The microwave circuit of the DDS-based chirped pulse spectrometer. Microwave reference connections are highlighted for clarity. See text for design details.
The pulse timing used for collecting multiple FIDs per gas pulse. See text for design details.
A comparison between the filtered chirped pulse outputs of an AWG (left) and a DDS (right) after 100 000 averages. The FFT of the waveforms are very similar, except for a small dip in the intensity of the DDS at 1.4 GHz. A spectrogram of the chirped pulses (top) indicates that the AWG and DDS have similar harmonic distortion in their output. The off diagonal content in the spectrogram is down by 30 dB from the main frequency sweep in both the DDS and the AWG.
The phase stability of the DDS spectrometer was compared to that of the AWG spectrometer via emission from the 321-312 AA transition in acetone (10762.6 MHz rest frequency, LO = 11 900 MHz). Both setups show little deviation from averaging.
The dual sideband convolved spectrum of acetone from 9.95 to 13.85 GHz (LO = 11.9 GHz) after 22 h of averaging (170 million FIDs). (a) The experimental spectrum (positive going) is plotted against a prediction of the normal species (negative intensity). The signal-to-noise is 18000:1 on the strongest transition (211-202 EE). Super-Nyquist content of the normal species at 1.5-1.9 GHz is plotted with a different scaling and highlighted in the dashed box. (b) An inset of the spectrum showing experimental and predicted 211-202 transitions of the carbonyl 13C substituted species of acetone at 2 K. (c) An inset of the methyl 13C substituted species of acetone at 2 K showing four of the 321-312 transitions.
The spectrum of acetone after 30 s of data collection (top), with an inset showing the detection of the 13C isotopomer at more than 3:1 signal-to-noise (bottom). LO = 11 900 MHz.
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