  We derive an analytical model describing the effect of filtering on amplified
spontaneous emission noise during or after opto-electronic conversion. In
particular, we show that electrical filtering results in a further reduction of
the signal quality factor associated with an effective increase of the noise
levels and can lead to counter-intuitive dependencies of the measured signal
quality on the characteristics of the test setup. Closed form equations are
compared with numerical models and experiments, showing excellent agreement.
