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February 2004 Charles A. DiMarzio, Northeastern University 10464-10-1
ECEG287 Optical Detection Course NotesPart 10: More on the Detector as
a Circuit ElementProfs. Charles A. DiMarzio
and
Stephen W. McKnight
Northeastern University, Spring 2004
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-2
Topics
• PMT Dynode Circuits– Conventional Voltage Divider– Cockroft-Walton
• Dynode Modulation– Application– Technique
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-3
PMT Concept
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-4
PMT Dynode Chain Currents (1)
V0=-HV
ic G12ic G13ic G14ic
R01 R12 R23 R34
V1 V2 V3
nnnncnnnnncn RVViGRVViG ,11,11,11,1 //
Assumption is made that all V/R are much larger that corresponding Gic.
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-5
PMT Dynode Chain Currents (2)
V0=-HV
ic G12ic G13ic G14ic
R01 R12 R23 R34
V1 V2 V3
nnnncnnnnncn RVViGRVViG ,11,11,11,1 //
At high cathode currents, higher dynode currents modify dynode voltages.
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-6
Cockroft-Walton Divider
Emitter-follower-like circuit maintains the dynode voltages without high power consumption.
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-7
DOT: Using Modulated Light
Laser
Demod
t
P
t
P
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-8
Physical Reason for Dispersion
-0.5
0
0.5
Sam
ple
200 MHz.
0 5 10
10
20
30
40
50
-0.5
0
0.5
Sam
ple
500 MHz.
0 5 10
10
20
30
40
50
0 5 10-50
0
50
Sig
nal
Time, ns0 5 10
-50
0
50
Sig
nal
Time, ns
Imaginary partof diffusive wavenumber increaseswith frequency
Easy to understand in terms of multiple paths.m100574a.m
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-9
TECHNOLOGY•Near-infrared light•Fiber optics•Computed Tomography
ADVANTAGES•Optical contrast•Portable - bedside, ambulance•Continuous•Inexpensive
•DISADVANTAGES•Resolution•Depth penetration
From David A. Boas - MGH NMR Center
DOT Instrumentation at MGH
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-10
DetectorsSources
6 cm
4 cm
Mid-line
Data Set I - 98-05-14
At RestPassive movement of
right armPassive movement of
right arm
From David A. Boas - MGH NMR Center
Functional Imaging of a Neonate
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-11
Demodulation Techniques
Laser
t
P
t
P
Transmitter
LO
300.00MHz
300.01MHz
10kHz Ref
10kHz Sig
DC Out
300MHz Sig
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-12
PMT Time Delay and Bandwidth
tvatx 02
2
1• Dynode Paths
• Transit Time
• Variations
tvtmx
Vex 0
2
2
1
012
10
2
x
tv
x
t
m
Ve
mVe
mVe
vvxt
o
2
220
00
t
x
tv
vxt
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-13
Long and Short Paths in PMT
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-14
Mixing at the Dynode
V0=-HV
ic G12ic G13ic G14ic
R01 R12 R23 R34
V1 V2 V3
V2
t
February 2004 Charles A. DiMarzio, Northeastern University 10464-10-15
PMT Dynode Modulation Example
-900 -850 -800 -750 -7000
2
4
6
8
10
V2
, Dynode Voltage, Volts
G13
, Tw
o-S
tage
Gai
n
Operating Point
m10464_10a.m
