HP 8753D - Page 98

Measuring Gain
and Reverse
Isolation Simultaneously
Since
an
amplier
will
have high
gain in
the forward
direction and
high isolation
in the
reverse
direction,
the
gain
(S
21
)
will
be
much
greater
than
the reverse
isolation (S
12
).
Therefore
,
the
power
you
apply
to
the
input
of the
amplier for
the forward
measurement (S
21
)
should
be
considerably
lower
than
the
power
you
apply to
the output
for the
reverse measurement
(S
12
).
By
applying
low
power in
the forward
direction, you'll
prevent the
amplier
from
being
saturated.
A
higher
power
in
the
reverse direction
keeps noise
from being
a
factor
in
the
measurement
and
accounts
for
any
losses
caused by
attenuators or
couplers on
the
amplier's
output
needed
to
lower
the
output
power into
the analyzer
. The
following
steps
demonstrate
the
features
that
best
accomplish
these
measurements.
1.
Press
4
MENU
5
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COUPLED
CH
ON
.
Coupling
the
channels
allows
you
to
have
the
same
frequency
range
and
calibration
applied
to
channel
1
and
channel
2
.
2.
Press
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POWER
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N
PORT
POWER
[UNCOUPLED]
.
Uncoupling
the
port
power
allows you
to
apply
dierent
power
levels
at
each
port.
In
Figure
2-44
,
the
port
1
power
is
set
to
0
25
dBm
for
the
gain
measurement
(S21)
and
the
port
2
power
is
set
to
0
dBm
for
the
reverse
isolation
measurement (S12).
3.
Press
4
CHAN
1
5
4
MEAS
5
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N
Trans:
FWD S21
(B/R) 4
MENU
5
N
N
N
N
N
NN
NN
NN
NN
N
N
N
N
POWER
and set
the power
level for
port
1
.
4.
Press 4
CHAN
2
54
MEAS
5
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N
NN
NN
N
N
N
Trans: REV
S12 (A/R)
4
MENU
5
N
N
N
N
N
NN
NN
N
N
N
N
N
N
N
N
POWER and
set the
power
level
for
port
2
.
5.
P
erform
an
error-correction
and
connect
the
amplier
to
the network
analyzer
.
Refer
to
the
\Optimizing
Measurement
Results"
Chapter
for
error-correction
procedures
.
6.
Press
4
DISPLA
Y
5
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N
DUAL
CHAN
ON
.
Y
ou
can
view
both
measurements
simultaneously
by
using
the dual
channel display
mode
.
Refer
to
Figure
2-44
.
If
the
port
power
levels are
in dierent
power
ranges
,
one
of
the
displayed
measurements
will
not
be
continually
updated
and the
annotation tsH
will appear
on
the left
side of
the display
.
Refer
to
\Source
Attenuator
Switch
Protection"section
in
Chapter
6, \Application
and Operation
Concepts,"
for
information
on
how
to
override
this
state
.
Making Measurements 2-49