# LTspice FFT Result Visualization Difficulty

Hi,

I am using LTspice on the circuit EMI spectrum study. Let's say I have two waveforms for V1 and V2. I wonder if in LTspice, I can plot the FFT results for :

(1) V1+V2

(2) FFT(V1) and FFT(V2) on the same figure

I tried but it does not seem LTspice support these functions. Could anyone help me with LTspice? Thanks for help!

Parents
• Hello Leey,

You can do math equations like V(a)+V(b) even with the FFT results in the FFT-waveform view.
Just enter V(a)+V(b) instead of V(a) in the FFT waveform viewer.

The FFT is a linear process. Thus FFT( V(V(a)+V(b) ) is equal to FFT(V(a)) + FFT(V(b)).

Helmut

• Hello Leey,

You can do math equations like V(a)+V(b) even with the FFT results in the FFT-waveform view.
Just enter V(a)+V(b) instead of V(a) in the FFT waveform viewer.

The FFT is a linear process. Thus FFT( V(V(a)+V(b) ) is equal to FFT(V(a)) + FFT(V(b)).

Helmut

Children
• Oh yes, I like this better.  The only way I could figure out how to do it was to plot one node, and then right-click on the node name to edit it to include the addition of a second node:

• Thanks Helmuts and AnneM, I tried this, but I still have problem. Every time I 'View -> FFT'， I am asked to select one node that's in the schematic, and when I select one, a NEW Window pops up and for a second signal, another window pops up, they are not showing in the same plot for me to do the summations. Please see the screen shot. But thanks for the information, I may have changed some configurations for the environment. Please let me know if you know what might have I done to my FFT window. Thanks.

• are you able to select more than one in each window?

• Oh right, AnneM, I just selected one node each time. Thank you, it works now.