User`s guide

7-44
Operating Concepts
Measurement Calibration
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.
Figure 7-27 Source Match E
SF
Frequency response (tracking) error is caused by variations in magnitude and phase
flatness versus frequency between the test and reference signal paths. These are due
mainly to coupler roll off, imperfectly matched samplers, and differences in length and loss
between the incident and test signal paths. The vector sum of these variations is the
reflection signal path tracking error, E
RF
as shown in Figure 7-28.