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Operating Concepts
Measurement Calibration
Measurement Calibration Measurement calibration is an accuracy enhancement procedure that effectively removes
the system errors that cause uncertainty in measuring a test device. It measures known
standard devices, and uses the results of these measurements to characterize the system.
This section discusses the following topics:
• definition of accuracy enhancement
• causes of measurement errors
• characterization of microwave systematic errors
• effectiveness of accuracy enhancement
• ensuring a valid calibration
• modifying calibration kits
• TRL*/LRM* calibration
What Is Accuracy Enhancement?
A perfect measurement system would have infinite dynamic range, isolation, and
directivity characteristics, no impedance mismatches in any part of the tes t s etup, and flat
frequency response. In any high frequency measurement there are measurement errors
associated with the system that contribute uncertainty to the results. Parts of the
measurement setup such as interconnecting cables and signal-separation devices (as well
as the analyzer itself) all introduce variations in magnitude and phase that can mask the
actual performance of the test device. Vector accuracy enhancement, also known as
measurement calibration or error-correction, provides the means to simulate a nearly
perfect measurement system.
For example, crosstalk due to the channel isolation characteristics of the analyzer can
contribute an error equal to the transmission signal of a high-loss test device. For
reflection measurements, the primary limitation of dynamic range is the directivity of the
test setup. The measurement system cannot distinguish the true value of the signal
reflected by the test device from the signal arriving at the receiver input due to leakage in
the system. For both transmission and reflection measurements, impedance mismatches
within the test setup cause measurement uncertainties that appear as ripples
superimposed on the measured data.
Error-correction simulates an improved analyzer system. During the measurement
calibration process, the analyzer measures the magnitude and phase responses of known
standard devices, and compares the measurement with actual device data. The analyzer
uses the results to characterize the system and effectively remove the system errors from
the measurement data of a test device, using vector math capabilities internal to the
network analyzer.
When you use a measurement calibration, the dynamic range and accuracy of the
measurement are limited only by system noise and stability, connector repeatability, and
the accuracy to which the characteristics of the calibration standards are known.