Why calibrated RF power meters are so important.

Why is traceability important?

While market-ready products for the D band will not be available for a while, the foundation for market approval needs to be laid much earlier. Calibrated RF power meters are needed to ensure that transmit power remains within legal limits. A standard needs to be created and made available to test equipment manufacturers that lets them calibrate measurement scales for power meters and make them traceable to national reference standards. This very challenging and expensive standard creation was completed for the D band in 2023.

Rohde & Schwarz was a partner in the joint European project. The company worked with Germany's national metrology institute (PTB) and other national metrology institutes. R&S NRP170TWG power meters were also calibrated for the D band. The meters are the only available RF power meters with a calibrated measurement range for frequencies above 110 GHz.

The role of Rohde & Schwarz in the calibration hierarchy

Fig. 2 illustrates the international principle underlying the calibration hierarchy and calibration chain.

As Germany’s national metrology institute, the PTB ensures that key measurands can be verified with extremely high accuracy. The PTB maintains high-precision test instruments and equipment for monitoring the measurement environment. Both together form the national standard. In the case of RF power, the national standard consists of an RF power meter and a microcalorimeter. For more information and to find out how Rohde&Schwarz traces RF power measurements, see [1] and [2].

The accredited calibration laboratories represent the next level. In Germany, the Deutsche Akkreditierungsstelle GmbH (DAkkS) is responsible for these accreditations. The company has a network of over 600 calibration labs. The labs maintain reference standards (RF power meters with measurement accuracy precisely derived from measurements based on the national standard and documented measurement uncertainty).

In this principle, measurement accuracy is transferred from the top to the bottom of the calibration hierarchy or calibration chain. Measurement uncertainty increases slightly with each additional link in the chain.

Rohde & Schwarz is situated directly below the PTB in the calibration hierarchy. The company operates accredited calibration laboratories in Memmingen, Munich and Cologne that are tasked with ensuring traceability to the national primary standard. These laboratories are a key element in our calibration infrastructure and industrial quality assurance.

Special reference standard features

The reference standards at the accredited calibration laboratories must be compatible with the calibration systems used by national metrology institutes. Only passive power meters based on thermistors or comparable technology can be used as a reference standard. In the industry, this sensor type has not been used for RF power measurements for a while. The excellent long-term stability of thermistors mean they are still used by national metrology institutes.

Rohde & Schwarz is one of only a few manufacturers worldwide that produces thermoelectric RF power meters that are suitable for tracing the RF power measurand. Such transfer standards can be used as reference standards by accredited calibration laboratories. Rohde & Schwarz now offers these transfer standards for the D band. The company is making a solid contribution to quality assurance and commercialization of new frequency bands for applications such as mobile radio.

Further reading on traceability of RF measurands

[1] Traceability of RF measurands. Judaschke, R., NEWS 199 (2009), pp. 28-33. Guest article by Germany's national metrology institute (PTB).

[2] Calibration of the RF power measurand at Rohde & Schwarz. Rösel, G., NEWS 199 (2009), pp. 34-37.