Agilent Technologies 8719D User Manual page 336

These three errors are mathematically related to the actual data, SIXA, and measured data,
If the value of these three "E" errors and the measured test device response were known for
each frequency, the above equation could be solved for S 11~
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 &IA is known at all frequencies
The lirst standard applied is a "perfect load," which makes &IA = 0 and essentially measures
directivity (see Figure 6-38). "Perfect load" implies a reflectionless termination at the
measurement plane. All incident energy is absorbed. With &A = 0 the equation can be solved
for Env, the directivity term. In practice, of course, the "perfect load" is difficult to achieve,
although very good broadband loads are available in the HP 8719D/ZOD/22D compatible
calibration kits.
Since the measured value for directivity is the vector sum of the actual directivity plus
the actual reflection coefilcient of the "perfect load," any reflection from the
represents an error. System effective directivity becomes the actual reflection coeflicient of the
near "perfect load" (see F'igure 6-39). In general, any termination having a return loss value
greater than the uncorrected system directivity reduces reflection measurement uncertainty.
6-62 Application and Operation Concepts
Figure 6-38. 'Terfect Load" 'Rmnination
(1
obtain the device
termination