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Agilent AN 1287-1 Understanding the Fundamental Principles of Vector Network Analysis Application Note Table of Contents 2 Introduction 2 Measurements in Communications Systems 4 Importance of Vector Measurements 5 The Basis of Incident and Reflected Power 5 The Smith Chart 6 Power Transfer Conditions 9 Network Analysis Terminology 11 Measuring Group Delay 12 Network Characterization Introduction Network analysis is the process by which designers and manufacturers meas- ure the electrical performance of the components and circuits used in more complex systems. When these systems are conveying signals with information content, we are most concerned with getting the signal from one point to another with maximum efficiency and minimum distortion. Vector network analysis is a method of accurately characterizing such components by measur- ing their effect on the amplitude and phase of swept-frequency and swept- power test signals. In this application note, the fundamental principles of vector network analysis will be reviewed. The discussion includes the common parameters that can be measured, including the concept of scattering parameters (S-parameters). RF fundamentals such as transmission lines and the Smith chart will also be reviewed. Agilent Technologies offers a wide range of both scalar and vector network analyzers for characterizing components from DC to 110 GHz. These instru- ments are available with a wide range of options to simplify testing in both laboratory and production environments. Measurements in Communications Systems In any communications system, the effect of signal distortion must be consid- ered. While we generally think of the distortion caused by nonlinear effects (for example, when intermodulation products are produced from desired carrier signals), purely linear systems can also introduce signal distortion. Linear sys- tems can change the time waveform of signals passing through them by alter- ing the amplitude or phase relationships of the spectral components that make up the signal. Let's examine the difference between linear and nonlinear behavior more closely. Linear devices impose magnitude and phase changes on input signals (Figure 1). Any sinusoid appearing at the input will also appear at the output, and at the same frequency. No new signals are created. Both active and passive nonlinear devices can shift an input signal in frequency or add other frequency compo- nents, such as harmonic and spurious signals. Large input signals can drive normally linear devices into compression or saturation, causing nonlinear operation. A * Sin 360