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230 PI-MAX
®
4 System Manual Issue 4
15.2 Signal Delay
For the camera to see a transient signal, it is essential that it be gated on when the signal
arrives at the camera. If this requirement is not given careful consideration, it is possible to
set up an experiment in which the signal will come and go before the camera is gated on. If
this happens, no proper data can be taken. Depending on the nature of the experiment and
the specific equipment involved, a number of different factors may need to be considered to
be assured that the camera gates at the right time.
15.2.1 Time Budgets
A time budget is a listing of all the delays in the system that affect coincidence of the signal
and gate at the camera. Given a system that, in addition to the PI-MAX4 and an internal
timing generator, contains a low-jitter pulsed laser triggered from an external timer and an
external trigger source that is also triggering the pulse generator, a time budget for this
system might be developed as follows.
PI-MAX4 Internal Timing Generator Time Budge Calculation
• Signal Delay
—
Cable Delay from External Timer to Laser: 10 ns (6 ft cable is assumed)
— Delay (at laser); Trigger to Laser Pulse: 10 ns
— Delay; Laser Pulse to Sample: 10 ns
— Delay; Fluorescence Signal to Detector: 5 ns
Total Signal Delay: 35 ns
• Gate On Delay
—
Cable Delay from External Timer to PI-MAX4: 15 ns (10 ft cable is assumed)
— PI-MAX4 Insertion Delay; Trigger to Start of Gate Open Pulse: 25 ns
Total Gate On Delay: 40 ns
In this example, although the Signal Delay and the Gate On Delay are close, if the signal is
a pulse lasting only a few nanoseconds, it will have come and gone before the Gate opens,
and no valid experimental data could be taken. Obviously, this sample time budget is
unlikely to match any actual system and the values for both the Signal Delay and the Gate
On Delay could be very different from those indicated here. Nevertheless, it illustrates the
importance of making a record of the delays that will be encountered in any system to
determine their possible impact on experimental results.