A digital correlator for use in intensity fluctuation spectroscopy : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University

Copyright is owned by the Author of the thesis. Permission is given for

a copy to be downloaded by an individual for the purpose of research and

private study only. The thesis may not be reproduced elsewhere without

the permission of the Author.

A DIGITAL CORRELATOR FOR USE IN

INTENSITY FLUCTUATION SPECTROSCOPY

A thesis presented in partial fulfilment of the requirements for the degree of

Doctor of Philosophy in Physics at Massey University

ROBERT CHARLES O ' DRISCOLL 1982

. •. H

ll

ABSTRACT

A digital correlator suitable for appl ications in intensity fluctu­

ation spectroscopy is described . Intensity fluctuation spectroscopy is a technique 1n which temporal fluctuations in the intensity of scattered laser l ight are analysed in order to obtain information about the mot ion of the scatterers . However a drawback of intensity fluctuation spectroscopy is that even very small amounts of contam­

inant dust can make the measured data difficult , if not impossible , to interpret .

To help overcome this problem a device , known as the "bl inker" , is incorporated in the correlator . This device enables l ight scattering measurements to be made on less than scrupulously clean samples by monitoring the scattered l ight intens ity and inhibiting data col ­

lection whenever the presence of dust in the scattering sample i s sus ­ pected .

An outl ine of intensity fluctuation spectroscrpy is given followed by a review of correlation techniques with particular emphasis on photo ­ count correlation , and on methods of reducing the complexity of the correlator circuits . The reasons for adopt ing the single clipping technique are noted , and the specification and design of the instru­

ment discussed . The electronic circuits and their operation are described .

The instrument has several different modes of operation . These include : s ingle-cl ipped , double -clipped, or scaled and cl ipped auto­

correlation ; cross-correlation ; probability density and distribution analysis; and multichannel signal averaging .

The effect of dust on the measured intensity correlation function is examined and techniques which have been developed to minimise this effect are reviewed . The bl inker technique is described in detail to ­ gether with a description of the required electronic circuits .

The procedure for testing the correlator is given. Since much of the

111

testing was performed on the complete intensity fluctuation system, this system is described and details given of the sample preparation and correlation data analysis techniques . Results are presented to demonstrate the correct operation of the instrument .

Experimental results are also presented to show how the blinker was used to minimise the effect of dust contaminat ion in a dilute solution of 9lnm diameter polystyrene latex spheres . Examples are included of the appl ication of the bl inker in the study of concentrated latex sphere solutions , and in the study of dilute and concentrated solu­

tions of polystyrene random coils .

Fina l ly , some proposals are made for future developments which include a software correlator and a hardware full correlator , both of which

.

are based on the existing . instrument .

}

. I

lV

ACKNOWLEDGEMENTS

First and foremost , I would l ike to thank my supervisor Dr D N Pinder for his guidance and encouragement throughout this proj ect .

I am grateful for the support of Professors R D Batt and G N Malcolm and many of the staff in the Department of Chemistry, Biochemistry and Biophysics at Massey Univers ity for making this work possible . In particular , I am indebted to the technical staff for their cooperation and advice , and for free access to workshop facilities . The assis­

tance of Mr

K

R Smith in preparing the artwork for three of the printed c ircuit cards is also appreciated .

Thanks are due to Mr A S Craig and Dr D A D Parry for their help with the electron microscopy .

Final ly , I wish to thank Mrs L McDowall for the care and interest taken in typing this thesis , and my wife for her patience and assis­

tance with proof reading .

)

TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES

CHAPTER

1 -

INTRODUCTION

1 . 1

BACKGROUND

1 . 2

INTENSITY FLUCTUATION SPECTROSCOPY

1 . 3

SYNOPSIS

CHAPTER

2 -

CORRELATION TECHNIQUES

2 . 1

INTRODUCTION

2 . 2

FREQUENCY ID1AIN ANALYSIS

2 . 3

TIME ID1AIN ANALYSIS

2 . 4

ANALOGUE CORRELATION

2 . 5

DIGITAL CORRELATION

2 . 5.1

Sampling

2 . 5 . 2

Quantisation

2 . 5 . 3

Digital correlator for analogue

2 . 5 . 4

Hard l imiting

2 . 5 . 5

Ergodic conversion

2 . 5 . 6

Comparator- trigger correlator

2 . 5 . 7

Multibit correlation

2 . 6

PHOTOCOUNT CORRELATION

s ignals

2 . 6 . 1

Optical detection with a photornultiplier tube

2 . 6 . 2

Photon counting

2 . 6 . 3

Arrival times of photoelectron pulses

2 . 6 . 4 An

ideal photocount correlator

2 . 6 . 5

Single cl ipping

2 . 6 . 6

Double cl ipping

2 . 6 . 7

Add- subtract correlator

PAGE

ll

iv

V

ix xiii

1 3 12

14 1 5 1 8 1 9 2 2 2 2 2 3 2 5 2 7 33 34 36 38

38 40 43 4 7 4 9 56 5 7

V

Vl

PAGE

2 . 6 . 8

Random cl ipping

58

2 . 6 . 9

Scal ing

59

2 . 6 . 10

Multibit correlat ion

62

2 . 7

SOFTWARE SYS'ffi.1S

63

CHAPTER

3 ^-

CORRELATOR DESI GN

3 . 1

THE DECI SION TO BUI LD

64

3 . 2

DESIGN CONSIDERATIONS

65

3 . 3

SPECIFICATIONS

66

3 . 4

CONTROLS

66

3 . 5

HARDWARE

71

3 . 5 . 1

Card frame

7 1

3 . 5 . 2

Cooling

72

3 . 5 . 3

General layout

7 3

3 . 5 . 4

Printed c ircuit cards

76

3 . 6

ELECTRONICS

7 7

3 . 6 . 1

Choice of logic family

7 7

3 . 6 . 2

Construction

78

3 . 6 . 3

Signal transmission

79

3 . 6 . 4

Control wiring

85

3 . 6 . 5

Power supply

85

CHAPTER

4 ^-

C I RCUIT DESCRIPTION

4 . 1

SYSTEM OVERVIEW

86

4 . 2

CLOCK

88

4 . 2 . 1 20MHz

master clock

89

4 . 2 . 2

Sample tim� clock

89

4 . 2 . 3

Samples timer

91

4 . 3

INPUT AMPLIFIER

93

4 . 4

DERANDOMISER AND SYNCHRONISER

95

4 . 4 . 1

Derandomiser

95

4 . 4 . 2

Synchroniser

98

4 . 5

PULSE DELAY CIRCUIT

98

4 . 6

CLIPPER- SCALER

4 . 7

DELAY LINE ,

AND

GATES ,

AND

ACCUMULATORS

4 . 8

OPERATION IN OTHER MODES

4 . 8 . 1

The function selector

4 . 8 . 2

Correlation modes

4 . 8 . 3

Probability modes

4 . 8 . 4

Signal average modes

4 . 9

DATA

OliTPliT AND

DISPLAY

4 . 9 . 1

Accumulator scanning circuit

4 . 9 . 2

Analogue output circuit

4 . 9 . 3

Computer interface CHAPTER

5 ^-

DUST

5 . 1

THE EFFECT OF DUST ON THE MEASURED INTENSITY

vii

PAGE

99 103 103 106 109 109 113 115 115 1 1 7 117

CORRELATION FUNCTION

¹²⁰

5 . 2

TECHNIQUES USED TO MINIMISE THE EFFECT OF DUST

124 5 . 2 . 1

Analysis techniques

5 . 2 . 2

Instrumentation techniques

5 . 3

THE BLINKER TECHNIQUE

5 . 3 . 1

"Clean" samples

5 . 3 . 2

Dirtier samples

5 . 4

DESCRIPTION OF

THE

BLINKER

5 . 4 . 1

Pulse delay circuit

5 . 4 . 2

Intensity monitor c ircuit

5 . 5

OPERATING THE BLINKER

CHAPTER

6 ^-

TESTS

AND

MEASUREMENTS

6 . 1

INTRODUCTION

6 . 2

PRELIMINARY TESTING

6 . 2 . 1

Constant input

6 . 2 . 2

Periodic inputs

6 . 3

LIGHT SCATTERING MEASUREMENTS

J 124

1 2 7

1 3 0 1 3 0 1 3 1 132 134 137 142

144 144 145 148 159

PAGE

6 . 3 . 1

The intens ity fluctuation spectrometer

159

6 . 3 . 2

Base l ine check

162

6 . 3 . 3

Polystyrene latex spheres

165

6 . 3 . 4

Sample preparation

167

6 . 3 . 5

Data analys is

¹⁶⁸

6 . 3 . 6

Experimental results

1 7 0

6 . 4

MEASUREMENTS WI'IH

1HE

BLINKER

6 . 4 . 1

Dilute monodisperse scatterers

6 . 4 . 2

Other measurements using the blinker CHAPTER

7 ^-

PROPOSED DEVELOPMENTS

7 . 1

LIMITATIONS OF A CLIPPING CORRELATOR

7 . 2 SOFI'WARE

CORRELATOR

7 . 3 HARDWARE

FULL CORRELATOR

7 . 4 AN

ALTERNATIVE BLINKER M)DE

7 . 5

CONCLUDING REMARKS

APPENDIX I SPECIFICATIONS

APPENDIX I I

AN

ALTERNATIVE PREAMPLIFIER

AND

DISCRIMINATOR APPENDIX I I I CIRCUIT DIAGRAMS

APPENDIX IV

AN

AUDIO SPECTRUM ANALYSER

APPENDIX V A PLOTIER INTERFACE FOR SERIAL CCMPUTER TERMINALS

APPENDIX VI A DIGITAL CORRELATOR WITH BLINKER FACILITY APPENDIX VI I A ME'IHOD OF MEASURING

1HE

LIQ-IT' SCATIERING

OF SOLUTIONS CONTAINING DUST PARTICLES BIBLIOGRAPHY

1 7 3 1 73 1 8 0

1 8 3 184 187 191 191

1 93

196 200 2 1 9 230

234 244

249

viii

Figure 1 . 1

Figure 1 . 2 Figure 1 . 3

Figure 2 . 1

Figure 2 . 2 Figure 2 . 3

Figure 2 . 4 Figure 2 . 5 Figure 2 . 6 Figure 2 . 7 Figure 2 . 8 Figure 2 . 9 Figure 2 . 10 Figure 2 . 11 F igure 2 . 12 Figure 2 . 13 Figure 2 . 14 Figure 2 . 15 Figure 2 . 16

Figure 3 . 1 Figure 3 . 2 Figure 3 . 3 Figure 3 . 4

ix

LIST OF FI

GURE

S

PAGE Schematic representation of a laser light

scattering experiment 4

Detection of scattered light 8

Autocorrelation function and power spectrum for a typical homodyne light scattering experiment 1 1 Functional block diagram of a single channel

autocorrelator

Quantiser transfer functions

Block diagram of generalised parallel channel

2 0 2 4

digital autocorrelator 26

The effect of hard limiting 28

1 -bit by 1 -bit correlator 30

1 -bit correlator employing optical processing 32

3-bit ( 7 -le�el) quantiser 37

Output current at photornultiplier tube anode 39

Block diagram. of a photon cmmt ing system 39 Measurement of probability of coincidence 4 5 Measurement o f time - interval distribution 4 5 Block diagram of ideal photocount correlator 48

Clipping and clipper 5 1

Single-clipping correlator 54

Ramped clipping 60

Scaling 60

Control panel 68

Front view of correlator 74

Rear view of correlator 74

Circuit constructed on VERO DIP board 75

Figure 3.5 Figure 3 . 6

Printed circuit card

Circuit of longest signal path in the correlator

PAGE

75

81

Figure 3 . 7 Crosstalk waveforms 8 1

Figure 3 . 8 Measured input and output characteristics of

DM74S04 inverter 8 2

Figure 3 . 9 Predicted driver and receiver· waveforms 8 3 Figure 3 . 10 Measured driver and receiver waveforms 8 3 Figure 4 . 1 Block diagram of the correlator 8 7

Figure 4 . 2 20MHz master clock 90

Figure 4 . 3 Sample time clock pul se generation 90

Figure 4 . 4 Sample time clock 92

Figure 4 . 5 Samples timer and enable circuit 94

Figure 4 . 6 Input ampl ifier 94

t

Figure 4 . 7 Derandomiser and synchroniser 96 Figure 4 . 8 Derandomiser and synchroniser pulse sequences 9 7

Figure 4 . 9 Cl ipper - scaler 1 0 0

Figure 4 . 10 Cl ipper and scaler pulse sequences 1 0 1 Figure 4 . 11 Comparison o f c lipper and scaler outputs 1 0 2 Figure 4 . 12 Delay l ine ,

AND

gates , and accumulators 104 Figure 4 . 13 Single clipping correlator pulse sequences 1 0 5 Figure 4 . 14 Function selector circuit 1 08 Figure 4 . 1 5 Zero dead-time circuit for probabil ity modes 1 1 0 Figure 4 . 16 Zero dead-time loading of shift register delay

l ine 1 1 0

Figure 4 . 1 7 Probabil ity density analyser pul se sequences 1 1 2 Figure 4 . 18 Signal average trigger circuit 1 14 Figure 4 . 19 Signal averager pulse sequences 1 14

X

-:

Figure 4 . 20 Figure 4 . 21 Figure 4 . 22 Figure 5 . 1 Figure 5 . 2 Figure 5 . 3 Figure 5.4 Figure 5 . 5

Figure 6 . 1

Figure 6 . 2 Figure 6 . 3 Figure 6 . 4 Figure 6 . 5 Figure 6 . 6

Figure 6 . 7

Figure 6 . 8

PAGE

Block diagram of accumulator scanning circuit 116

Analogue output circuit 118

Principle of computer interface 118

Block diagram of bl inker 133

Pulse delay circuit 135

Pulse sequences for pulse delay c ircuit 136

Intensity monitor c ircuit 139

Pulse sequences for correlator stopping and

starting 141

Experimental arrangement for measurements on sine or square wave

Signal average mode , 504Hz square wave Probability mode , 504Hz square wave Correlation mode , 504Hz square wave Correlation mode , 504Hz sine wave

Variation of single-clipped correlation function basel ine and cosine term amp l itude with cl ipping level

Variation of scaled correlation function basel ine and cos ine term ampl itude with scal ing level

Schematic diagram of the intensity fluctuation spectrometer

149 150 1 5 1

153 1 5 5

1 5 7

1 58

Figure 6 . 9 Baseline check

160 164 166 Figure 6 . 10 Photograph of 9lnm latex spheres

Figure 6 . 11 Diameter distribution of polystyrene latex spheres

Figure 6 . 12 Semilogarithmic plot of correlator output for data collected from a dilute solution of latex spheres in water

166

1 7 1

Xl

Figure 6 . 13 Figure 6 . 14

Linear dependence of decay rate on sin2 (e/2) Comparison of semilogarithmic plots of corre ­ lator output obtained from one continuous

experiment and the aggregate of 5000 experiments

PAGE

1 72

174 Figure 6 . 15 Semilogarithmic plot of correlator output with

and without the blinker 1 7 5

Figure 6 . 16 Probability dens ity functions measured with

and without the blinker 177

Figure 6 . 1 7 Measured translational diffus ion coefficient Figure 6 . 18

F�gure 7 . 1 Figure 7 . 2 Figure 7 . 3

Figure 7 . 4

versus s in2 (e/2) 1 78

Estimates of the decay rate of the intensity autocorrelation function of light scattered from a purposely dirtied solution of latex spheres in water

Block diagram of hardware Flow chart of software

First channel of proposed 8 -bit 'full ' correlator

Pulse sequences for 8 -bit multiplier and accumulator

179 1 8 5 185

189

1 90

xii

LIST OF TABLES

Table 3 . 1 Summary of correlator characteristics

Table 4 . 1 Connections for enabling different modes

PAGE 6 7 1 0 7

xiii