Measuring S-parameters with a vector network analyser

The scattering parameters can be converted into the perhaps more intuitive Y, Z or chain circuit parameters, which are familiar from standard courses on electronic circuit design. S-parameter measurements in coaxial systems are predominantly carried out at a characteristic impedance of 50 Ω, or in waveguide systems at the characteristic impedance of the waveguide. In the case of a 50 Ω coaxial measurement system, the S-parameters therefore provide a very good description of the behaviour of the DUT at a source and load impedance of 50 Ω.

It is the developer’s task to select different source and load impedances or to modify the input and output matching networks so that the component performs even better – for example, by providing higher gain and being better matched to the actual source and load.

The vector network analyser, or VNA for short, can best be understood as the ‘RF engineer’s multimeter’. It measures the S-parameters of active and passive components in the RF, microwave and mmWave ranges. Using this data, the engineer can begin the circuit design or assess whether a sub-component meets the specifications and will not impair system performance when subsequently deployed.