  A pulsed source of entangled photons is desirable for some applications. Yet,
such a source has intrinsic problems arising from the simultaneous arrival of
the signal and noise photons to the detectors. These problems are analyzed and
practical methods to calculate the number of accidental (or spurious)
coincidences are described in detail, and experimentally checked, for the
different regimes of interest. The results are useful not only to measure
entanglement, but to all the situations where extracting the number of valid
coincidences from noisy data is required. As an example of the use of those
methods, we present the time-resolved measurement of the Concurrence of the
field produced by spontaneous parametric down conversion with pump pulses of
duration in the ns-range at a repetition of kHz. The predicted discontinuous
evolution of the entanglement at the edges of the pump pulse is observed.
