Gas sensor

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Transmitter Array Helps Detect 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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