Clock and Trigger

[creillyucla]

Hello,

My colleague posted a similar question, but I wanted to rephrase it in an alternative way to see if it will be more helpful.

We are using a PXI-5105 Digitizer with LABView.  For simplicity we are only using one channel (ch0).  The input to this channel is a PMT.

A different PMT is fed through a discriminator.  We want to feed the logic output of the discriminator into PFI-1 to trigger the Digitizer to take a single sample of the input to ch0.

This is what we want to do.  We want LABView to do this as quickly as possible.

We are able to do this using the "trigger" function of NI-Scope.  However, we have been told that using the "trigger" will drastically slow performance (limits acquisition to ~100 events per second).  The right way to do it is to use the "trigger" once to start a "clock".  The "clock" is then triggered by the PFI-1, which is a signal to digitize a single value of ch0.  This process fills a buffer ( size ~ 1000 events ).  When it fills up, you need to start the trigger again which in turn will start a clock.  This method is apparently much faster.

The previous paragraph is simply a repetition of something I was told; is this correct?  If so, how do we set up this situation?

Thanks,

Chris

[Paul_C.]

Hello Chris,

It sounds like you simply want a multi-record acquisition with each record only being 1 sample.  With the 5105, the rearm time on the referance trigger (when using the internal clock) is 2.4us.  This would allow you to get something close to 416666 samples per second.  The problem is that you will miss triggers while the device is rearming.  You cannot use the input to the PFI-1 line as your sample clock because it isn't free-running.  Our digitizers, such as the 5105, require a free-running clock.  This type of application, as I understand it, might be more suitable to a multi-function DAQ device such as our M-series cards.  These don't require a free-running clock and would allow you to specify the output of the discriminator as the sample clock.  This would allow you to catch "triggers" at a rate up to the maximum external sample clock rate of the M series device (1.25MS/s for 1 channel on some).  

If you plan on using the 5105, I would recommend starting with the niScope EX Multi Record.vi example or if you are taking records for a large period of time, you might take a look at the niScope EX Multi Record Fetch More Than Available Memory.vi example.  Both of these examples are setup to acquire multiple records from a reference trigger.  The more than available memory example simply allows you to specify to acquire more records than can be stored in the onboard memory.  Which, for you, is quite a lot considering each record is only 1 sample.  Both examples can be found using the Example Finder in LabVIEW (Help » Find Examples) and navigating to Hardware Input and Output » Modular Instruments » NI-SCOPE.

I hope this helps,
Paul C.