7- 43
Operating Concepts
Measurement Calibration
only of the signal reflected by the test device (S
11A
, for S
11
actual).
Figure 7-25 Reflection Coefficient
However, all of the incident signal does not always reach the unknown. Refer to Figure 7-26. Some of (I) may
appear at the measurement system input due to leakage through the test set or through a signal separation
device. Also, some of (I) may be reflected by imperfect adapters between a signal separation device and the
measurement plane. The vector sum of the leakage and the miscellaneous reflections is the effective
directivity, E
DF
. Understandably, the measurement is distorted when the directivity signal combines
vectorally with the actual reflected signal from the unknown, S
11A
.
Figure 7-26 Effective Directivity E
DF
Since the measurement system test port is never exactly the characteristic impedance (50 ohms), some of
the reflected signal bounces off the test port, or other impedance transitions further down the line, and back
to the unknown, adding to the original incident signal (I). This effect causes the magnitude and phase of the
incident signal to vary as a function of S
11A
and frequency. Leveling the source to produce a constant
incident signal (I) reduces this error, but since the source cannot be exactly leveled at the test device input,
leveling cannot eliminate all power variations. This re-reflection effect and the resultant incident power
variation are caused by the source match error, E
SF
as shown in Figure 7-27.
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