Instabilities diagnosis and the role of K in microwave circuits

Many papers and textbooks and the leading CAD (computer-aided design) packages state that a two-port is stable if and only if K>1 and mod Delta /sup s/ mod <1 (or an equivalent set of conditions). The authors point out that the stipulation that the statement is rigorous only if no poles of the unloaded circuit lie in the right half plane is sometimes overlooked by current microwave designers who rely on K and mod Delta /sup s/ mod for determining the stability of their designs. Two examples are presented to illustrate the limited role of K in determining the stability of microwave circuits. The first example is an amplifier which was designed and manufactured in MMIC (monolithic microwave integrated circuit) form and the other is a simple study model of a ring oscillator. Both circuits fulfill all the above conditions for all omega but nevertheless exhibit strong microwave oscillations.<>