Including Source and Sensor Mismatch in RF Power Uncertainty

A calibrated RF power sensor can give repeatable readings while the power delivered to it differs from the intended matched-load quantity. Reflections between the source and sensor contribute an uncertainty that depends on both interfaces. Include that contribution before interpreting a small power difference as a real change in the product.

Define the power reference

The Rohde & Schwarz RF level uncertainty application note explains how source and sensor mismatch contribute to a power measurement uncertainty budget. Sensor calibration and connector mismatch address different parts of the measurement.

State the connector plane and the power quantity being reported. Distinguish delivered power into the actual sensor from a quantity referenced to an ideal matched load.

Collect the relevant interface information

Record sensor calibration factor, sensor reflection information, source match, adapters, and frequency. Use the actual configuration rather than a nominal connector impedance alone.

If only reflection magnitudes are known, use an appropriate uncertainty bound or statistical method rather than assuming a favorable phase relationship. If complex correction is used, preserve its calibration and reference-plane assumptions.

Check the effect of adapters and pads

An attenuator can improve the match presented to the source but also adds loss, uncertainty, and thermal dependence. Characterize it at the required frequency and power before using it as a measurement improvement.

For example, replacing a direct sensor connection with a padded connection may change the reading after simple loss correction. That difference is evidence to examine source-load interaction, not a reason to select whichever result is closer to the target.

Separate repeatability from accuracy

Repeat connections using controlled torque and cable support. Record the spread, but do not use that spread as the entire uncertainty budget. A systematic mismatch contribution can remain similar across every reconnection.

Check source stability over the measurement interval and sensor zeroing or range behavior as applicable. Changes in source output can otherwise become mixed with the mismatch investigation.

Evaluate uncertainty at each important frequency. Connector and device match can vary considerably across a band, so one center-frequency estimate may not represent the worst condition.

When comparing two boards, keep the sensor path and operating point controlled and assess whether their different output matches affect the comparison.

Save the reference-plane definition, interface data, correction method, and uncertainty budget. The reported power should carry precision consistent with that budget. A sensor’s calibration certificate is essential evidence, but it does not remove every uncertainty created by connecting that sensor to a real source and adapter network.