Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 133

Even though a VNA measures S‐parameters as its fundamental information, many other figures of merit may be computed directly from these measurements, through the use of transformations, found in several references (Hong and Lancaster 2001; Keysight Technologies n.d.-a). Most of these common parameters relate the voltage and current at the ports, rather than the a and b waves. Many of these transformations arise out of different definitions of terminal conditions as applied to Figure 1.2. These definitions arise out of DC or low‐frequency measurements, where it is an easy matter to short a terminal, meaning Z_L = 0, or open a terminal, meaning Z_L → ∞. An often confusing point is that it is not necessary that the terminals actually be opened or shorted, but most commonly the parameter is described in those terms. Just as it is most common to terminate the a 2‐port network in Z₀ to define S21, making a₂ = 0, it is not necessary to do so, and S21 can be determined with any terminal impedance as long as sufficient changes in a1 and a2 are made to solve Eq. (1.17), as shown in Eq. (1.21). Since the voltage and current relationships on the terminals of a DUT are easily determined from the S‐parameters, many other linear parameters can be determined as well. Unless otherwise noted, these transformations apply to the simple case where the S‐parameters are defined with a single, real‐valued reference impedance.