7- 49
Operating Concepts
Measurement Calibration
Figure 7-36 Load Match E
LF
The measured value, S
21M
, consists of signal components that vary as a function of the relationship
between E
SF
and S
11A
as well as E
LF
and S
22A
, so the input and output reflection coefficients of the test
device must be measured and stored for use in the S
21A
error-correction computation. Thus, the test setup is
calibrated as described for reflection to establish the directivity, E
DF
, source match, E
SF
, and reflection
frequency response, E
RF
, terms for reflection measurements on both ports.
Now that a calibrated port is available for reflection measurements, the thru is connected and load match,
E
LF
, is determined by measuring the reflection coefficient of the thru connection.
Transmission signal path frequency response is then measured with the thru connected. The data is
corrected for source and load match effects, then stored as transmission frequency response, E
TF
.
NOTE It is very important that the exact electrical length of the thru be known. Most calibration kits
assume a zero length thru. For some connection types such as Type-N, this implies one male
and one female port. If the test system requires a non-zero length thru, for example, one with
two male test ports, the exact electrical delay of the thru adapter must be used to modify the
built-in calibration kit definition of the thru.
Isolation, E
XF
, represents the part of the incident signal that appears at the receiver without actually passing
through the test device. See
Figure 7-37. Isolation is measured with the test set in the transmission
configuration and with terminations installed at the points where the test device will be connected. Since
isolation can be lower than the noise floor, it is best to increase averaging by at least a factor of four during
the isolation portion of the calibration. The
softkey under the
menu allows a calibration sequence to resume after a change to the averaging factor.
If the leakage falls below the noise floor, it is best to increase averaging before calibration. In this case,
omitting isolation is better than measuring the isolation standards without increasing the averaging factor.
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