R&S^®NRP Power Meter

Handling of external components through Γ and S parameter correction

Γ correction – taking into account the source mismatch

The most important source of error in power measurements on RF and microwave signals is the mismatch of source and sensor. Due to reflections that cannot be eliminated, it is not the nominal power P_GZ0 of the source that is transmitted to the power sensor, but the power P_i that deviates to a certain extent from the nominal value. To minimize the influence of mismatched sources, the standing wave ratio (SWR) at the sensor end was reduced to the extent technically feasible. A signal source with an SWR of 2, for example, still leads to an additional uncertainty of the measurement result of 4 % (0.17 dB). Although this error normally is decisive for total measurement uncertainty, it was frequently not taken into account because it could not be specified for the sensor alone.

Γ correction function: By taking into account the complex reflection factor Γ_G of the source,the measurement result (P_i ) is corrected in such a way that the nominal power of the source P_GZ0 is displayed.

Here the R&S NRP sensors boast another innovation: To reduce the mismatch uncertainty to almost zero, the complex reflection coefficient of the source is transmitted to the sensor via the USB data interface, and the sensor corrects the matching error by means of correction, taking into consideration its own impedance mismatch. This approach yields a measurement result of significantly higher precision.

Here, too, the user is offered a straightforward, standardizable solution. With the help of a small software tool that runs on any PC, the complete s-parameter data set of the twoport connected ahead can be loaded into the sensor's memory via the USB data interface. The data format required (s2p/Touchstone) is provided by any vector network analyzer.

S-parameter correction – taking into account additional components

A similar mismatch problem is encountered in test setups where the sensor cannot be connected directly to the source to be measured. Especially in production facilities, it is often necessary to connect a cable or an attenuator for level matching. In this case, the interactions between three components must be taken into account – a non-trivial bit of mathematics involving complex numbers.

After the source's complex reflection coefficient has been transmitted (optionally), a perfectly corrected reading is obtained; the sensor practically behaves as if it were connected directly to the source.

Shifting the reference plane from plane 1 to plane 2 by means of s-parameter correction. The influence of the additional component is compensated for, so that the measurement result output corresponds to the nominal power P_GZ0 of the source.