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DAQ Assistant can't lock the signals?

Now I'm using Function Generator to generate a signal, and using DAQ assistant to acquire that signals

But I find the signals is moving, I want make it locked and when I changed the frequency or phase, it can also displayed in the panel, What should I do?


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Message 1 of 12

I don't understand what you are asking for.


What do you mean by "lock the signals" and "I find the signals [are] moving"?


Perhaps you can post a screenshot of what the signals look like and what you want them to look like.

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Message 2 of 12

I think he is mistaking the DAQ device as an osziloscope which could "lock to an analog trigger" (e.g. rising slope of a sine at 0V).


There are DAQ devices where you can do such things, but not in continuous mode.

Continuous mode (as the name implies) will continue to acquire data without any gap as long as no error occurs or you stop the acquisition. Because chances are against you in order to acquire a complete integer number of periods of your signal within a "package" you read and display, the display will nearly always start to "move left or right". This is expected behavior.


The only "solution" for your request is to use finite acquisition, purchasing (if not already available) a device capable of analog trigger and configure it properly.


hope this helps,

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Expert: Geometry
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Message 3 of 12

Sorry for my mistake and poor english.


What I mean by"locked signal" is like using Simulate,  when you set the frequency to 50 HZ, it can be locked. But it will "moving" when you set the frequency to 50.1 HZ


So I want my acquired signals can be locked.

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Message 4 of 12

Then I think Norbert's reply pretty well explains what you are seeing.


What you will need to do is analyze your signal's data to find a point of interest such as a negative to positive zero crossing and use a subset of the data starting at that point to display your data on the XY or waveform graph.

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Message 5 of 12

And I also find that the trigger can't be used, why?


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Message 6 of 12

Trigger can be used just fine.  You have to select something.  I believe the Reference Trigger option is greyed out if you don't have compatible hardware installed but I'm sure you don't need it.


As far as the "locking" a signal goes, this has been explained to you.  The 50Hz signal is NOT locked in any way.  It is an integer value so it doesn't move but it is not locked any more than the 50.1Hz signal is.  What you're seeing is just a display artifact.  You can't trigger on a signal the way you're thinking without advanced programming techniques that are beyond the abilities of the DAQ Assistant (I could be wrong since I don't use the Express stuff).

LabVIEW Pro Dev & Measurement Studio Pro (VS Pro) 2019 - Unfortunately now moving back to C#, .NET, Python due to forced change to subscription model by NI. 8^{
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Message 7 of 12

Ok, even though it is not locked, but I still wanna see the acquired wavefrom stop "moving".

Do you have any ideas about this?


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Message 8 of 12

As I said in my post, you will have to analyze the data to find a point of interest such as the positive to negative zero crossing.  Take a subset of the data to eliminate the elements before that point and send the remaining data to the graph.  Now the first point shown on the graph should always look like the same part of your waveform.


There are probably some subVI's in the waveform and analysis palettes to help you find that point.

Message 9 of 12

It sounds complicated.


Maybe I should say how to locked the phase, because it will always change with time goed by.

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Message 10 of 12