HP 8753D - Page 327

Figure
6-41.
Major
Sources
of
Error
The
transmission coecient
is measured
by
taking
the
ratio
of
the
incident
signal
(I)
and
the
transmitted
signal (T)
(see
Figure
6-42
).
Ideally
,
(I)
consists
only
of
power
delivered
by
the
source
,
and
(T)
consists
only
of
power
emerging
at
the
test
device
output.
Figure 6-42. Transmission Coecient
As in the reection model, source
match can cause the incident signal to vary as a function
of test device S
11A
.Also
, since 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
(see Figure 6-43).
6-60 Application and Operation Concepts