Chapter 7 Gated Operation with a PTG 87
will mean inserting an additional delay of 500 ns to accomplish coincidence at the detector.
MCP bracketing should only be used in experiments where it is going to make a difference.
Note that background light need not be the limiting factor in measurements where MCP
bracket pulsing is unable to provide the required degree of rejection. In such
measurements, the option remains of installing an external shutter ahead of the PI-MAX.
Impact of Bracket Pulsing on Delay
If operating in the UV when bracket pulsing is activated (Gen II Intensifier only), the MCP
gate automatically brackets the photocathode gate pulse to further enhance the on/off ratio.
There is, however, a limitation of bracket pulsing that can complicate the coincidence of
the signal and gate at the camera. Because MCP bracket gating is much slower than
photocathode gating (500 ns is required to gate the MCP on and another 200 ns to gate the
MCP off at the end of the photocathode gate). As a result, MCP bracket pulsing should not
be used in experiments where the delay between the trigger and the photocathode gate is
less than 1 µs.
Note: The 1 µs restriction does not apply when you are using a PTG with Anticipator
mode selected or when you are using a PTG, a pre-trigger is available, and the Pre-
Trigger mode is selected.
Setup
MCP Bracket pulse implementation in a system having a PTG is accomplished by
selecting Bracket Pulsing ON from the host software and selecting the Bracket Start
parameter appropriate to your experiment (see choices below). The Timing Gen cable
carries the signal that gates the MCP on and off if bracket pulsing has been turned on.
Figure 35, page 88, is a timing diagram for a PTG that has been set up for bracket pulsing
with a "Main" bracket start. Note that the insertion delay is 25 ns instead of the 85 ns
insertion delay characteristic to the DG535.
The four Bracket Start choices are:
Main: Main times the bracket start from the main trigger (either an external or an
internal trigger). Select this if the event of interest will occur at least 1 µs after
the main trigger (the 1 µs includes the 500 ns required to turn the MCP fully on).
Main+Burst: Select this if Burst mode is active. Brackets an entire burst but not
the individual pulses within a burst.
Pre-Trigger: Allows you to capture an event that would otherwise occur while
the MCP was turning on. This selection requires that, in addition to the main
trigger at Ext Trig In, you trigger the PTG by a pulse applied to the Pre-Trig In
BNC. The pre-trigger must be at least 525 ns before the main trigger. After an
insertion delay of approximately 25 ns, the MCP will be gated on before the main
trigger occurs. The main trigger plus the gate delay and width parameters
determine the photocathode gating and the MCP turn off. Because the MCP is
gated on earlier, it is on for a longer period of time and therefore there is a slight
loss of on/off ratio. However, you would be able to capture a non-repetitive
event.
Anticipator: Select this if the experiment is repetitive, driven by an external
trigger at a constant frequency, and the event of interest would occur before the
MCP has fully turned on. It is critical that the trigger pulse be as jitter free as
possible, since the anticipator circuit will look at the pulse repetition rate (for the
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