Agilent Technologies 8504B - Page 68

FINAL TRIM SIZE : 7.5 in x 9.0 in
P
erforming
Guided
Setups
and
Calibrations
Performing
Manual
Calibrations
The
values
listed
above
are
derived
from
the
following
formulas:
0 15
dB
=
10
Log
0
:
0316
0
14
:
7 dB
=10
Log 0
: 0337
Y
ou can
direct the
precision reectometer
to
use
your
own
calibration-
standard
values during
a magnitude
calibration. Using
your
own
optical
device
of known
reectance for
a standard
can
improve
dynamic
accuracy
.
Ensuring
accuracy
of
low-level
reections
When
observing
reections
at
low
levels
, perform
the balance
receiver
calibration
periodically
after
a
full
calibration
to ensure
better accuracy
.
P
eriodically
balancing
the
receiver
helps
to
remove
small \drifts"
when
displaying
very
small
signals
.
This
drifting
occurs
most
often when
the
instrument
is not
allowed
to
temperature
stabilize
for
at
least
one
hour
prior
to
use.
P
olarization
balance calibration:
the inside
story
The
receiver
contains
a
polarization
splitter
that
splits
an
y
incoming
light
into
two
orthogonal
polarizations.
(Refer
to
\Block
Diagram
"
in
Chapter
5
for
a
block
diagram
of
the instrument.)
A
photodiode
mixes
the
optical
elds
reected
from
the
two
arms
of
the
interferometer
and
generates
the
interference
signal
in
each
polarization.
T
o
ensure
proper
operation
of
the
receiver
,the
reference
arm
reections
needs
to
be
about
equally
split
between
the
two
polarizations.
The
balance
receiver
calibration
ensures
that
this
condition
is
met.
During
balance
receiver
calibration,
the
TEST
PORT cable
is terminated
with
a
low
reection
termination.
This
means
substantially
that
the
only light
that hits
both receiver
detector
diodes
is
from
the
mirror
in
the
reference
light
path. The
polarization of
this light
is then
adjusted
b
y
the
front
panel
reference
polarization
balance
knobs
such
that
the responses
from each
detector are
equal, or
balanced.
Subsequent
measurements
of
polarization-independent
return
loss
are then
insensitive to
polarization
transformations of
the ber
in the
test
arm.
2-21
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