Development of a Reduced-Cost CP-FTMW Spectrometer Using Direct Digital Synthesis Ian Finneran Daniel Holland Brandon Carroll Geoffrey Blake California Institute of Technology Divisions of Geological & Planetary Sciences and Chemistry & Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA

Motivation: Maximize Bandwidth/Sensitivity for Given Budget B. C. Dian et al., Science 320, 924 (2008). G. G. Brown et al., Review of Scientific Instruments 79, (2008).

G. G. Brown et al., Review of Scientific Instruments 79, (2008). D. A. Obenchain et al., Journal of Molecular Spectroscopy 261, 35 (2010). B. Reinhold et al., Journal of Molecular Spectroscopy 270, 89 (2011).

Main Costs for CP-FTMW Spectroscopy at 8-18 GHz AWG/AFG Amplifier Digital Sampling ~$10k-$40k per GHz bandwidth ~$1k/W – low end $100-$500/W – high end ~$10k-$20k per GHz bandwidth Total Cost: ~$120k 2 GHz BW 1 W – 200 W per GHz

Direct Digital Synthesis (DDS) Frequency Time Chirped pulse specs needed: ~1 μs pulse length High bandwidth Phase stable Low trigger latency Dan Holland: Asynchronous Optical Sampling (ASOPS) Time Domain THz Spectrometer

A Small, Lightweight, Low-Power 2 GHz BW DDS Solution for <$ GS/s AWGDDS/PLL Solution DDS Percent of AWG Bandwidth2 GHz 100% Price~$50,000 (as low as $10,000 used) $775~1.5% (new vs. new) Power~200 W3 W~1.5% Size1,400 cm 3 42,000 cm 3 ~3.3% Weight14.1 kg0.24 kg~1.7% PLL board DDS board DDS Drawbacks Not arbitrary 1 chirped pulse/trigger Requires low end computer/arduino to load control word Less bandwidth (for now)

4 GHz PLL Sample Clock Source 6 GHz PLL LO Source DDS Chirp Source

Super-Nyquist content with 1 GHz chirped pulse 2 GHz sample clock

~$70,000, 40 W, 3.8 GHz (DSB) solution 2.1 kHz repetition rate (70 FIDs per gas pulse) I. Peña et al., The Journal of Physical Chemistry Letters 4.1 (2012):

LO = 11.9 GHz 31 seconds of data collection or 65,000 averages PLL is internally clocked (for now) S:N = ~350:1 (CH 3 ) 2 CO EE (CH 3 ) 2 13 CO

170 million Averages (22 hours), LO = 11.9 GHz (CH 3 ) 2 13 CO ( 13 CH 3 ) 2 CO (CH 3 ) 2 CO S:N = ~18,000: EE P. Groner et al., The Astrophysical Journal Supplement Series 142, 145 (2002). J. Swalen and C. Costain, The Journal of Chemical Physics 31, 1562 (1959). F. Lovas and P. Groner, Journal of Molecular Spectroscopy 236, 173 (2006).

Very Tentative Super-Nyquist (CH 3 ) 2 C 18 O Detection ( ) = MHz MHz EE S:N = ~0.2% of super- Nyquist normal species R. Nelson and L. Pierce, Journal of Molecular Spectroscopy 18, 344 (1965).

Chirp Source: DDS/PLL - $775 LO source*: PLL - $150 Chirp Source: AWG ($50k-$100k) LO source: MW Synthesizer (~$7k) *Doubler = $360, not needed for newest PLL

Chirp Source: AWG ($50k-$100k) LO source: MW Synthesizer (~$7k) Chirp Source: DDS/PLL - $775 LO source*: PLL - $150 *Doubler = $360, not needed for newest PLL (CH 3 ) 2 CO LO=11.9 GHz 10 million averages (1.3 hours)

( MHz rest frequency, LO = MHz)

Conclusion DDS+PLL combination for 2 GHz chirped pulse: $775 – ~$170 if you make your own circuit board – Bandwidth limited to 2 GHz (for now) PLL for LO: $150 – $10+VCO if you make your own circuit board Super-Nyquist content could be used for bandwidth extension Size, weight, power reductions for in situ applications Future applications in segmented chirp spectroscopy are possible

Future directions: LO source for millimeter-wave segmented chirp pulse spectroscopy?

Acknowledgements NSF (grant CHE and the Graduate Research Fellowship Program) NASA (grant NNX09AM84G) The Pate lab Steve Shipman Jeff Groseth Bob Dengler The rest of the Blake lab – Brett McGuire