3-2 Traceability and Uncertainty
Chapter 3 — Performance Verification
3-2
PN: 10410-00730 Rev. D
MN469xC Series Multiport Test Set MM
3-2
Traceability and Uncertainty
Vector network analyzers (VNAs) are precision instruments for making high frequency and broadband
measurements in devices, components, and instrumentations. The accuracy of these measurements is affirmed
by demonstrated and adequate traceability of measurement standards. Metrological traceability, per
International vocabulary of metrology, JCGM 200:2008, is property of a measurement result whereby the
result can be related to a reference through a documented unbroken chain of calibrations, each contributing to
the measurement uncertainty. For the accuracy of VNAs and quality assurance by users, two standard
approaches were created to ensure sound metrology traceability. One is to construct tight uncertainty budget
and specifications in three tiers from the ground up, and the other is to develop a calibration hierarchy for
systematic verification. The three-tier process is depicted in the following sections.
First Tier of Uncertainty - The VNA Calibration
A traceable VNA itself for coverage from 70 kHz to 70 GHz requires proper calibration for several key
quantities, e.g., frequency, power level, and high level noise, via traceable standards to the SI units. Each
contributing uncertainty was evaluated at the time of instrument calibration.
The inception of a precision VNA is accuracy-enhanced 50 ohm impedance, which is characterized in lieu of
coaxial transmission lines all with proper propagation properties throughout the whole measurement system,
including the device-under-test. A transmission line for VNAs is best represented by a coaxial airline, which
was precisely selected and machined based on the electromagnetic properties such as conductivity and skin
depth, etc. Therefore, the dimensional measurement accuracy of the airline gives out the first tier of
measurement uncertainty of impedance quantity.
Second Tier of Uncertainty - Systematic Measurement Errors
The second tier of uncertainty, corrected or residual uncertainty, is the result of the accuracy enhancement of
VNA calibration to remove systematic errors. Systematic measurement errors are components of measurement
error that, in replicate measurements, remains constant or varies in a predictable manner. This accuracy
enhancement is usually the function of calibration kits. The choice of calibration kits used will dictate the level
of uncertainties for the intended measurements or applications.
Third Tier of Uncertainty - Random Measurement Error
The third tier of uncertainty is random measurement error that, in replicate measurements, varies in an
unpredictable manner. The examples are connector repeatability and cable stability, etc. Random
measurement error equals measurement error minus systematic measurement error.
Standards and Verification
Most often, instrument end users demand system verifications in order to provide a quality check or assurance.
This is accomplished by utilizing a set of known or characterized devices, e.g., verification kit, for comparison.
It can also be done by using devices that are different from the calibration kit. The calibration hierarchy of
verification uncertainty is built through unbroken chain comparisons with the national standards.
•
Physical standards
→
airline dimensionality
→
impedance standard
→
residuals and port parameters
•
Basic power standards
→
power sensors
→
power accuracy specifications
•
Basic time standards
→
frequency reference source
→
frequency accuracy