Time Budgets; Measuring Coincidence - Princeton Instruments PI-MAX 3 System Manual

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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-MAX3 and an
internal timing generator, contained 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 appear as follows.
PI-MAX3 Internal Timing Generator
Signal Delay




Gate On Delay


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.

Measuring Coincidence

In addition to preparing a Time Budget, it is advantageous if you can directly measure the
timing of the critical signals. A fast oscilloscope can be used for this purpose. Without an
oscilloscope to monitor the signals, it will be difficult to determine the timing with
sufficient accuracy.
Tips: If you have a set of matching probes, use them. Major brand oscilloscope probes
of the same model are usually matched to better than 1 ns. If you choose to use coaxial
cables, measure the delay of each and use that information in the calculations.
The PI-MAX3 Monitor BNC connector provides a pulse coincident with respect to the
actual intensifier photocathode gating by ± 5 ns. Note that this output is not designed for
good fidelity but rather for accurate timing. The amplitude is typically more than a volt
and we suggest that you monitor the pulse with a high impedance probe (you can also use
50 Ω coaxial cable to a 50 Ω oscilloscope and you will get a lower amplitude pulse).
The signal timing will probably be more difficult to measure. Typically, you might divert
a small portion of the laser beam using a pellicle mirror located near the sample position.
By directing the beam to a PIN diode module, you could obtain an electrical signal that
could be monitored with the oscilloscope to accurately indicate the arrival of the laser
PI-MAX
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
Cable Delay from External Timer to PI-MAX3: 15 ns (10 ft cable is
assumed)
PI-MAX3 Insertion Delay; Trigger to Start of Gate Open Pulse: 25 ns
Total Gate On Delay: 40 ns
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3 System Manual Version 1.A