The Calibration Standards

During measurement calibration, the analyzer measures actual, well-defined standards and mathematically compares the results with ideal “models” of those standards. The differences are separated into error terms which are later removed during error-correction. Most of the differences are due to systematic errors-repeatable errors introduced by the analyzer, test set, and cables-which are correctable.

The standard devices required for system calibration are available in compatible calibration kits with different connector types Each kit contains at least one short circuit, one open circuit, and an impedance-matched load. In kits that require adapters for interface to

the test set ports, the adapters are phase-matched for calibration prior to measurement of noninsertable and non-reversible devices. Other standard devices can be used by specifying their characteristics in a user-defined kit, as described later in this section under “Modifying Calibration Kits n

The accuracy improvement of the correction is limited by the quality of the standard devices, and by the connection techniques used. For maximum accuracy, ensure that the connectors are clean and use a torque wrench for final connections.

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:

n7-mmshort (with no offset)

ntype-N male short (with no offset)

There are two reasons why other types of reference standards show phase shift after calibration:

w The reference plane of the standard is electrically offset from the mating plane of the test port. Such devices exhibit the properties of a smaIl length of transmission line, including a certain amount of phase shift.

nThe standard is an open termination, which by deiinition 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 pour unal~z~ ti working -Zg and that it has successfully performed a calibration. Figure 6-48 shows sample displays of various calibration standards after calibration.

Application and Operation Concepts 6-73

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HP 8753E manual Frequency Response of Calibration Standards