The Stability Radius: A New Indicator of Unconditional Stability for N-Port Linear Networks

A new indicator of unconditional stability is proposed, which is well suited to N-port networks called the stability radius. When compared to previous work on this subject, the new indicator is unique, irrespective of N, and has a clear geometrical interpretation in terms of allowed reflectances in the termination planes. The latter aspect endows it with the ability to describe the (unconditional) stability margin quantitatively. The case for general N needs to be addressed, as in the previous literature, with numerical techniques, but closed-form expressions are derived for N ≤ 2. The present work brings to light a subtle error in a previous publication and provides a way to correct it.