HP 8753E User Manual page 382

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 B"J$:G$fl~ under the calibration menu.
a characteristic impedance of 51 Q (~~~~~~~~~ = 51 Q), it could still be used to calibrate
referenced to 50 Q, instead of 51 Q. When the line standard is remeasured, the center of the
Smith chart is at the current value of :$m$@' (in this case, 50 Q). Since only one value of
offset ZO can be selected for the line standard, the value of ZO should be a constant value over
frequency range of interest in order to be meaningful.
the
must have a known insertion 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 dehnition.
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 (~~~~~~~:..;~~~).
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
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 dell&ion. 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.
By
Application and Operation Concapts