Scattering parameters of RF components and circuits are commonly measured by a vector network analyser (VNA) [11, 12]. The term ‘vector’ underlines that the amplitude and phase of reflection and transmission coefficients are determined. Figure 5.20 shows a vector network analyser with high-precision coaxial measurement lines and a test-fixture holding the device under test (DUT), in this case a microstrip filter on an alumina substrate.
A vector network analyser usually has two coaxial measurement ports with a port reference impedance of Z0 = 50 Ω. A one-port network or a two-port network can be directly connected to the measurement ports using transmission lines. If the network has three or more ports all unconnected ports have to be terminated by the port reference impedance. (In Figure 5.21 the tapered symbol indicates a reflectionless termination (r = 0) of port 3 to N.) By systematically connecting all network ports (arranged in pairs) to the measurement ports of the VNA all scattering parameters of a multi-port network can be measured successively with a two-port network analyser.