Atek HP 8753D, Руководство пользователя, Страница: 322 / 677

Figure 6-35. Reection Tracking E
RF
These three errors are mathematically related to the actual data, S
11A
, and measured data,
S
11M
, by the following equation:
S
11 M
= E
D F
+
(S
11 A
E
RF
)
(1 0 E
S F
S
11 A
)
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
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 rst standard applied is a \perfect load," which makes S
11A
= 0 and essentially measures
directivity (see Figure 6-36). \P erfect load" implies a reectionless 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 dicult to achieve ,
although very good broadband loads are available in the HP 8753D compatible calibration kits .
Figure 6-36. \P erfect Load" T ermination
Since the measured value for directivity is the vector sum of the actual directivity plus
the actual reection coecient of the \perfect load," any reection from the termination
represents an error . System eective directivity becomes the actual reection coecient of the
near \perfect load" (see Figure 6-37 ). In general, any termination having a return loss value
greater than the uncorrected system directivity reduces reection measurement uncertainty .
6-56 Application and Operation Concepts