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

DDS-BASED FAST SCAN SPECTROMETER

Eugen A. AlekseevInstitute 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

BWOpower source

Reference synthesizer

1

Ucorr

fBWO

n·fref

Reference synthesizer

2

fsynth=n·fref1±fref2

directional coupler(beamsplitter)

Absorption cell

harmonic mixer

DDS can be used in both reference synthesizers

Direct Digital Synthesizer

DACΣ Phase register

ROM sine

lookup

fout

System clock fref (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. 119 10687-10695 (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 MHzΔf=1kHz

Quartz oscillator

50 MHz

Filter 8.7 – 11.8

MHz

Display

ADCGPIB Emulator

RS-232

Internal bus

fmod

microcontroller ADuC 842

f0+∆f

f0–∆fAD9834

external modulation signal

data from/to PC

GPS reference10 MHz

Frequency range : 8.7 – 11.8 MHzFrequency switching time (min) : 15 µs (serial data loading)

Board of DDS-based synthesizer

Quartz filter

Synthesizer 50 MHz DDS

GPIBRS-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 4τ 5τ

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

tfsw = 50 µs → τ = 10 µs → fmod>500 kHz fmod should not exceed linewidth!!!

tfsw = 500 µs → τ = 100 µs → fmod>50 kHz (typically 90 – 100 kHz)

time

DDS-based fast scan spectrometer in Lille

Scan over 100 MHz2000 points

Data transfer from DSP to PC

Adjusting high

voltage

Scan over 100 MHz2000 points

1 s 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

f0=643862.9289 MHzf0=643862.930 MHz

18O13CS

α=1.06·10-6 cm-1

Sensitivity test: • single acquisition• frequency switching time: 1 ms• time constant: 200 µs

OCS J=53←52f0=643862.9285(10) MHz

from CDMS database

simple Lamb dip

• 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

Square-wave modulation – lock-in detection on odd harmonics

Aziridine c-C2H4NH

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 HCOOCHD2

630 – 663 GHz→• single acquisition• frequency step: 0.05 MHz• total acquisition time: 18 min

645 – 645.6 GHz→

TJ05 by L. CoudertTJ11 by L. Margulès

Methanol CH2DOH

622 – 626 GHz→

624.4 – 624.6 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 CH2DOH

215 – 230 GHz→• single acquisition• frequency step: 0.035 MHz• total acquisition time: 15 min

J=5←4

Solide state spectrometer in Lille

Absorbing cell

SR7270DSP Lock-in

amplifier

InSbBolometer

Amplifier

SynthesizerAgilent E8257D

12.5-17 GHz

Active sextupler75 – 105 GHz//15 dBm

Ethernet hub

x6

x2 150- 215 GHz

x3 230- 315 GHz

x5 400- 530 GHz

x3500-645 GHz

x3700-945 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