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Noise analysis definition questions

Noise Analysis help says:

Another measure of noise, the “signal to noise ratio,” takes the signal magnitude into account. This is the ratio of signal power to noise power, according to the following formula:

S/N = 20*log(VSig/VTot)

where VTot is the total input-referred noise voltage and VSig represents the expected signal amplitude. VSig is a user-defined value that you enter in the Noise Analysis dialog. The default value is 1V.

1. VSig and VTot are rms voltages, right?  The "ratio of signal power to noise power" would be 10*log10(PSig/PTot)

2. The value you enter in this dialog is Vrms?  But the value you enter in the signal generator component is Vpeak?

3. So there's no way to analyze the total input noise except to calculate total output noise and subtract gain?  It would be nice if there were a way to output that number instead of SNR.

  • Jonathan,

    1. VSig and VTot are rms voltages, right?  The "ratio of signal power to noise power" would be 10*log10(PSig/PTot)
        - You are right, the equations should be 20*log10(Vsig/Vnoise) or 10*log10(Psig/Pnoise).

    2. The value you enter in this dialog is Vrms?  But the value you enter in the signal generator component is Vpeak?
        - The noise analysis is associated with AC analysis, and their nominal magnitude is in rms. The source entries are
        really oriented towards transient sims, where peak magnitudes are emphasized.

    3.  So there's no way to analyze the total input noise except to calculate total output noise and subtract gain?  It would be nice if there were a way to output that number instead of SNR.
       - There is an option in the Noise Analysis window that lets you specify Input Noise.
          An snapshot showing this will hopefully appear below. Is this what you are asking for?

    Regards,
    John
         

  • John Miller - WEBENCH Design Center said:

       - There is an option in the Noise Analysis window that lets you specify Input Noise.

    The "Input noise" and "Output noise" are spectral density plots.  I think the "Total noise" is the integral of the "Output noise" plot, so the peak value is the total output noise?  I would be interested in the peak value of the integral of the "Input noise" plot, which is total equivalent input noise (EIN).  I've been taking peak of "Total noise" plot and dividing by gain to get it, but it's inconvenient.