This is the gatekeeper concept. Van Valkenburg famously introduces . A rational function is "positive real" if it can be physically realized using passive elements (R, L, and C). If your math leads to a non-PR function, you cannot build it without active components or transformers.
Van Valkenburg presents network synthesis as a spectrum ranging from to a vital tool for daily engineering problem-solving. The text is specifically designed for advanced undergraduate or beginning graduate students, serving as a comprehensive introduction to the "pole and zero" approach to circuit design. Key Technical Concepts introduction to modern network synthesis van valkenburgpdf
M.E. Van Valkenburg’s Introduction to Modern Network Synthesis This is the gatekeeper concept
