HP 85108A, System Manual

Page: 71 / 136

View the S
12
measuremen t. If the thru is connected, the transmission co ecien t should b e
1
6
0 .
Normalization is turned o for the S
11
and S
22
measuremen ts, then the appropriate memory is
selected and normalization is turned on to view S
21
and S
12
.
Since these are accurate, rep eatable attenuators, this sequence may b e eective in y our
application. If it is necessary to c hange other parts of the test setup, esp ecially comp onen ts
connected to the test p orts or connected b et w een the rear panel links, this pro cedure may
require additional steps to adequately comp ensate for the c hanges.
Selecting Appropriate Measurement Calibration
Selecting appropriate measuremen t calibration signal lev els a v ailable for calibration and for
measuremen t may also inuence the decision of whic h error mo del to use for the measuremen t.
If appropriate signal lev els can b e ac hiev ed for all parameters during calibration and
measuremen t, the HP 8510 2-P ort accuracy enhancemen t error mo del, either the HP 8510 F ull
2-P ort or TRL 2-P ort, pro vides b est accuracy b y pro viding b est c haracterization and remov al
of the systematic errors in the test setup. If the device is noninsertable, the HP 8510 Adapter
Remo v al calibration pro cedure can b e implemen ted using the F ull 2-P ort and/or the TRL
2-P ort calibration.
Ho w ev er, there are applications in whic h the 2-P ort error mo del cannot b e used. There are
others in whic h b etter results are pro duced using other calibration tec hniques describ ed later.
These are the reasons that 2-P ort correction cannot b e used eectively:
1. V ery low signal lev els of the S
22
and S
12
measuremen t will probably reduce the accuracy of
the S
11
and S
21
measuremen ts.
Using 2-P ort correction, the v alues of all four parameters are used to nd the corrected
v alue for the display ed parameter. If, for example, the rev erse parameters are not
represen tativ e of their actual v alues due to insucien t signal lev els, then their
con tribution to the accuracy enhancemen t algorithm will cause errors. This leads to noisy ,
nonrep eatable data for the other parameters, ev en if their measured data is represen tativ e.
2. Equipmen t external to the HP test set m ust b e switched dep ending up on the parameter
b eing measured.
There is no means of automatically switching equipmen t external to the test set dep ending
up on the S-parameter b eing measured.
Using 2-P ort correction, all four S-parameters are automatically selected in sequence,
and there is no signal a v ailable to indicate whic h parameter is b eing measured. If
dynamic range considerations make it necessary to c hange the test set conguration for
measuremen t of dieren t parameters, the automatic switching used in the 2-P ort error
mo del cannot b e used.
3. T est set conguration c hanges error terms other than frequency resp onse b et w een
calibration and measuremen t.
If c hanges to the test set b et w een calibration and measuremen t signicantly c hange
the directivit y , isolation, source or load match, or frequency resp onse error co ecien ts
measured during the calibration, then the corrected data will b e in error. If only the
frequency resp onse c haracteristics of the test set c hange, comp ensation can b e pro vided
using the display memory features of the HP 8510B. (See the Changing Signal P ath After
Calibration section in this c hapter.)
Operation 3-35
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