How do you select only a portion of the data file to run an FFT on?

[DLC]

Collecting data on an explosive pulse power experiment.  Collecting RF waveform data on o-scope (100K points).  Collecting in a wide time window in order to get the entire event.  The resulting waveform shows that the interesting part is only a very small portion of this entire data stream.   I don't want to run an FFT against the entire collection because I am concerned that I am trying to figure out frequency content of the noise that I know is not in the real signal.  Is there a way in the DIAdem FFT analysis function to specify the range of time that I want to look at.  Since my scope data is time vs amplitude, I can figure out the time range.  I think I can accomplish what I want to do if I first make a seperate data set of just the data that I want to look at.  That is an extra series of steps that I don't want to have to do.  As it is I have a LabVIEW program that is collecting my data and then I suck it into DIAdem to generate a report. I would like to run the FFT on a subset of the entire data set in each channel.

Any suggestions?

[Manooch_H]

Hello DLC,

I believe the solution you are looking for can be achieved using the Time Intervals feature of the FFT with One Time Signal. If you go to the Analysis Pane in DIAdem and select Signal Analysis»FFT (One Time Signal), this will open up the FFT with One Time Signal dialog window. Towards the lower right of this window, you will see the Time Intervals... button. If you click this button, and enable the All bullet, you can select which data point you want to start with and how long of an interval you want the FFT to be performed on.

[Brad_Turpin]

Hi DLC,

If the ranges in your time series data where you have noise are easiest to determine by having a human look at them, you might consider doing this in VIEW.  In VIEW you can interactively highlight the noise regions and delete them from the time seried data, leaving only the non-noise data to run the FFT on, etc.

Brad Turpin
DIAdem Product Support Engineer
National Instruments

[DLC]

Brad, I don't see how you can run the FFT on anything in VIEW.  When you go to the FFT function dialog box all you can choose from is your data portal entries.  I think from what you described, you would have to create another entry in the data portal that only included the points you were interested in and then run the FFT on that.  That is what I was hoping to avoid.

Manooch suggested a solution that I think will work for me.  It does require me to figure out what data points within the 100K points my desired time interval covers.  I record my data in micro seconds so I have to convert time into points and intervals.  In the FFT dialog box the Time Interval is where I put in the starting point and the number of points in the interval.  When I ran the FFT with just my desired set of points and compared it to the FFT that I ran over the entire 100K data set I found that it gave me vurtually the very same results.  Since I was mainly interested in determining what the main frequency content of the waveform was, it showed me that I could run the FFT over the whole data set and live with the results I got from that.

Thanks for all your help.

DLC

[Brad_Turpin]

Hi DLC,

The interactive X range delete action in VIEW removes the data points from the existing channel(s), it does NOT create new channel(s).  The only question is whether you can most easily determine the X range of valid frequency content by dragging cursors in VIEW or by supplying a data point number range.  Both approaches will work and will produce the same result, I just wanted to point out that there were two approaches available.

Brad Turpin
DIAdem Product Support Engineer
National Instruments

[myra]

I read DLC's last comment, and apparently I'm hoping to be able to do what he didn't want to...  I have fairly large data files, and I would like to be able to make multiple subsets of this data that I can individually take the FFT of.  I was wondering if there is a way to select a data subset in VIEW and make a new channel in the data portal with it?  I would prefer this over trying to select a time interval in the FFT window.

Thanks, Myra

[Otmar]

Hello Myra,

Your request is a pretty simple procedure in DIAdem VIEW. I'll list the steps out below:

1. Drag and drop the channel(s) you want to create subsets of into a DIAdem VIEW "2D axis" window.
2. In the VIEW toolsbar, select the "Band Cursor".
3. Move the band so that the data you would like to create as a new subset channel is inside the band.
4. In the "2D axis" window, select the"Set Flags" icon - that will highlight the data within the band curso.
5. In the "2D axis" window, select the "Flags: Copy Data Points" icon

DIAdem will create new data channels in the DataPortal, there name will being with "Copy...". You can now use these data channels to run an FFT.

You can create as many copies of subsets as you wish. Please remeber to turn off Flags before you highlight another section of the data to copy, since you can flag multiple sections of a data set at the same time.

Let me know if you have any additional questions,

[myra]

Thanks!

I followed your instructions, and they worked great.  I had tried to do something similar with the flags yesterday but without any success.  I really appreciate your assistance.

Thanks,

Myra