HP HP 8753E, User Manual, Page: 343 / 699

Figure 6-34. Effective Directivity
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 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, (see 6-35).
S o u r c e
I
1 I A
R
l
U n k n o w n
Figure 6-35. Source Match
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 paths. The vector sum of these variations is the reflection signal path tracking
error, (see Figure 6-36).
Application and Operation Concepts