1
\$\begingroup\$

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?

\$\endgroup\$
4
  • \$\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\$ – Bimpelrekkie Oct 30 '20 at 11:10
  • 1
    \$\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\$ – a concerned citizen Oct 30 '20 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\$ – Vinzent Oct 30 '20 at 13:33
  • \$\begingroup\$ @aconcernedcitizen Thank you for clarifying that those functions only work in the time domain. \$\endgroup\$ – Vinzent Oct 30 '20 at 13:34
4
\$\begingroup\$

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

enter image description here

\$\endgroup\$
1
1
\$\begingroup\$

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).

\$\endgroup\$
1
  • \$\begingroup\$ @Vinzent you are asking the wrong person - I am only reporting what I find to be true about simulators. \$\endgroup\$ – Andy aka Oct 30 '20 at 11:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.