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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;

enter image description here

However when I simulate transient response I see the noise:

enter image description here

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?

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  • \$\begingroup\$ 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/… ? \$\endgroup\$ Commented Oct 30, 2020 at 11:10
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    \$\begingroup\$ 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. \$\endgroup\$ Commented Oct 30, 2020 at 13:18
  • \$\begingroup\$ @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.. \$\endgroup\$
    – user173292
    Commented Oct 30, 2020 at 13:33
  • \$\begingroup\$ @aconcernedcitizen Thank you for clarifying that those functions only work in the time domain. \$\endgroup\$
    – user173292
    Commented Oct 30, 2020 at 13:34

2 Answers 2

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This is (most easily) achieved in LTSpice by using a resistor as input to a voltage-controlled-voltage-source (VCVS).

enter image description here

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How do I include custom noise voltage sources in the noise simulation?

When doing an AC analysis, it scans the spectrum with a sinewave source hence your noise source is irrelevant and ignored in an AC analysis. If you want to see the spectral effects that the noise creates you'll need to run a transient analysis then, set up a Fourier analysis based on the transient results (between two points in time or the whole transient response).

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  • \$\begingroup\$ @Vinzent you are asking the wrong person - I am only reporting what I find to be true about simulators. \$\endgroup\$
    – Andy aka
    Commented Oct 30, 2020 at 11:45

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