User`s guide

7-43
Operating Concepts
Measurement Calibration
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.
Figure 7-28 Reflection Tracking E
RF
These three errors are mathematically related to the actual data, S
11A
, and measured
data, S
11M
, by the following equation:
If the value of these three "E" errors and the measured test device response were known
for each frequency, this equation could be solved for S
11A
to obtain the actual test device
response. Because each of these errors changes with frequency, their values must be
known at each test frequency. These values are found by measuring the system at the
measurement plane using three independent standards whose S
11A
is known at all
frequencies.
The first standard applied is a "perfect load," which makes S
11A
= 0 and essentially
measures directivity. See Figure 7-29. "Perfect load" implies a reflectionless termination at
the measurement plane. All incident energy is absorbed. With S
11A
= 0 the equation can be
solved for E
DF
, the directivity term. In practice, of course, the "perfect load" is difficult to
achieve, although very good broadband loads are available in the compatible calibration
kits.
S
11M
E
DF
S
11A
E
RF


1
E
SF
S
11A
–


------------------------------------------+=