6-6
Calibrating for Increased Measurement Accuracy
Calibration Considerations
Frequency Response of Calibration Standards
In order for the response of a reference standard to show as a dot on the smith chart display format, it must
have no phase shift with respect to frequency. Standards that exhibit such "perfect" response are the
following:
• 7-mm short (with no offset)
• type-N male short (with no offset)
There are two reasons why other types of reference standards show phase shift after calibration:
• The reference plane of the standard is electrically offset from the mating plane of the test port. Such
devices exhibit the properties of a small length of transmission line, including a certain amount of phase
shift.
• The standard is an open termination, which by definition exhibits a certain amount of fringe capacitance
(and therefore phase shift). Open terminations which are offset from the mating plane will exhibit a
phase shift due to the offset in addition to the phase shift caused by the fringe capacitance.
The most important point to remember is that these properties will not affect your measurements. The
analyzer compensates for them during measurement. As a result, if these standards are measured after a
calibration, they will not appear to be "perfect" shorts or opens. This is an indication that your analyzer is
working properly and that it has successfully performed a calibration.
Figure 6-1 shows sample displays of
various calibration standards after calibration.
Electrical Offset
Some standards have reference planes that are electrically offset from the mating plane of the test port.
These devices will show a phase shift with respect to frequency.
Ta b le 6-1 shows which reference devices
exhibit an electrical offset phase shift. The amount of phase shift can be calculated with the formula:
= (360 f l)/c where:
f = frequency
l = electrical length of the offset
c = speed of light (3 x 10
8
meters/second)
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