NI 6259 photon counter min pulse detection

Question

Hello,
I have a PCI 6259 DAQ card which I'm using as a counter. Is there a limit to the minimum pulse width it can count while operating as a counter? My photon counter gives out TTL pulses with widths of 50ns which I think is below what the counter can read unless I am mistaken. I'm using the 100kHz internal clock as the clock source. Is there anyway to get the counter to pick up such small pulses?
Would the NI PCI 6120 DAQ card be a better choice for this type of measurement? I should be able to get my hands on one shortly.
Thanks for your time,
Mark.
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Answer 1

A 50 ns pulse corresponds to a 20 MHz freq.  You should be able to capture/measure such a TTL pulse with a 6259, but I don't think you'll really want to be measuring pulse *widths*.   You'd only be able to get a nominal count of 4 timebase edges per width with a possible +/- 1 count uncertainty, i.e., 25% measurement error.
More likely, you'd either want to:
        1. measure the period/frequency of successive rising edges of these 50 ns pulses, effectively timestamping every detected photon.   Brief fast bursts of photons may produce an unrecoverable FIFO error.   A newer X-series device with a bigger counter FIFO would be a better choice if this is an issue.
OR
        2. perform "binning" by counting the # of these 50 ns pulses that occur during equal time intervals.   Here you are pretty immunce to brief bursts of photons, but you need to sustain long-term throughput over the PCI bus.  You're probably pretty safe sampling at <= 100 kHz or so, but if you need better precision on the bins & thus faster sustained throughput, I'd again recommend a PCIe X-series board. 
   You might also have to watch out for a feature known as "Duplicate Count Prevention" which could impact your measurement data if you ever have 0 photons in a given interval.  The faster you sample, the more likely this is to come up.
-Kevin P