FFT Zoom

[RobtheVib]

Hello Everybody,

Hope you had a nice Easter holiday (away from LabView!)...

Could anybody have a look at my VI and explain to me why my zoom doesn't work? My VI is  based on the example VI in LabView7.1, which seems to do exactly the same thing but, of course, does work. Why mine doesnt?

It's a simple VI that finds the index of the peak value of an FFT array, cuts out a certain length of the signal (where the peak lies) and displays it on a graph. The problem is that the graph doesn't show the correct freq. values and, instead, starts always from zero although the length of the zoom is correct.

Thanks in advance...

Roberto

[johnsold]

Roberto,

Look at the context help for the Waveform Graph: It shows how to use a Bundle node to provide an Xo and dX values to the graph. When you extract the subset near the peak, that information gets lost. The new Xo is the index of the start of the subset multiplied by the dX (actually dF since your data ahs been transformed to the frequency domain) plus the original Xo (which can be obtained from the FFT data before the peak finder).

I cannot easily put it into LV7.1 for you, sorry.

Lynn

[Dennis_Knutson]

Try this. While the Extract Portion of Waveform is getting the zoomed subset, it does not change the t0 of the waveform. So, what I did, was just change the min and max of the x axis. This is the same thing that happens when you use the zoom tool on the graph palette.