Comment
Author: Admin | 2025-04-28
S12. In fact, almost all active circuits are non-reciprocal since transistors are inherently non-reciprocal devices.Anisotropic materials have different electrical properties depending on the direction a signal propagates through them. One example of an anisotropic material is the class of materials known as ferrites, from which circulators and isolators are made. Two classic examples of non-reciprocal networks are RF amplifiers and isolators. In both cases, the scattering parameter S21 is much different from S12. In fact, almost all active circuits are non-reciprocal since transistors are inherently non-reciprocal devices.A reciprocal network always has a symmetric S-parameter matrix. That means that S21=S12, S13=S31, etc. All values along the lower-left to upper right diagonal must be equal. A two-port S-parameter matrix (at a single frequency) is represented by:where S21 is identical to S12.If you are measuring a network that is known to be reciprocal, checking for symmetry about the diagonal of the S-parameter matrix is one simple check to see if the data is valid. Here is an example of S-parameters of a network that is either a non-reciprocal network, or your technician has a drinking problem.Although the data shows the part is well matched (S11 and S22 magnitudes are low), and low loss (S21 and S12 magnitudes are high). The magnitudes of S12 and S21 are equal, so what is the problem? The phase angles of S12 and S21 are significantly different. That can't be right.Properties of lossless networksFor a network to be lossless, all of the power (or energy) of a wave incident at any one port must be equal to the sum of the powers of the waves exciting from the other ports plus the power of the reflected wave at the incident port. Within a lossless network, no power is dissipated, i.e., converted to heat or radiation. Note that an active device is not in the same category as a lossless part, since power is added to the network through an external power source by its bias connections.Within the S-parameter matrix of a lossless network, the sum of the squares of the magnitudes of any column must total unity (unity is a fancy way of saying "one"). If any of the columns' sum-of-the-squares is less than one, then there is a lossy element within the network, or something is radiating.Why are we looking at sum of the squares instead of sum of the elements themselves? Because the S parameters are
Add Comment