  We address in this work the phase sensitivity of a Mach-Zehnder
interferometer with Gaussian input states. A squeezed-coherent plus squeezed
vacuum input state allows us to unambiguously determine the optimal
phase-matching conditions in order to maximize the quantum Fisher information.
Realistic detection schemes are described and their performance compared in
respect with the quantum Cram\'er-Rao bound. The core of this paper discusses
in detail the most general Gaussian input state, without any apriori parameter
restrictions. Prioritizing the maximization of various terms in the quantum
Fisher information has the consequence of imposing the input phase-matching
conditions. We discuss in detail when each scenario yields an optimal
performance. Realistic detection scenarios are also considered and their
performance compared to the theoretical optimum. The impact of the beam
splitter types employed on the optimum phase-matching conditions is also
discussed. We find a number of potentially interesting advantages of these
states over the coherent plus squeezed vacuum input case.
