We have an experiment in which fire a laser, and in the subsequent few hundred microseconds we detect events that ultimately give a TTL pulse for each one. The TTL pulses arrive at random time intervals after the laser has fired (which we can obtain a trigger pulse fro mif we need to), and there may be a few hundred TTL pulses after each laser shot. We want to determine the time of arrival of each of these pulses, and had figured that the counter/timer input should be able to do the job, giving a time-resolution of ca. 50ns (more than adequate). I should add that we are basically trying to get the card to act as a multichannel scaler -or MCS.
We had tried to use a modified version of the Measure Buffered Period example provide d in the DAQ library, but found that the clock appeared to 'lose' some time awhich loosely depended upon the count rate.
Has anyone got any experience of using a 602X seies card as a mutlichannel scaler for timeing pulses in this fashion and if so would they be willing to share info?
Andy Beeby Dept of Chemistry University of Durham Durham, UK email@example.com
This would have been the VI I would have recommended, and I have used it before to measure the time between pulses to get frequency. How is the clock losing time? Are you using the internal 20 MHz or 100 KHz clock? And what do you mean by the count rate?
We run the card with the 20MHz internal clock - our TTL pulses are clean (when we look at them on a scope, and are all terminated correctly) and have a duration of ca. 90ns. Our program, based upon the exmaple records the number of clock 'ticks' between the rising edges of the TTL pulses. When we put a 10Hz pulse train in the interpulse duration comes out at 100ms - spot on. When we start to try and time our random pulses, along with the 10Hz pulses we find that summing all of the time intervals between the 10Hz pulses gives random values - ie they fall between ca. 95 and 99ms, and there is a general trend that the larger the number of counts recorded in the 100ms period the larger the discrepancy, although I haven't found an exact correl ation. ( I refer to the mean number of counts per interval./laser shot as the 'count rate'.).
I should add that we know which pulses are which - ie which ones come from the laser, becuase we have a gate on the pulse train that only allows pulses that arrive at the detector for the period 0-10ms after the laser shot, giving a quiet 90ms interval befroe the next pulse, which is the laser TTL pulse. All of the pulses - ie from the laser and the PMT are generated by the same chip.
The buffered period measurement is definitely the one you want to use for your application. If you are loosing pulses, the only thing I can think of might be happening is the pulse itself is out of the board specifications. It would be good if you could monitor the pulses with a Scope to double check if the pulses are well behaved, and don't have any glitches. Other than that, you can test the counter of your board by hooking up a nice TTL square wave to the source pin and the same signal you are using at the gate pin to do the buffered operation. Hope this helps. Filipe