Rollett's stability factor, calculated from the S-parameters by
k = { 1 - |S11|^2 - |S22|^2 + |D|^2} / {2 |S12| |S21| }
where D is the Determinant, D = S11 S22 - S12 S21
Devices with a k Factor > 1 and a non-negative determinant at a given frequency
are said to be unconditionally stable at that frequency, meaning they will not
oscillate due to load mismatch.
see also stability
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