stability factor for lower frequencies?

Hello everyone.
I am designing a VGA for signals up to 3Ghz.
It seems that everytime I put a feedback in my circuit the K factor gets lower than 1, though the phase margin and gain margin calculations would seem to suggest that the system is stable.
I was wondering whether I can still trust the K factor for lower frequencies and in systems with feedback.
Does anybody know where I could find some info about that?

Thank you

You can try Mu and MuPrime, that can give you better indications.