# LTSpice noise simulation with custom noise voltage source

I'm trying to simulate noise of a circuit in LTSpice.

I need to add custom white-noise voltage sources to the circuit, but when I do a noise simulation using the ".noise" command I don't see the contribution of these sources in the output.

I can't share my actual circuit but I created this example which shows my problem;

However when I simulate transient response I see the noise:

So the question is; How do I include custom noise voltage sources in the noise simulation?

Or am I forced to use transient simulation and then calculate the RMS voltage of the output?

• I think that you're trying to do a "noise" simulation (with a random signal) in the time domain (your 2nd plot shows time on the X-axis). The .noise statement doesn't work like that, .noise is a small signal analysis similar to the .ac analysis. Have you read: allaboutcircuits.com/technical-articles/… ? – Bimpelrekkie Oct 30 '20 at 11:10
• white() (and its cousins, rand() and random()) is a time-domain function, so it has no effect in the frequency domain. See Vinzent's answer for a common solution. – a concerned citizen Oct 30 '20 at 13:18
• @Bimpelrekkie I'm aware that noise is a small signal analysis similar to .ac.. This is the simulation I was intending. The reason for showing the second plot (transient) was to show that the white-noise source was working in the time domain. I understand now that the white(..) function only works in the time domain.. – Vinzent Oct 30 '20 at 13:33
• @aconcernedcitizen Thank you for clarifying that those functions only work in the time domain. – Vinzent Oct 30 '20 at 13:34