# Noise density analysis

I like that in LTSPICE during a noise analysis when I use the same frequency for the first and last frequencies (e.g., .noise V(out) V1 dec 1 1K 1K) it switches into a voltage noise density analysis and gives you both the output and input referred noise, the noise contribution by every circuit element, as well as the gain of the circuit.

It looks like QSPICE displays the noise at the output and the noise referred to the input in the output window but only when the analysis is performed over a band of frequencies, when using a single frequency it won’t give you the voltage noise density in the output window; it won’t give you anything, only a dot in the waveform window. Is there, currently, a way to implement something similar to LTSPICE with regards to voltage noise density? If not, it would be a nice addition for future updates.

P.S. This simulator is on a whole other level.

You can’t calculate voltage noise density with only one point. Here I have the formula.
Best regards

Ivan, I am well aware of that equation. The fact is that it is much simpler if the simulator gives you everything you need without having to compute it yourself everytime you run a simulation. Also, that is the equation for the integrated noise. That is exactly what the simulator gives you, what I want is not the integrated noise but the noise density

Try to use: .noise V(out) V1 list 1K

Best regards

Hello, ivan1.

Thank you.

I am not entirely sure about what it is supposed to happen with that command. The only difference I can see is that I get a line in the waveform window rather than a single point, but the results are the same than if I type .noise V(out) V1 1 1k 1k. Which is indeed the noise voltage density, however, it is still the output-referred noise density, not the input-referred noise density. Of course, I can divide by the gain, but in some circuits it is not straight-forward. Is there a way I could get the input-referred noise density directly?

Here I have an example

Thank you, ivan1. That is exactly what I was looking for.