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Getting closer to the electronic nose, via a 250-GHz route

We have low-cost, high-performance sensors for so many real-world phenomena: light, sound, images, motion, pressure, heat, temperature, energy, power…the list goes on. But good sensors which replicate the versatile human or animal nose are still elusive.

Researchers see this gap, of course, and merging extreme RF capabilities and abstract molecular principles to advance the state of the art, such as for the detection and analysis of breath-exhaled gas. A new device developed by a team at the University of Dallas (Texas) in conjunction with a team from Ohio State University ( Columbus, Ohio) is electronic nose can detect gas molecules with more specificity and sensitivity than so-called “breathalyzers,” which can confuse acetone for ethanol in the breath. The researchers note that the distinction is important, for example, in cases of patients with Type 1 diabetes, who often have high concentrations of acetone in their breath.

To make this electronic, molecule-specific nose required extremely sophisticated >250-GHz ICs and circuitry. This was combined with a “rotational spectroscopy” arrangement, which measures the energies of transitions between quantized rotational states of molecules in the gas phase. The spectra of polar molecules can be measured in absorption or emission by microwave spectroscopy or by far infrared spectroscopy, Figure 1 . Certainly, this is a very advanced project to plan and then implement.

Figure 1

The rotational spectroscopy setup (using a receiver from Virginia Diodes, Inc.) shows the complexity and advanced technologies needed for this type of 250-GHz gas-sensing arrangement (from University of Texas/Dallas).

The rotational spectroscopy setup (using a receiver from Virginia Diodes, Inc.) shows the complexity and advanced technologies needed for this type of 250-GHz gas-sensing arrangement (from University of Texas/Dallas).

The University of Texas press release, “Scientific Gains May Make Electronic Nose the Next Everyday Device,” Is an interesting overview of the team and project, but obviously relatively non-technical. Fortunately, the IEEE paper which the researchers published is available online “200-280 GHz CMOS RF Front – End of Transmitter for Rotational Spectroscopy” and is quite informative and readable, with useful graphics, despite its intensity and short, two-page length. The authors make the objective and situation clear in the paper.

“Rotational spectroscopy enables detection of gas molecules with absolute specificity and excellent sensitivity as well as determination of concentration. Presently, rotational spectrometers are implemented with compound semiconductor devices that are bulky and costly. A transmitter for the spectrometer should generate an FM signal that can be scanned over ~100-GHz frequency range with a 10-kHz step. The transmitted power level should be -30 to -10 dBm to avoid the saturation of gas molecules in a sample.”

All I can say is this: “wow, those are very tough objectives and very impressive results.” The level of RF system, circuit, modeling, and analytical insight needed for at these frequencies, combined with CMOS process understanding and capability, is just astounding. The final circuit board, Figure 2 , is an amazing aggregation of advanced, diverse technologies and know-how.

Figure 2

The transmitter PC board of the rotational spectroscopy system measures approximately 47 × 110 mm; 'CHIP1' contains most of the GHz-range active circuitry and transmission lines (from University of Texas/Dallas).

The transmitter PC board of the rotational spectroscopy system measures approximately 47 × 110 mm; “CHIP1” contains most of the GHz-range active circuitry and transmission lines (from University of Texas/Dallas).

Perhaps in a decade or two we'll have wide-range electron noses using MEMS-based sensors which are able to quantitatively sense and characterize thousands of odors at very low levels, rather than be targeted at a specific class of molecule. After all, that's what a bloodhound’s nose can do so very well, with the 300 million olfactory receptors in their noses, compared to about six million in humans, Figure 3 . But until then, we'll look at CMOS devices in the hundreds of GHz to do what they can.

Figure 3

When a bloodhound dog breathes in, the air separates into distinct paths, one (red) flowing into the olfactory area and the other (blue) passing through the pharynx (black) to the lungs (from PBS and Brent Craven).

When a bloodhound dog breathes in, the air separates into distinct paths, one (red) flowing into the olfactory area and the other (blue) passing through the pharynx (black) to the lungs (from PBS and Brent Craven).

Have you had any experience with gas sensors? Did you have to sense multiple gases, or just one?

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