Rollett's stability factor, calculated from the Sparameters 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 nonnegative 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
