improvements in semiconductor materials and processing. Such high frequencies have tremendous benefits for medical treatments and in different types of sensors, such as gas sensors. To offer a hint of some of the uses for higher-frequency signals—notably from 238 to 252 GHz—researchers from several German institutions, including IHP in Frankfurt, Germany, developed a transmitter array based on silicon-germanium (SiGe) BiCMOS semiconductor technology. The function blocks included a transmitter, receiver, and a gas Golay absorption cell fabricated in 0.13-μm BiCMOS with SiGe:C heterojunction bipolar transistors (HBTs) characterized by transition frequency of 300 GHz and maximum frequency of oscillation of 500 GHz.

The transmitter array was designed with four transmitters for spatial power combining. Each transmitter includes a two-stage power amplifier, a frequency doubler, and an integrated antenna. The inputs of the transmitters are connected to a Wilkinson power divider, which is fed by a local oscillator (LO). The LO is a 120-GHz voltage-controlled oscillator (VCO) and 1/64 frequency divider with two-stage differential amplifier and external phase-locked loop (PLL). The companion antenna array integrates with the transmitters by means of microstrip interconnections on a silicon substrate. The antenna array is formed of four identical double-folded dipole antenna elements.

A commercial test receiver was used to measure the signal levels from the transmitter array, and the estimated gain based on measurements was 6 dBi at 245 GHz, which compares well with the simulated 7 dBi value. The transmit array module was formed by bonding the semiconductor chip with antennas and transmitters to a plug-in board, which was then mounted on a carrier board with a commercial PLL. The gas spectrometer was developed by combining the transmit module with a separate SiGe receiver module operating from 238 to 252 GHz. See “245-GHz Transmitter Array in SiGe BiCMOS for Gas Spectroscopy,” IEEE Transactions on Terahertz Science and Technology, Vol. 6, No. 2, March 2016, p. 318.

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About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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