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3.12
10780A Receiver
The 10780A Receiver (Figure 3-9) converts the Doppler-shifted laser light into electrical sig-
nals that can be processed by the rest of the laser system. The receiver contains a photo-
detector, an amplifier and level translator, a line driver, a level sensor (comparator), and
local voltage regulators.
A
lens
on
the
front
end
of
the
receiver
focuses
the
laser
light
onto
the
active
chip
of
a
silicon
PIN
photodiode.
Between
the
lens
and
the
diode
i s
a
small
piece
of
polarizing
material
oriented
at
4 5
degrees
to
the
horizontal
and
vertical
axes
of
the
receiver.
When
the
receiver
is
mounted
properly-vertical
axis
parallel
or
perpendicular
tb
the
axes
of
the
laser
head-the
polarizer
passes
one-half
the
incident
power
from
each of
the
two
incoming
orthogonally
polarized
components
of
the
received
laser
beam.
The
resulting
power
on
the
photodiode
chip
is
an
amplitude-modulated
sine
wave;
its
frequency
is
the
Doppler-shifted
split
frequency,
and
its
amplitude
is
proportional
to
the
product
of
the
incident
powers
of
the
two
orthogonal
com-
ponents.
The
photodiode
generates
an
ac
current,
which
is
converted
to
an
ac
voltage
at
a
frequency
of
100
kHz
to
5
MHz.
High-Gain
I
Amplltier
Photodiode
Differential
+5v
Reference
Level
External
Level
Test
Point
L
(
comparator
Figure
3-9. 10780A
Receiver Block Diagram
The detected signal voltage goes through a stage of impedance transformation, two stages of
voltage gain, and a stage of level translation. The result, a TTL-level signal, goes to a TTL
differential line driver, which
is ac-coupled to the rest of the 5501A system by a shielded
twisted-pair cable. The output of the line driver is a differential square wave at the Doppler-
shifted split frequency (measurement signal).
The level sensor (comparator) disables the line driver unless the incident laser power is four
microwatts or more.
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