Gated event counting for a finite time

Question

I am using a PCI 6110 card for gated photon counting. The digital signal is coming from a single photon counting module, which generates a single TTL pulse upon receiving a photon.  It also generates ~100 dark count /sec.  To get rid of the dark counts, I use gate pulses with very small duty cycle.  In this way, the dark counts are suppressed.  I want to operate for a fixed time interal with a resolution of ~1ms.  My gate pulses are at 800kHz.  Their duration is on the order of 10ns. Is there a way that I can perform gated counting for only 10000 gate pulses?  Thanks.

Answer 1

Hi UMeng,
Your question stirred up an interesting discussion at the office.  You can't do gated, buffered counting as the gate input is used to trigger the latch to the buffer.  We came up with a couple of ideas here, but neither are perfect.  It is important to also consider that you are running right at the specifed minimum pulse durations for the gate and source pulse durations for the 6110, which is 10 ns. 
NI PCI-6110/6111 Specifications
http://digital.ni.com/manuals.nsf/websearch/A9694D34EF10469386256E3000539747
With that in mind, you could use an external high speed AND gate to generate a signal for a simple event counter to read using the laser gate and the photon detector output as input to the AND gate.
Otherwise, you could try doing gated event counting on the signal and simple event counting on the gate as long as the gate frequency is consistent.  Then you could use the frequency of the gate and number of counts of the gate to get the total acquisition time.  This will require that you start and stop both counter tasks at as close to the same time as possible.  The start could be triggered with an Arm Trigger.  Getting the values from the counters would always require 2 software calls, but the longer the acquisition, the less important this error would become.
Let me know if either of these sound appealing.
Regards,
John Bongaarts
AE Specialist - Test Products