Synchronize sampling with external trigger

[stefanox]

Starting from the Example "Configured Acquisition", using an NI-5122 device, I have sapled an external test signal (triggering the acquisition with the signal itself)
The first sample acquisition time shows a random delay on the triggering instant. The delay value is between zero and the sampling interval.
I saw the effect that I could see if I had (in the sampling device) an internal clock, running at the set SAMPLE RATE frequency and the sampling would take place at the first clock transition after receiving the external trigger.
The random phase relation between the external trigger and this hypothetical "SAMPLE CLOCK" leads to a random delay between the trigger and the acquisition instants
How can I avoid this and make the two events simoultaneous?
Thanks a lot for your help
Stef

[AlessioD]

Hi stef,

the fact that the trigger is randomly delayed between 0 and (Sampling Interval) is due to the fact that the samples can be acquired only on edges of the Sampling clock and the trigger condition can occur any time between two consecutive sample clock edges.
Basically, if you are acquiring at 100 MS/s, the sampling period would be 10 ns. The trigger condition can occur any time during this 10 ns period, causing the delay you are seeing.

What you may want to try is to enable the TDC property for the trigger circuitry on the NI-5122 Scope. The TDC is a method that allows for highly accurate trigger position calculation. It has a resolution of 100ps. In the documentation for the NI-5122 (see link below) you can read that Disabling the TDC (which is disabled by default) may reduce the rearm time for the trigger but it forces the trigger to be approximated to the nearest sample clock edge (basically the behaviour you are seeing).

Here you can find all the documentation available for NI HiSpeed Digitizers

http://digital.ni.com/manuals.nsf/websearch/44EEAE2AD6F15D2086256EEC006942E3?OpenDocument&node=132100_US

As far as the CLK OUT is concerned, you should refer to the niScope Configure Clock.vi to be able to route the clock to the external PIN.

Best regards,

AlessioD
National Instruments

[stefanox]

dear Alessio

thanks a lot for your help. But unfortunately seems like some of the 5122 properties cannot be handled by the 7.3 Ni-Scope functions. Both the "Enable TDC" and "Export Sample Clock" properties are present in the device features and in the Ni-scope Help, but are not related to any function (like NI-scope Config clock).
How can I access those properties?

Beside that, my WINDOWS 2000 recognize correctly the 5122 as a "NI Data Acquisition Device". Contemporarely recognize it as a common PCI device but cannot find the drivers. Is this the reason why when I run the DAQ Assistant the system doesn't recognize any Physical Channel?

Thanks again
Stef

[AlessioD]

Hi,

first of all I would like to make sure that you have the latest drivers installed to run your 5122 correctly.
Here you can download the latest version of the NI-Scope driver which is ver 2.7

http://digital.ni.com/softlib.nsf/websearch/2BB24B1FE891AE8F86256EF50054700E?opendocument&node=132060_US

In this document you can read that there is a list of required software that is necessary for your instrument to run correctly

- NI-DAQ
- NI-VISA
- IVI Compliance Package

Make sure you meet all these requirements on your win2000 machine.

Once you have all these Drivers installed, in LabVIEW functions under All Functions>>NI Measurements>>NI-Scope>> you should find all the functions necessary to your development. In particular there is a function called NI-Scope Property Node. This is the VI that lets you configure the TDC. You find the TDC Enable Property under the Horizontal category.

Also NI-Scope has a function called NI-SCOPE Express which serves the same purposes of the DAQ Assistant.

AlessioD
National Instruments

[stefanox]

Hello Alessio

I have locked the 5122 INTERNAL CLOCK to an EXTERNAL 10 MHz REFERENCE CLOCK connected to the CLK_IN connector. Exporting the INTERNAL CLOCK to the CLK_OUT connector I can see that it is in phase with the EXTERNAL REFERENCE CLOCK, but exporting the SAMPLE CLOCK I see it is not in phase with the REFERENCE CLOCK. Isn't the one signal a division of the other one? How can I anyway sinchronize the SAMPLE CLOCK with the REFERENCE CLOCK?

Thanx for your help
Stefano