DDS-BASED FAST SCAN SPECTROMETER Eugen A. Alekseev Institute of Radio Astronomy of NASU, Kharkov, Ukraine Roman A. Motiyenko and Laurent Margulès Laboratoire PhLAM, Université Lille 1, France

BWO-based frequency synthesizer BWO PLL BWO power source Reference synthesizer 1 U corr f BWO n·f ref Reference synthesizer 2 f synth =n·f ref1 ±f ref2 directional coupler (beamsplitter) Absorption cell harmonic mixer DDS can be used in both reference synthesizers

Direct Digital Synthesizer DAC Σ Phase register ROM sine lookup f out System clock f ref (up to 1 GHz) Tuning data word M (24-48 bits)

DDS : advantages and application in spectroscopy  Extremely high resolution (10 -6 Hz)  Very fast frequency switching (10 ns)  Frequency switching with continuous phase PTSx10: DDS-based synthesizer without FM typical frequency switching time: <2 µs B.A. McElmurry et al. J.Chem.Phys (2003) TAMU fast scan submillimeter-wave spectrometer Narrow band fast scan Our objective: wide band fast scan with frequency synthesis + lock-in detection minimum distortion in lineshape high measurement accuracy

DDS-based reference synthesizer Q-filter 10MHz Δf=1kHz PLL 50 MHz Δf=1kHz Quartz oscillator 50 MHz Filter 8.7 – 11.8 MHz Display ADCGPIB Emulator RS-232 Internal bus f mod microcontroller ADuC 842 f 0 +∆f f 0 –∆f AD9834 external modulation signal data from/to PC GPS reference 10 MHz Frequency range : 8.7 – 11.8 MHz Frequency switching time (min) : 15 µs (serial data loading)

Board of DDS-based synthesizer Quartz filter Synthesizer 50 MHz DDS GPIB RS-232 Analog adapter ADuC842

5τ rule  In a system which is characterized by a time constant τ the response to a transient process can be determined at 99% level of confidence after the period of 5τ 1τ 2τ 3τ 4τ5τ frequency switching measurement In a system with FM signal and lock-in detection with time constant τ, the modulation frequency should be at least 5/τ or higher t fsw = 50 µs → τ = 10 µs → f mod >500 kHz f mod should not exceed linewidth!!! t fsw = 500 µs → τ = 100 µs → f mod >50 kHz (typically 90 – 100 kHz) time

DDS-based fast scan spectrometer in Lille Scan over 100 MHz 2000 points Data transfer from DSP to PC Adjusting high voltage Scan over 100 MHz 2000 points  1 s  500 ms Typical measurement cycle (0.05 MHz frequency step) Square-wave modulation detection on odd harmonics (n=1, 3,…)

Performances of the spectrometer f 0 = MHz f 0 = MHz 18 O 13 CS α =1.06·10 -6 cm -1 Sensitivity test: single acquisition frequency switching time: 1 ms time constant: 200 µs OCS J=53 ← 52 f 0 = (10) MHz from CDMS database simple Lamb dip for a strong isolated line the measurement uncertainty can be estimated at – MHz level but typically, taking into account experimental conditions we estimate the measurement uncertainty as 0.01 – 0.03 MHz Square-wave modulation – lock-in detection on odd harmonics

Aziridine c-C 2 H 4 NH 585 – 660 GHz → single acquisition frequency step: 0.05 MHz total acquisition time: 48 min – GHz → – GHz →

Methyl formate HCOOCHD – 663 GHz → single acquisition frequency step: 0.05 MHz total acquisition time: 18 min 645 – GHz → TJ05 by L. Coudert TJ11 by L. Margulès

Methanol CH 2 DOH 622 – 626 GHz → – GHz → (TJ05 by L. Coudert) 577 – 660 GHz → single acquisition frequency step: 0.05 MHz total acquisition time: 56 min J=12 ← 11 J=13 ← 12

Methanol CH 2 DOH 215 – 230 GHz → single acquisition frequency step: MHz total acquisition time: 15 min J=5 ← 4

Solide state spectrometer in Lille Absorbing cell SR7270 DSP Lock-in amplifier InSb Bolometer Amplifier Synthesizer Agilent E8257D GHz Active sextupler 75 – 105 GHz//15 dBm Ethernet hub x6 x GHz x GHz x GHz x GHz x GHz Perspectives: extension of the frequency range up to 1 THz new solid state detectors W-band BWO at first stage of frequency multiplication chain

Akcnowledgements  Programme National de Physique Chimie du Milieu Interstellaire  Université Lille 1