Why does df of FFT changes after i change the input from dynamic data into scalar waveform?

[How81]

Hi all,

I have 2 questions:

Question 1:

I have a problem with the data I obtain from DAQ. Here's the scenario, At the (1000 sample/second, 2000 samples) DAQ data output, I fed into the FFT, I get a df of 0.5 seconds.

But when I change the dynamic data into scalar waveform, and fed it into FFT, the df value becomes 0.0005 instead. How is that so?

Question 2:

Assuming I used dynamic data from DAQ as input for the FFT block, I want to get the first 60 readings from the FFT output, I could not.

I tried :

Unbundling by name,

converting cluster into array,

Both tell me the sink and the source are incompatible. I look for advice in this field, could anyone be kind enough to tell me what exactly should I use to capture the first 60 readings after FFT ? I need to manipulate the readings later on, thus need to extract them out, possibly in array? or any better suggestions are welcomed!

Thanks!!

How 

[How81]

And can someone please tell me why I can't uncluster that power spectrum after the FFT?

Thanks!

[CharlieM]

Hi How,

I placed an FFT Power Spectrum.vi on a blank Block Diagram and played around with it for a bit. I think most of your questions could be answered by the LabVIEW Help for this VI and by diving into the VI (double clicking on it and viewing the Block Diagram). For example, I found:

1. The df is directly proportional to the inverse of the dt of the input signal. So it programmatically changes when you change the input.

2. The FFT Power Spectrum.vi (I’m assuming this is what you are using) is polymorphic. There are two instances and they occur based on the signal wired in.  Again, you can read about this in the LabVIEW help and see it by investigating the VI. For a quick test: take your dynamic signal and “Convert from Dynamic Data” before you input the signal to the FFT VI. You will than notice that you can “Unbundle” the output.