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Enhancing Microwave
Spectroscopy in Application Note
Astrophysics Applications
Overview
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 micro-
wave 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
C ho o se the p erfo rmanc e and f or m
emitted by the atoms and molecules present in the ISM. In this facto r yo u need :
case, the goal is to precisely determine the structure of gas-
phase molecules. 8 1 1 8 0 B and M 8 1 9 0 A W i d eb and
A rb i trary W avefo rm G enerator
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. This application brief describes an approach
that is faster and more accurate than typical methods. The
key element of the solution is an arbitrary waveform generator
(AWG) that can provide a fast broadband signal such as a linear
frequency sweep or "chirp."
Pr ob lem So l uti o n
Our application engineering organization worked with a customer to The revised system addresses the root problem in the original
enhance a system that includes absorption cells. The medium in the system by eliminating the long, slow sweep of a CW signal at a
cells absorbs those parts of the frequency spectrum that resonate series of discrete frequencies. Instead, the system now uses a
with the molecules in the sample material; a detector behind the cell high-performance AWG to provide a fast broadband signal that
makes the actual measurement. stimulates the full range of frequencies in a matter of microsecond
or milliseconds - and then the source is switched off. Because the
In the original system, supersonic jets of gas were being stimulated detector sees only the relaxation spectrum emitted by the absorption
with a continuous-wave (CW) source. It would typically take about 30 cell, measurements have greater sensitivity and dynamic range. This
minutes to step the CW source - one frequency at a time - across the configuration also makes it easy to quickly calibrate the system before
required range of frequencies. or between measurements.
Because the source was running for the duration of the measurement Figure 1 shows a block diagram of the system, which handles experi-
process, the detector would be exposed to both the "relaxation ment control, data acquisition and data analysis. Starting with the
spectrum" from the absorption cell and the emanations from the AWG at the left, system operation proceeds as follows under the
"hot" source and its imperfections such as fluctuations in frequency control of a PC:
and amplitude. These issues were limiting the dynamic range and
sensitivity of the measurement apparatus.
Amplification &
Upconversion
polarization
Antenna &
Switching parabolic
reflectors
Switching Downconversion
I&O
outputs I & O signals
AWG Microwave signal Low-pass filtering
generator
Pxi chassis with
10 MHz
reference
high-speed
digitizers
Figure 1. This simplified system block diagram shows the major signal flows to and from the sample chamber, which contains the antenna and parabolic reflectors.
2
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