Noise analysis: what do output variables mean?

[glw]

I am trying to understand the basic "noise analysis example" from the Multisim  (v11.0) user manual, for an inverting opamp circuit with gain G= 5. (R1 = 1k, R2 = 5k). I am offered a choice of output variables to plot: onoise_rr1, onoise_rr2, inoise_spectrum, onoise_spectrum.

What do these mean? From the plot it looks like onoise_rr1 gives the noise spectrum for R1 multiplied by G^2 as expected. Similarly onoise_rr2 seems to give the correct output noise for R2. What is the naming convention supposed to mean?

 What is inoise_spectrum? Current noise? Input noise?

What is onoise_spectrum? Total output noise? It seems larger than the the sum of the individual components plotted (R1 and R2). Maybe it includes opamp noise? If so can I just plot that noise component as well? (e.g.. onoise due to input voltage and current noises at opamp inputs?)

I also point out that the expected noise calculations in the "noise analysis example" section of the manual are incorrect . You should not multiply by bandwidth -- MS gives an answer in V^2/Hz. Also the values given for R1 and R2 do not match the plot in MS because the noise from R1 gets multiplied by G^2 . To obtain an RMS voltage noise, as the manual attempts to do, you have to multiply by the noise equivalent bandwidth which is somewhat larger than the 3dB analog bandwidth of the system and nowhere near the 1GHz bandwidth used in the example.

Another question: why do I need to specify a noise "reference source" for purposes of calculating noise power? For noise figure you need an input noise source (=reference?) but for just noise power analysis I'm not sure what this is supposed to mean. The manual gives no information on what goes into the calculations.

Thanks in advance

GW

[Tien_P]

Hi,

Please have a look at this link:

http://www.ecircuitcenter.com/Circuits/Noise/Noise_Analysis/res_noise.htm

I hope it will answer many of your questions.