  We address the information/disturbance trade-off for state-measurements on
continuous variable Gaussian systems and suggest minimal schemes for
implementations. In our schemes, the symbols from a given alphabet are encoded
in a set of Gaussian signals which are coupled to a probe excited in a known
state. After the interaction the probe is measured, in order to infer the
transmitted state, while the conditional state of the signal is left for the
subsequent user. The schemes are minimal, {\em i.e.} involve a single
additional probe, and allow for the nondemolitive transmission of a continuous
real alphabet over a quantum channel. The trade-off between information gain
and state disturbance is quantified by fidelities and, after optimization with
respect to the measurement, analyzed in terms of the energy carried by the
signal and the probe. We found that transmission fidelity only depends on the
energy of the signal and the probe, whereas estimation fidelity also depends on
the alphabet size and the measurement gain. Increasing the probe energy does
not necessarily lead to a better trade-off, the most relevant parameter being
the ratio between the alphabet size and the signal width, which in turn
determine the allocation of the signal energy.
