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Figure 6-41. Major Sources of Error
The transmission coecient 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 Coecient
As in the reection 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 reected 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-reected at port 2, thus aecting S
21M
,
or part may be transmitted through the device in the reverse direction to appear at port 1,
thus aecting 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