Feature: RF and microwave


Why calibrated RF test stations disagree


By Tatiana Krasik, PhD – RF Measurement and Automated Test Engineer T


wo stations are calibrated, healthy and running the same test program on the same device. One reports -18.7dBm and the other -19.4dBm. Te difference


may come from the station architecture rather than a fault. At millimetre-wave frequencies, those details become much harder to ignore.


What a 0.7dB station offset means A familiar problem appears when an RF test moves from the laboratory bench to production: A device passes in engineering and fails in manufacturing. Te same problem can show up between two production stations built from the same bill of materials, where a difference of a few tenths of a decibel may go unnoticed until a batch of units approaches a test limit. Te first response is usually to look for a


fault: Repeat the calibration, swap a cable, check the operator procedure. Sometimes that finds the problem. In other cases no single fault appears, and the practical fix is simply to apply an offset to one station without establishing why it is needed. A 0.7dB offset is large enough to matter.


Against a ±1.5dB test window, it leaves only 0.8dB on the affected side instead of 1.5dB. Nearly half of the one-sided allowance has been used before variation in the device is considered. Te offset can come from several choices


that are easy to leave undocumented: the location of the reference plane, the adapters present during calibration, analyser configuration and the instrument state assumed by the automated sequence. Frequency doesn’t change the algebra of


any of this. A device with 12dB return loss interacting with a path having 16dB return loss produces the same mismatch bound at 2.4GHz as at 28GHz. What commonly changes with frequency is the practical sensitivity of the implementation: loss in an uncorrected path is oſten larger, connector and fixture discontinuities occupy a larger fraction of a wavelength, and maintaining stable match and phase through cables, switches and transitions becomes more demanding. Tose effects can matter at any frequency, but their control generally becomes less forgiving through the millimetre-wave bands. In this article, correlation means


agreement between stations, not a statistical correlation coefficient. Measurement uncertainty remains essential: it quantifies the values that can reasonably be attributed to a measurand and can reveal important contributors. It doesn’t, by itself, guarantee that two stations realise the same measurand, use the same reference plane, or establish the same configuration. Two results can each carry defensible uncertainties and still be unsuitable for direct comparison if those definitions differ.


16 November 2026 www.electronicsworld.co.uk


The measurement system Two identical signal analysers, with the same model, firmware and calibration status, do not constitute two identical measurement systems. Te instrument is one component in a chain and need not be the dominant contributor. Te measurement system is the complete assembly of: • Te instrument and its internal state; • Everything between the instrument connector and the device: cables, adapters, switch matrices, attenuators, fixtures, waveguide transitions, probes and antennas;


• Te correction data applied, and the plane at which it was established;


• Te soſtware that configures, sequences, triggers and interprets;


• Te thermal environment, and the connection event itself. An absolute amplitude accuracy


specification applies to the instrument under stated conditions, at its defined connector and in a defined configuration. It describes one component, not the complete station. Using it as though it were a station-level specification can make an otherwise explainable station-to-station difference look surprising. Te first point to fix is the measurement


reference. In practice, a result may be referenced to the instrument port, the end of the test cable, or the DUT connector aſter de-embedding. Each choice can be


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