counting ttl pulses

[CJKS]

Hello Everyone,

I am attempting to continuously record (i.e. time stamp) only positive (i.e. 'Logic High' event) TTL pulses from a PMT using a PCI-6602 board via a BNC-2121.

From reading the LabVIEW tutorials/examples it seems that I should be able to get very high precision (15 digits) from the "Get Date/Time In Seconds" VI. I need to be able to record times to the microsecond (at least). Yet when I look at the data it appears that for every digit less than a millisecond either a 9 or a 0 is recorded, which leads me to believe that I can't get better time resolution than a millisecond. Is there anything I can do to change this?

Assuming that I can get this to work I would also like to simultaneously monitor and record a second PMT channel. Any hints or tips would be great!

I've attached my very basic VI below.

Cheers,

CJ

Message Edited by CJKS on 05-22-2006 11:53 AM

[falkpl]

are you trying to count time between photon events or are you trying to count the number of photons during a time interval?  the 6602 has a 80Mhz clock so you will be able to get time resolutions of 12.5ns not miliseconds.  The "Get Date/Time In Seconds" VI refers to the system clock and is not nearly as good as using the 6602 's clock.  what exactly  are you trying to do? I couldnt open you vi since I am using 7.0. I have written many programs using photon counting and the 6602 in both traditional and daqmx.

Paul

[CJKS]

Hello Paul,

Thanks very much for the reply. To answer your question, (in the broadest sense) I am trying to build a software correlator, more specifically I want to determine the time of every photon event.

Currently I have a DAQ Assistant VI in a while loop accquiring (1 sample on demand) Digital In. The output of the VI is then run through an "index array" where I wire the resulting boolean to a true-false case structure.

The idea being when I have a photon event (i.e. Boolean = True) it executes the code in Case = True, namely a "Format Date/Time String" that writes the time to a file via a "Get Time/Date In Seconds" VI.

I'll see if I can upload a pdf image of the diagram.

So if I understand you correctly, by using the "get time in seconds" VI  I am limited by the system clock....? What other options for counting time do I have?

I can see how I could get the information I need by counting time between photons too.

Thanks again,

CJ

[CJKS]

Hi Paul,

I screen grabbed the LabVIEW diagram and put it into a word file.

[falkpl]

Yes the simpliest way to do what you want is to count the time between rising edges using a counter (the 6602 has 8 80 Mhz 32-bit counters).  Yo will not use digital io but time counting.  The gate of your counter will be the TTL/Cmos pulse from your PMT and the source of the counter will be the 6602 master clock 80Mhz, so you can count clock ticks between photons, now you have a 12.5ns time resolution.  You can use the system clock to get a time zero if you would like to use absolute time.  The olny thing to keep in mind is that you will be limited to a maximum time between pulses of 2^32/80M or 53 seconds, you will get an overflow of your counter but usually this is not a problem with photon counting.  You will be also limited to a maximum photon rate of about 20Mphoton events.sec this is also not a problem since the interesting photon events seem to happen between these time scales.  if you want a longer time scale you cvan always casceds 2 counters to get 64bit resolution but this is probablu not necessary.  Look at the buffered period measurment examples.  You can easily scale this to 8 counters with 8 PMTs if desired with your curent hardware but this gets expensive.  Goodluck.

Paul

[CJKS]

Hi Paul,

YES! You pointed me in the right direction and I think I got it! Thanks, for all the help.

CJ

[falkpl]

No problem.