Agilent Technologies 8719D User Manual page 374

CAL 20: SYSTEM 20 is selected when the desired measurement impedance differs from the
impedance of the line standard. This requires a knowledge of the exact value of the Z0 of
the line. The system reference impedance is set using SET 20 under the calibration
The actual impedance of the line is set by entering the real part of the line impedance as the
OFFSET 20 within the define standard menu. For example, if the line was known to have
a characteristic impedance of 51 fl (OFFSET 20 = 51 Q), it could still be used to calibrate
for a 50 D measurement (SET Xi = 50 Q). After a calibration, all measurements would be
referenced to 50 Q, instead of 51 61. When the line standard is remeasured, the center of the
Smith chart is at the current value of SET 20 (in this case, 50 Q). Since only one value
offset Z0 can be selected for the line standard, the value of ZO 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 SET REF: TER?J and
SET REF: REFLECT. By default, the reference plane is set with the thru standard which
must have a knowninsertion phase or electrical length. If a non-zero length thru is specified
to have zero delay, the reference plane will be established in the middle of the thru. The
reflect standard may be used to set the reference plane instead of the thru provided the phase
response (offset delay, reactance values and standard type) of the reflect standard is known
and is specified in the calibration kit definition.
Note
Dispersion Effects
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. Fortunately, the TRL calibration technique accounts for
dispersive effects of the test fixture up to the calibration plane, provided that:
1. The thru (zero or non-zero length) is defined as having zero electrical length
and is used to set the reference plane (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
defined as having zero length in the TRL standards dehnition, 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 definition. 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 configuration, the measurement
will be properly calibrated up to the point of the device.
6-100 Application and Operation Concepts
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