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Principles and Care of Calibration Standards
Principles and Care of Calibration Standards
This section explains the special care required by network analyzer calibration standards ,
principles behind their use , and common problems associated with them.
Calibration Standards Require Careful Handling
F or best accuracy and repeatability , use great care in handling and storing the cal standards .
Their performance and accuracy depend on very precise mechanical tolerances , sometimes on
the order of a few ten thousandths of an inch. Therefore , cal standards must be handled and
stored more carefully than ordinary devices .
Proper Inspection, Cleaning, and Connection
Inspect and clean the connectors using the methods recommended in the calibration kit
manuals . Before using cal standards , gage the test port connectors , standards , cables , and
the test device to verify that the mating plane dimensions of all connectors are within the
allowable tolerances . T o minimize repeatability errors , use an appropriate torque wrench when
tightening connections . Cal Kit manuals supply detailed information on cal standards , and
explain proper techniques for using them.
Principles of Operation
Network analyzer accuracy-enhancement is accomplished by measuring known cal standards .
The measured response is compared to the predicted response , then error terms are derived
from the magnitude and phase dierence between the measured response and the predicted
response . The predicted response is determined by using a complex mathematical model which
predicts the magnitude and phase response of the cal standard over its entire frequency range .
Thus , the accuracy improvement which can be expected is directly related to how well the
models predict the response of the standard. The model for each standard is specied in a data
le on the disc supplied with the calibration kits .
Examples of \perfect" standards are shown in the assumptions made for the xed and sliding
loads used in reection calibration. The device impedance is assumed to be exactly the system
characteristic impedance , Z
0
, usually 50 ohms .
Quality of the Standards Aects A ccuracy
The quality of the load used for calibration determines the eective directivity for reection
measurements . A high quality xed load exhibits the lowest repeatable return loss . The quality
of the sliding load is determined by the return loss of:
The connector .
The transmission line between the connector and the sliding element.
Standard Models Dier Depending on Connector Type
Standard models dier according to connector type . F or example , the short circuit in the
Hewlett-P ackard 7 mm calibration kit is modeled as a perfect zero ohm termination, having a
reection coecient of 1
6
6180 positioned at the reference plane . The short circuit in the
Hewlett-P ackard 3.5-mm calibration kit is modeled as a perfect short displaced about 1 cm from
the reference plane .
Specications for the shielded open circuits add a reactive phase shift to the modeled response
characteristic . In order to model the typical non-linear phase shift, the shielded open circuit is
assumed to exhibit a phase shift with frequency that can be approximated using the equation
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