DDSBASED FAST SCAN SPECTROMETER Eugen A Alekseev Institute

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

BWO-based frequency synthesizer directional coupler (beamsplitter) BWO power source Ucorr BWO Absorption cell f. BWO n·fref PLL Reference synthesizer 2 Reference synthesizer 1 harmonic mixer fsynth=n·fref 1±fref 2 DDS can be used in both reference synthesizers

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

DDS : advantages and application in spectroscopy Extremely high resolution (10 -6 Hz) Very fast frequency switching (10 ns) Frequency switching with continuous phase PTSx 10: • DDS-based synthesizer • without FM • typical frequency switching time: <2 µs Narrow band fast scan TAMU fast scan submillimeter-wave spectrometer Our objective: wide band fast scan with frequency synthesis + lock-in detection • minimum distortion in lineshape • high measurement accuracy B. A. Mc. Elmurry et al. J. Chem. Phys. 119 10687 -10695 (2003)

DDS-based reference synthesizer Frequency range : 8. 7 – 11. 8 MHz Frequency switching time (min) : 15 µs (serial data loading) external modulation signal fmod GPS reference 10 MHz Q-filter 10 MHz Δf=1 k. Hz PLL 50 MHz Δf=1 k. Hz Quartz oscillator 50 MHz f 0+∆f f 0–∆f Filter 8. 7 – 11. 8 MHz AD 9834 Display Internal bus RS-232 data from/to PC GPIB Emulator ADC microcontroller ADu. C 842

Board of DDS-based synthesizer Analog adapter RS-232 ADu. C 842 Quartz filter GPIB Synthesizer 50 MHz DDS

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 frequency switching measurement time 1τ 2τ 3τ 4τ 5τ • 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 tfsw = 50 µs → τ = 10 µs → fmod>500 k. Hz fmod should not exceed linewidth!!! tfsw = 500 µs → τ = 100 µs → fmod>50 k. Hz (typically 90 – 100 k. Hz)

DDS-based fast scan spectrometer in Lille Square-wave modulation detection on odd harmonics (n=1, 3, …) Typical measurement cycle (0. 05 MHz frequency step) Scan over 100 MHz 2000 points 1 s Data transfer from DSP to PC 1 s Adjusting high voltage 500 ms Scan over 100 MHz 2000 points

Performances of the spectrometer Square-wave modulation – lock-in detection on odd harmonics Lamb dip simple OCS J=53← 52 f 0=643862. 9285(10) MHz from CDMS database f 0=643862. 930 MHz f 0=643862. 9289 MHz Sensitivity test: • single acquisition • frequency switching time: 1 ms • time constant: 200 µs 18 O 13 CS α=1. 06· 10 -6 cm-1 • for a strong isolated line the measurement uncertainty can be estimated at 0. 001 – 0. 005 MHz level • but typically, taking into account experimental conditions we estimate the measurement uncertainty as 0. 01 – 0. 03 MHz

Aziridine c-C 2 H 4 NH 585 – 660 GHz→ • single acquisition • frequency step: 0. 05 MHz • total acquisition time: 48 min 611. 5 – 613. 5 GHz→ 612. 6 – 612. 8 GHz→

Methyl formate HCOOCHD 2 630 – 663 GHz→ • single acquisition • frequency step: 0. 05 MHz • total acquisition time: 18 min 645 – 645. 6 GHz→ TJ 05 by L. Coudert TJ 11 by L. Margulès

Methanol CH 2 DOH 577 – 660 GHz→ • single acquisition • frequency step: 0. 05 MHz • total acquisition time: 56 min 622 – 626 GHz→ 624. 4 – 624. 6 GHz→ J=12← 11 J=13← 12 (TJ 05 by L. Coudert)

Methanol CH 2 DOH J=5← 4 215 – 230 GHz→ • single acquisition • frequency step: 0. 035 MHz • total acquisition time: 15 min

Solide state spectrometer in Lille Active sextupler 75 – 105 GHz//15 d. Bm Synthesizer Agilent E 8257 D 12. 5 -17 GHz x 6 x 2 150 - 215 GHz x 3 230 - 315 GHz x 5 400 - 530 GHz x 3 500 -645 GHz x 3 700 -945 GHz SR 7270 DSP Lock-in amplifier Absorbing cell In. Sb Bolometer Amplifier Ethernet hub 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