  It is well known that the Husimi Q-function of the signal field can actually
be measured by the eight-port homodyne detection technique, provided that the
reference beam (used for homodyne detection) is a very strong coherent field so
that it can be treated classically. Using recent rigorous results on the
quantum theory of homodyne detection observables, we show that any phase space
observable, and not only the Q-function, can be obtained as a high amplitude
limit of the signal observable actually measured by an eight-port homodyne
detector. The proof of this fact does not involve any classicality assumption.
