I am performing noise simulations in Qucs-S and plotting the variable

onoise_spectrum

. I want to confirm the unit of

onoise_spectrum

. Does it represent the output noise spectral density in V^2/Hz (noise power) or in V/sqrt(Hz)​ (root spectral density)? Also, if the default is V^2/Hz, do I need to manually take the square root to obtain V/sqrt(Hz)​? I would appreciate any clarification or examples from Qucs-S or Ngspice users!

See the documentation on ngspice regarding noise. The units depend on settings made in the control block. According to the documentation, by default the value is

V/sqrt(Hz)

(voltage noise spectral density). But if you have the command

set sqrnoise

in the control block, then the value is in

V^2/Hz

(power spectral density). Also note that you can get the integrated noise over the frequency range from the value

onoise_total

.