HP 8753D, User Manual, Page: 361 / 678

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CAL ZO: SYSTEM ZO is selected when the desired measurement impedance diers from the
impedance of the line standard. This requires a knowledge of the exact value of the Z
0
of
the line . The system reference impedance is set using
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SET ZO under the calibration menu.
The actual impedance of the line is set by entering the real part of the line impedance as the
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OFFSET ZO within the dene standard menu. F or example , if the line was known to have
a characteristic impedance of 51 (
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OFFSET ZO = 51 ), it could still be used to calibrate
for a 50 measurement (
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SET ZO = 50 ). After a calibration, all measurements would be
referenced to 50 , instead of 51 . When the line standard is remeasured, the center of the
Smith chart is at the current value of
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SET ZO (in this case , 50 ). Since only one value of
oset Z
0
can be selected for the line standard, the value of Z
0
should be a constant value over
the frequency range of interest in order to be meaningful.
The location of the reference plane is determined by the selection of
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SET REF: THRU and
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SET REF: REFLECT . By default, the reference plane is set with the thru standard which
must have a known insertion phase or electrical length. If a non-zero length thru is specied
to have zero delay , the reference plane will be established in the middle of the thru. The
reect standard may be used to set the reference plane instead of the thru provided the phase
response (oset delay , reactance values and standard type) of the reect standard is known
and is specied in the calibration kit denition.
Note
Dispersion Eects
Dispersion occurs when a transmission medium exhibits a variable propagation
or phase velocity as a function of frequency . The result of dispersion is a
non-linear phase shift versus frequency , which leads to a group delay which
is not constant. F ortunately , the TRL calibration technique accounts for
dispersive eects of the test xture up to the calibration plane , provided that:
1. The thru (zero or non-zero length) is dened as having zero electrical length
and is used to set the reference plane (
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SET REF: THRU ).
2. The transmission lines used as calibration standards have identical dispersion
characteristics (i.e ., identical height, width and relative dielectric constant).
When a non-zero length thru is used to set the reference plane , it should be
dened as having zero length in the TRL standards denition, even though it
has physical length. The actual electrical length of the thru standard must then
be subtracted from the actual electrical length of each line standard in the TRL
calibration kit denition. The device must then be mounted between two short
lengths of transmission line so that each length is exactly one-half of the length
of the non-zero length thru standard. In this conguration, the measurement
will be properly calibrated up to the point of the device .
6-94 Application and Operation Concepts