Tech Briefs

Enhancing Microwave Spectroscopy in Astrophysics Applications

An arbitrary waveform generator is the key element in this faster and more accurate method.

In popular perception, the vastness of space is an empty vacuum dotted with stars, planets, black holes, and other celestial formations. In reality, astrophysicists have shown that space contains low-density matter — gas clouds, dust grains, and more — existing in ionic, atomic, or molecular phases.

Here on Earth, astrophysicists use techniques such as microwave spectroscopy to investigate the chemical composition of the matter that exists within this so-called interstellar medium (ISM). Similar to the spectrum analyzers used to characterize the content of wireless communication signals, a spectrometer connected to a radio telescope displays the unique spectra emitted by the atoms and molecules present in the ISM. In this case, the goal is to precisely determine the structure of gas-phase molecules.

The basic approach is to use spectroscopy to create a database of “fingerprints” from known gases and then compare the stored readings to those captured with a spectrometer. In most cases, the process of creating the database can be quite time-consuming. The approach described here is faster and more accurate than typical methods. The key element is an arbitrary waveform generator (AWG) that can provide a fast broadband signal such as a linear frequency sweep or “chirp.”

The enhanced system includes absorption cells. The medium in the cells absorbs those parts of the frequency spectrum that resonate with the molecules in the sample material; a detector behind the cell makes the actual measurement. In the original system, supersonic jets of gas were being stimulated with a continuous- wave (CW) source. It would typically take about 30 minutes to step the CW source — one frequency at a time — across the required range of frequencies.

Because the source was running for the duration of the measurement process, the detector would be exposed to both the “relaxation spectrum” from the absorption cell, and the emanations from the “hot” source and its imperfections such as fluctuations in frequency and amplitude. These issues were limiting the dynamic range and sensitivity of the measurement apparatus.

The revised system addresses the root problem in the original system by eliminating the long, slow sweep of a CW signal at a series of discrete frequencies. Instead, the system uses a high-performance AWG to provide a fast broadband signal that stimulates the full range of frequencies in a matter of microseconds or milliseconds — and then the source is switched off. Because the detector sees only the relaxation spectrum emitted by the absorption cell, measurements have greater sensitivity and dynamic range. This configuration also makes it easy to quickly calibrate the system before or between measurements.

Figure 1 shows a block diagram of the system, which handles experiment control, data acquisition, and data analysis. Starting with the AWG at the left, system operation proceeds as follows under the control of a PC:

• An AWG creates a pair of linear frequency chirps, each having a bandwidth of 500 MHz to 1 GHz. These chirps provide the same high resolution of the CW approach, but in much less time.
• A microwave signal generator is used to provide a precise carrier signal for the upconversion. The target frequency range covers 2 GHz to 26 GHz.
• A switch directs the signal generator output to the upconverter during the stimulus phase, and to the downconverter during the measurement phase.
• After amplification and polarization, the upconverted broadband stimulus is directed to the antenna, and the signal is beamed at the sample gas using a parabolic reflector. When exposed to a microwave field, the sample experiences molecular polarization.
• The return signal is reflected back to the antenna, which detects the relaxation spectrum.
• Within the switching system, the return signal is directed to the downconverter for translation into the frequency range of the digitizers. The downconverter outputs are low-pass-filtered to attenuate out-of-band signals and minimize the likelihood of signal fold-back or “aliasing” into the frequency range of interest.
• High-speed PXI digitizers sample the incoming signals. These are converted into the frequency domain via fast Fourier transform (FFT), and the spectral composition of the transient emission is obtained.

Using a frequency chirp as the stimulus has another important benefit: it has inherently phase-invariant repeatability. Because the molecular signal is extremely weak, it is usually far below the level of thermal noise in the system. Phase-synchronized repetitions of the stimulus are essential to improving the signal-to-noise ratio through the phase-correct addition of individual measurements (Figure 2).

In astrophysics applications, the net result of the AWG-based measurement solution is a deeper understanding of the chemical composition of the ISM. This technique is equally applicable, and similarly beneficial, to measurements in other fields. These include molecular physics, allowing basic research such as the fundamentals of molecular physics and the analysis of molecular structures to be performed. In biomolecular spectroscopy, one can investigate and understand the behavior and structure of biological molecules. In environmental research, one can compare measurements to results from satellite-based measurements of chemical activity in Earth’s atmosphere and oceans.

This work was done by Michael Benzinger of Agilent Technologies, Inc. For more information, visit