Agilent Technologies 4395A - Page 430

Calibration
for
Network
Measurement
Two-P
ort Error
Model
The
error model
for
measurement
of
the
transmission
coecients
(magnitude
and
phase)
of
a
two-port device
is
derived
in
a
similar
manner
.
The
major
sources
of
error are
frequency
response
(tracking), source
match,
load
match,
and
isolation
(
Figure
A
-36
).
These
errors are
eectively
removed using
the
full
two-port
error
model
Figure
A
-36.
Major
Sources
of
Error
Measuring Transmission
Coecient.
The
transmission
coecient
is
measured
by
taking
the
ratio
of the
incident
signal
(I)
and
the
transmitted
signal
(T)
(
Figure
A
-37
).
Ideally
,
(I)
consists
only
of power
delivered
by
the
source
,
and
(T)
consists
only
of
power
emerging
at
the
test
device
output.
Figure A-37. Transmission Coecient
Load Match Error
. As in the reection model, source match can
cause the incident signal to
vary as a function of test device S
11A
. Also
, because the test setup transmission return port is
never exactly the characteristic impedance
, some of the transmitted signal is reected from
the
test set port 2, and from other mismatches between the test device output and the receiver
input, to return to the test device. A portion of this signal may be re-reected at port 2, thus
aecting S
21M
, or part may be transmitted through the device in the reverse direction to
appear at port 1, thus aecting S
11M
. This error term, which causes the magnitude and phase
of the transmitted signal to vary as a function of S
22A
, is called load match, E
LF
(Figure A-38).
A-54 Basic Measurement Theory