How do you create side band cursors on a spectrum plot? How do I subtract out a waveform without altering other frequencies amplitudes?

[LabViewer35242]

I'm trying to create sideband cursors on a frequency spectrum plot.  I have used DAQ software that allows you to select a primary frequency and drag sideband cursors to whatever df you want.  I would need about 7 cursors on each side of my 60Hz primary frequency.  I would then need a window or something that would show all 14 cursors and their frequencies and amplitudes.

I'm also trying to do subtract out the primary 60Hz frequency and display this on a frequency spectrum.  I'm doing this by generating a 60Hz sine wave at 110Amps (pk) amplitude, and multiplying this by my waveform. I think I should get out waveA+waveB and waveA-waveB.  I then put the waveform through a lowpass filter and do an FFT to display the spectrum.  I must have forgotten too much of my 9th grade trig though, because my amplitudes at 2Hz and 4Hz (were 62Hz and 64Hz) are now wrong and nonlinear (they are off by different factors). What is wrong with my logic here?

[charris]

Would you be willing to post a screen shot of the block diagram? If you're simply working with arrays or waveform data types, you can simply subtract the 60Hz sine wave from the input waveform. You may also want to consider using some of the built-in filter VIs (located in the "Signal Processing --> Waveform Conditioning" palette).

I'm probably oversimplifying, so take this with a grain of salt:

Judging by the image you posted, it looks like you've successfully gotten one cursor onto the graph. If you want more, simply right-click in the cursor legend window and select "Create Cursor --> Single Plot" to add another one.

[LabViewer35242]

Demodulation:

I attached 3 screen shots.  On the 2nd jpg I am trying to do the demod (subtract out 60Hz) two different ways.  The way on top I got from sample code a few years back, and the way I did it on bottom is very simple and the way I would imagine you should subtract out a signal.  I am just multiplying my waveform by the frequency (60Hz) I want to subtract out and then applying a filter.  Can someone mathematically explain why/how I can correct for the amplitude changes (Refer to: Demod Viewer Print Screen3.bmp, and what I am doing in the bottom part of Demod Viewer Print Screen2.bmp)? "Demod Viewer Print Screen3.bmp" shows the spectrum from the original waveform and spectrum after multiplied by 60Hz and put through a low pass filter.

Cursors:

I could put more cursors on the graph, but the idea is to be able to move the cursors together by dragging just one.  So, you would set your primary cursor at 60Hz and then have 7 more cursors to the right of it and 7 to the left. I would like to be able to drag the first cursor to the right to about 62Hz and then the other cursors would be at 64,66,68,70,72,&74 Hz. This makes it much easier to see if a peak is at a modulating frequency.  For instance if I wanted to look at the modulating frequencies 3.27Hz away from 60, I wouldn't have to figure out if a peak at 70.1 Hz is a modulating frequency.... Which it wouldn't be, but 63.27, 66.54, 69.81, 73.08 etc..would be.

[LabViewer35242]

Attachments

[charris]

Well, turns out you're not the only one struggling with 9th grade trig

I'm not seeing anything immediately wrong with your current demod code. Is there a chance you could post your VIs on here?

Also, can you provide any more info on the source signal that you're attempting to demodulate? I'd like to take a look at the signal itself, if possible.