Agilent Technologies 8753ET User Manual page 381

Figure 7-26 Effective Directivity E
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 and frequency. Leveling the source to produce a constant incident signal
11A
(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
Figure 7-27.
Figure 7-27 Source Match E
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
DF
SF
as shown in
RF
Measurement Calibration
Figure 7-28.
Operating Concepts
as shown in
SF
7-43