Two-port Measurements and S-Parameters

Network analyzers are the fundamental instrument for characterization of the devices and components used in RF and microwave systems. Network analyzers were briefly introduced in the previous laboratory for their use in impedance matching; they now will be examined in more detail within the context of component measurement. Transfer functions are more naturally expressed in the Laplace or frequency domain, and swept frequency instruments, like network analyzers, can provide a direct measurement of these transfer functions by sweeping a signal source and a tuned receiver across a range of frequencies. Two-port measurements provide explicit characterization of linear system blocks with a single input and a single output, by far the most common type of element. A two-port network could be as simple as a section of cable, or as complex as a complete transmitter-receiver link. In its simplest format, a reference excitation is applied to one port, and the response from the other port is recorded as a function of the sweep frequency. More complex elements with more than two ports can be characterized by extension, and the measurement techniques extend easily by simple multiplexing of the stimulus and response ports. Scattering parameters, or S-parameters for short, are the working language of network analyzers (NA). They provide a complete description of any linear, time-invariant element which then fully represents the behavior of that element within any system that it may be connected to. Scattering parameters involve phase information, and they are thus complex phasor quantities, and they are also frequency dependent. Once known, they can be transformed into other network parameters for circuit design, optimization, or tuning.