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. Table 6-1
shows which reference devices exhibit an electrical offset phase shift. The amount of phase
shift can be calculated with the formula:
Φ = (360° × f × l)/c where:
f = frequency
l = electrical length of the offset
c = speed of light (3 x 10
8
meters/second)
Fringe Capacitance
All open circuit terminations exhibit a phase shift over frequency due to fringe
capacitance. Offset open circuits have increased phase shift because the offset acts as a
small length of transmission line. Refer to Table 6-1.
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