Microwave spectroscopy is an invaluable tool for studying the structure, dynamics, and even the handedness of gas phase species. In particular, the specificity of microwave spectroscopy has been central to the unambiguous identification of the great majority of molecules detected in the interstellar medium. Applications of microwave techniques to problems in physical chemistry and molecular astrophysics have been greatly accelerated by developments in laboratory techniques.
There are three major components of CP-FTMW spectrometers that enable the collection of broadband spectra: high-samplerate digitizers for acquisition of the microwave emission, broadband high-power amplifiers (based on either solid-state or traveling wave tube technology) to ensure sufficient power for sample polarization, and arbitrary waveform generators (AWGs) for producing the chirped polarization pulse. In state-of-the-art, multi-GHz-bandwidth instruments, each of these major components carries significant cost. AFGs only lower the overall cost by significantly truncating the instrument bandwidth. AFGs and AWGs are expensive in part because they are designed to generate extremely complex pulse sequences.
Chirped pulse Fourier transform microwave spectrometers have become the instrument of choice for acquiring rotational spectra due to their high sensitivity, fast acquisition rate, and large bandwidth. However, the high cost of the required circuitry has hindered the truly widespread adoption of this approach. A newly constructed CP-FTMW spectrometer using direct digital synthesis was developed for chirped pulse generation.
Direct digital synthesizers (DDS) are Nyquist devices with good frequency agility and low phase noise. Using an external sample clock and digital control word, a DDS generates a tunable digital signal with a numerically controlled oscillator, which is then converted into sinusoidal output with a digital-to-analog converter (DAC). Their frequency agility and low phase noise capabilities have been utilized for longer broadband frequency sweeps at millimeter-wave frequencies for radar imaging applications, as well as slow, narrowband frequency sweeps (3 MHz) in a millimeter-wave fast scan absorption spectrometer.
In this work, a DDS chip was used to generate short (~1 μs), broadband (1.9 GHz) linear frequency sweeps. The DDS and phase locked loop (PLL) frequency synthesizer clock source evaluation board combination used here reduces the chirp generation cost to well under $1,000/GHz. A second PLL board functioning as the local oscillator (LO) source is also one to two orders of magnitude less expensive than the microwave synthesizers and phase locked oscillators used in other CP-FTMW spectrometers. The total power requirement of the DDS and PLL boards is only 3W, considerably less than the power draw of an AWG, which is 100- 500W. The DDS and PLL boards are also much smaller and lighter. These improvements could further enable the development of compact, in situ instruments for (inter)planetary measurements, especially if integrated into a mmwave spectrometer.