Power spectral density accuracy in Chirp Transform Spectrometers

149 Pages·2008·8.08 MB·English

Power spectral density accuracy in
Chirp Transform Spectrometers
Dissertation
zur Erlangung des Doktorgrades
der Fakultät für Angewandte Wissenschaften
der Albert-Ludwigs-Universität Freiburg im Breisgau
vorgelegt von
Lucas Paganini
aus Mendoza / Argentinien
Freiburg 2008
Bibliografische Information Der Deutschen Bibliothek
Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen
Nationalbibliografie; detailliertebibliografischeDatensindimInternetüber
http://dnb.ddb.de abrufbar.
D7
Referent: Prof. Dr. Leonhard Reindl
Korreferent: Prof. Dr. Oskar von der Lühe
Tag der mündlichen Prüfung: 27. März 2008
ISBN 978-3-936586-83-1
Copernicus Publications 2008
http://publications.copernicus.org
c Lucas Paganini
(cid:13)
Printed in Germany
To my family

Contents
Abstract ix
Zusammenfassung xi
ListofFigures xv
ListofTables xvii
ListofSymbols xix
1 Introduction 1
1.1 ThesisOverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Microwaveheterodynespectroscopy . . . . . . . . . . . . . . . . . . . . 3
1.2.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1.1 Blackbodyradiation andthebrightnesstemperature . . 4
1.2.1.2 The relationship of thermal blackbody radiation and
antennatemperature . . . . . . . . . . . . . . . . . . . 5
1.2.2 Heterodynereceivers . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.4 Theradiometerformula . . . . . . . . . . . . . . . . . . . . . . 7
1.2.5 Scientificapplications . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Aimsofthisthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 The400-MHzbandwidthCTS 13
2.1 Introduction toCTSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 TheChirptransform principle . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Developmentanddesign . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.1 Thedispersivefilters . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3.2 Digital chirpgeneration, theDDSboard . . . . . . . . . . . . . . 23
2.3.2.1 Thedirectdigitalsynthesizers . . . . . . . . . . . . . . 23
2.3.3 Timesynchronization . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.4 Operatingmultiple AD9858sinstances . . . . . . . . . . . . . . 26
2.3.5 Recommendation onparallelAD9858sarchitecture . . . . . . . . 27
2.3.5.1 BypassingtheREFCLKdivide-by-2 . . . . . . . . . . 27
2.3.5.2 SynchronizingSYNCLKamongallDDSs . . . . . . . 27
2.3.5.3 MeetingsetupandholdtimesbetweenFUDandSYN-
CLK . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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Contents
2.3.6 TheDDSandASICboardssynchronization . . . . . . . . . . . . 28
2.4 Characterization. Testmeasurements . . . . . . . . . . . . . . . . . . . . 29
2.4.1 Analysisofthechirpsignal . . . . . . . . . . . . . . . . . . . . . 29
2.4.2 Frequencyandtimedomainanalyses . . . . . . . . . . . . . . . 30
2.4.3 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.4.4 Spectralresolution . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.4.5 Powerlinearity anddynamicrange . . . . . . . . . . . . . . . . . 37
2.5 Observationsandresults . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.5.1 Ozonemeasurements . . . . . . . . . . . . . . . . . . . . . . . . 40
2.5.2 Astronomical observations at the Heinrich Hertz Submillimeter
Telescope(HHSMT) . . . . . . . . . . . . . . . . . . . . . . . . 42
2.5.2.1 Comets . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.5.2.2 Marsandothersources . . . . . . . . . . . . . . . . . 47
2.6 Analysisandperformance . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.7 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3 Impactsofnonlinearityinheterodynesystems 55
3.1 Ground-basedmicrowavespectroscopyoftheEarth’satmosphere . . . . 55
3.1.1 Theatmosphereasphysicalsystem . . . . . . . . . . . . . . . . 55
3.1.2 Physicalproperties. Composition andstructure . . . . . . . . . . 56
3.1.3 Absorption andemissionbygases . . . . . . . . . . . . . . . . . 58
3.1.4 Theshapeofaspectralline . . . . . . . . . . . . . . . . . . . . . 58
3.1.5 Theoryofradiativetransfer . . . . . . . . . . . . . . . . . . . . . 59
3.1.6 Inversion technique . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2 Numericalanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3 SimulationsontheEarth’satmosphere . . . . . . . . . . . . . . . . . . . 66
3.4 Othersourcesofinstrumental error . . . . . . . . . . . . . . . . . . . . . 67
3.4.1 Skywindow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.4.2 Referenceloads . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.4.3 Singlesidebandfilter . . . . . . . . . . . . . . . . . . . . . . . . 68
3.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4 Onthehighaccuracyofmeasuredspectra 77
4.1 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.2 Intercomparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.3 Linearitymeasurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.3.1 A3-dimensional analysismethod . . . . . . . . . . . . . . . . . 81
4.3.1.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . 84
4.3.1.2 Improvements providedbythismethod . . . . . . . . . 84
4.3.2 Anovelhigh-accuracymethod . . . . . . . . . . . . . . . . . . . 87
4.3.2.1 Gainfluctuations . . . . . . . . . . . . . . . . . . . . . 90
4.3.2.2 Measurementresults . . . . . . . . . . . . . . . . . . . 92
4.3.2.3 MethodologyappliedtoCTSs . . . . . . . . . . . . . . 92
4.3.2.4 Improvements . . . . . . . . . . . . . . . . . . . . . . 96
4.4 Theimportance forfuturedevelopments . . . . . . . . . . . . . . . . . . 99
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Contents
5 Concludingremarks 101
A Receivercalibration 103
A.1 Determination oftheantennatemperature,T . . . . . . . . . . . . . . . 103
A
A.2 Determination ofthereceivertemperature,T . . . . . . . . . . . . . . . 103
R
B Circuitdiagrams 105
C Observationsanddatareduction 109
C.1 Calibrationandtelescopeefficiency . . . . . . . . . . . . . . . . . . . . 109
D Detailsofdevicesundertest 113
Bibliography 115
Publications 125
Acknowledgements 127
Lebenslauf 129
vii

Abstract
Heterodyne spectroscopy is a technique providing practically unlimited spectral resolu-
tion. Eventhesmallestfeaturesofatmosphericspectrallinescanberesolvedinfrequency
using this technique. This is especially important, for instance, in the study of planetary
atmospheres where the structure of molecular transition lines provides detailed informa-
tionaboutmoleculardistribution,temperatureandpressureprofilesalongthelineofsight.
This thesis aims to address the specific properties required to maximize the reliability in
heterodyne-systemresponsesfocusingmainlyontheeffectsofnonlinearbehaviorinchirp
transform spectrometers.
In this investigation, a comprehensive description and characterization of a new 400-
MHz bandwidth Chirp Transform Spectrometer (CTS) with 100 kHz spectral resolution
are presented. In order to achieve the 400-MHz bandwidth, a newly developed DDS
board,drivenbya1-GHzfixedfrequencyclocksource,createsachirpsignalusingdigital
techniques. Novel methods have been applied to the RF section in the CTS, since the in-
trinsicpropertiesoftheSAWfilter(withabandwidthequalto400MHz)requiresaninput
signal two times larger than the SAW filter’s bandwidth. Furthermore, this spectrometer
has been applied to atmospheric science, i.e. a 142-GHz ozone system by detecting the
142.175-GHz rotational transition of ozone in the Earth’s atmosphere. In addition, the
CTS system was used for astronomical observations at the Heinrich Hertz Submillime-
terTelescopeduringtheobservationrunofthe73P/Schwassmann-Wachmann3cometin
May2006,duringthecomet’sclosestapproachtotheEarth.
Any deviation from the spectrometer’s (ideal) linear dynamic range may induce sig-
nificant effects in the spectra, therefore, it is essential to model these deviations such that
theyareaccountedfor. Theseanalysesareperformedbymeansofnumericalcalculations
and simulations which show how these deviations in the spectra might produce consider-
ablechangesintheretrievedverticalprofileoftracegasesinplanetaryatmospheres.
In several examples, deviations are evidenced in the measured spectra of heterodyne
systems. Thus, two novel experiments were conducted in order to analyze the behavior
ofnonlinearityinbackendspectrometers. Basedonadifferentialapproach,bothanalyses
provideaneffectivesolutionintheidentificationofnonlinearprocesses.
Finally, experimental results in the ozone-radiometer test facility at the Max Planck
Institute for Solar System Research (MPS) show the improvements in the 400-MHz-BW
CTSresponse. Itisconfirmedthattheoveralldeviationsintroducedbynonlinearityinthe
spectrometer have been decreased and thus the performance of the backend instrument
hasbeenimproved.
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