Adjusting The Signal Delay - Princeton Instruments PI-MAX System Manual

Chapter 11 Tips and Tricks 137
The PI-MAX Monitor BNC connector provides a pulse which, although delayed with
respect to the actual intensifier photocathode gating by 4-6 ns, quite accurately indicates
the intensifier photocathode gating time. 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.
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
beam at the sample position. Note that the indicated time would have to be corrected for
the insertion delay of the path from the pellicle mirror to the oscilloscope, including the
insertion time of the PIN diode, which might be on the order of the 10 ns. This correction
would have to be compared with the delays that would normally exist between the sample
position and the detector to determine the actual time the signal would arrive at the
detector. Also, the oscilloscope will have its own insertion delay, perhaps 20 ns, and an
uncertainty of nominally 1% of the time base.
Another consideration is how to trigger the oscilloscope. If there is a common trigger
source for the sample position signal and for the gating, that trigger could also be used to
trigger the oscilloscope, allowing both signals to be observed simultaneously. Another
possibility is to trigger the oscilloscope from the PIN diode signal to observe the Monitor
signal, or to trigger the oscilloscope from the Monitor signal to observe the PIN diode
signal. The signal that occurs first would have to be used as the oscilloscope trigger. This
is not necessarily always the case. Digital oscilloscopes can display signals that occur
before the trigger.
For even more precise measurement of the gate time, one can use the noise burst on the
Pulse Monitor output. The "noise" is actually induced voltage resulting from the fast rise
time gate pulse and is within ~2 ns of the true optical gate.

Adjusting the Signal Delay

The PTG and DG535 give the user wide latitude with respect to adjusting the delay
between the time the timing generator is triggered and the time the Gate On and Off
edges are generated. This being the case, as long as the light signal applied to the detector
occurs after the minimum delay time of the timing generator (25 ns for the PTG; 85 ns
for the DG535), there will be no problem establishing the necessary coincidence.
On the other hand, if the light signal applied to the detector occurs before the minimum
delay time of the timing generator, then no amount of adjusting the delay at the timing
generator can rectify the problem. The light signal itself will have to be delayed.
If a common source is triggering both the timing generator and the laser, a very
convenient solution is to insert electrical delay (long cable) between the trigger source
and the laser. This is generally preferable to establishing the necessary delay optically via
mirrors or fiber optic cable.
Alternatively, pass the laser output through a length of optical fiber cable. By using
different lengths of fiber, almost any desired signal delay can be achieved. Yet another
solution would be to set up two separated parallel mirrors with a small angle between
them. Typically, it will be easily possible to bounce the laser beam back and forth
between the mirrors half a dozen times to obtain the necessary delay. In any case, once
the light signal is arriving at the detector after the minimum gate time, the timing
generator delay adjustments can be used to bring them into coincidence. Keep in mind