Error and Uncertainty

Sources of error are minimized by two different types of calibration: factory (external) calibration and user calibration. Sources of error can be characterized as either random or systematic. Random sources of error are not repeatable or predictable and can typically be reduced by narrowing IF bandwidth settings or through averaging. If you understand the systematic sources of error, however, and apply user calibration to reduce the errors, your VNA can make measurements with excellent accuracy.

The following figure shows the six systematic error terms associated with a full S-parameter dual source network analyzer measurement of forward S-parameters only.

The following systematic error terms are shown in the preceding figure:

• Transmission and Reflection Tracking—The first uncertainty is the transmitted and reflected signal loss across frequency, or transmission and reflection tracking, respectively.
• Load and Source Match—The difference between the DUT input impedance and the network analyzer or system impedance are the source match and load match, respectively. The same concept applies to the DUT output impedance.
• Directivity—An ideal directional coupler produces an output signal in the coupled arm that is proportional to the measured signal traveling in one direction of the main arm, while producing no output for a signal traveling in the opposite direction. The difference between the coupler output (coupled arm) and the measured input signal (through arm) is the coupling factor. Values of 10 dB to 30 dB are common, which means that the coupler output RF power is 10 dB to 30 dB less than the input signal passing in the through arm in the proper direction. In theory, a directional coupler should produce no output for a signal traveling in the opposite direction; however, in reality, a signal traveling in the opposite direction through the coupler produces an unwanted response at the coupler's output, which is known as coupler leakage. The difference between the coupling factor and the coupler leakage is known as coupler directivity.
• Isolation—A small amount of incident signal is radiated or conducted from port one and is detected at the port two receiver, which is known as isolation or crosstalk. An isolation calibration can correct for internal crosstalk for measurement of DUTs with high loss.

These forward terms combined with the six other terms from measuring the reverse S-parameters, total 12 error terms. User calibration needs to adequately account for these 12 error terms so that the VNA can apply the proper correction factors to the measured data. This correction is a primary contributor to the remarkable accuracy of vector network analyzers.