  When a quantum system is monitored in continuous time, the result of the
measurement is a stochastic process. When the output process is stationary, at
least in the long run, the spectrum of the process can be introduced and its
properties studied. A typical continual measurement for quantum optical systems
is the so called homodyne detection. In this paper we show how the Heisenberg
uncertainty relations give rise to characteristic bounds on the possible
homodyne spectra and we discuss how this is related to the typical quantum
phenomenon of squeezing.
