Av
alanc
he
Photo
dio
de
Camera
Imager
The
sensor
for
the
system
w
as
a
EG G
Opto
electronics
CA E
a
v
alanc
he
pho
to
dio
de
APD
The
APD
op
erates
in
a
v
alanc
he
breakdo
wn
mo
de
and
creates
a
high
gain
junction
In
order
to
op
erate
in
the
a
v
alanc
he
mo
de
the
photo
dio
de
requires
a
v
olt
bac
k
bias
This
is
accomplished
b
y
using
a
EMCO
High
V
oltage
Co
step
up
in
v
erter
It
is
coupled
to
the
photo
dio
de
b
y
a
simple
passiv
e
lo
w pass
lter
stage
to
atten
uate
noise
The
photo
dio
de
camera
con
tains
t
w
o
circuit
b
oards
connected
bac
k to bac
k
The
rst
b
oard
is
the
rst
ampli cation
stage
The
second
b
oard
con
tains
the
second
stage
of
the
fron
t
end
ampli er
an
A
GC
a
tuned
lter
and
a
line
driv
er
The
b
oards
are
moun
ted
in
a
mac
hined
blo
c
k
of
alumin
um
A
Thor
Labs
optics
moun
t
is
attac
hed
to
the
the
blo
c
k
A
delron
ring
w
as
mac
hined
to
cen
ter
the
photo
dio
de
in
the
optical
assem
bly
This
is
imp
ortan
t
as
the
total
activ
e
area
of
the
photo
dio
de
is
only
mm
making
alignmen
t
critical
The
rst
b
oard
requires
only
V
olts
The
second
b
oard
runs
with
and
v
oltage
rails
These
v
oltages
are
regulated
on
the
b
oard
b
y
and
series
regulators
The
photo
dio
de
is
directly
coupled
in
to
a
cascade
of
Minicircuits
MAR
GHz
lo
w noise
monolithic
ampli ers
The
MAR
stages
are
coupled
together
as
w
ell
as
coupled
to
the
second
b
oard
b
y
F
capacitors
The
t
w
o
MAR
stages
pro
vide
ab
out
dB
of
gain
eac
h
MAR
pro
viding
appro
ximately
dB
The
rst
comp
onen
t
of
the
second
ampli er
b
oard
is
an
Activ
e
Gain
Con
trol
A
GC
circuit AD
The
A
GC
is
built
using
t
w
o
Analog
Devices
AD
v
ariable
gain
ampli ers
They
pro
vide
a
lo
w noise
high gain
circuit
circuit
that
can
run
at
the
frequency
w
e
desire
A
switc
h
allo
ws
this
circuit
to
b
e
ipp
ed
b
et
w
een
an
adjustable
constan
t
gain
or
the
A
GC
mo
de
with
adjustable
reference
The
A
GC
ampli er
is
able
to
pro
vide
up
to
dB
of
gain
In
actual
use
though
the
A
GC
w
as
alw
a
ys
on
and
the
constan
t
gain
w
as
nev
er
used
although
handy
for
debugging
The
feedbac
k
lo
op
follo
wing
the
A
GC
is
closed
after
an
additional
tuned
amplifer
stage
The
tuned
ampli er
is
constructed
out
of
a
Maxim
MAX
wideband
transconductance
ampli er
It
is
tuned
to
the
mo
dulation
frequency
using
an
iron
core
inductor
This
allo
ws
the
Q
of
the
circuit