THORLABS SPCM A Series, Руководство по эксплуатации, Страница: 31 / 61

3 Operating Instructions
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3.5.5 External Gating
The External Gating is similar to Externally Triggered Counter mode. The significant difference
is that the APD is not always biased beyond breakdown voltage, but the APD bias is controlled
by an external gating signal. The advantage of this active gating is in a lower probability to miss
a photon: In previously described modes, the APD is always biased beyond breakdown voltage.
That means, it reacts to incident photons at any time. This reaction includes also the activity of
the quenching circuit; during the quenching time (35 - 45ns) the APD is "blind" for incident
photons. When the APD is gated actively, it is armed with the arrival of the gating signal so that
every first incoming photon can be counted.
In External Gating mode, the counter is not triggered on a selectable edge, but enabled as long
as a gating signal is present. The gating signal must be TTL level compatible, HI = active. As
this signal is not a trigger signal, in Alignment display the Trigger State will show up as dis-
abled.
In External Gating mode, an Array measurement is possible as well. It allows high measure-
ment speed by limiting the number of bins; after completing the count is stopped ( Single Array )
or restarted ( Continuously ) by the GUI. The max. array size depends on the measurement
speed, see section Definitions . The time diagram below illustrates the external gating mode:
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