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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