Noise is electrical or electromagnetic energy that reduces the quality of a signal. Noise affects digital, analog and all communications systems. Noise Analysis calculates the noise contribution from each resistor and semiconductor device at the specified output node. Multisim creates a noise model of the circuit using noise models of each resistor and semiconductor devices and then performs AC-like analysis. It calculates the noise contribution of each component and propagates it to the output of the circuit sweeping through the frequency range specified.
Multisim can model three different kinds of noise:
Multisim performs Noise Analysis using the following approach:
Figure 1 shows a basic operational amplifier with a gain of 5. You will use Noise Analysis to obtain results for noise voltage for R1 and R2 and display a graph of the noise spectrum across a frequency range between 1 Hz and 10 GHz.
Figure 1. Inverting amplifier circuit.
Complete the following steps to configure and run a Noise Analysis:
Table 1. Parameters used in Noise Analysis.
Input noise reference source
Displays nodes contained within subcircuits or hierarchical blocks. There are three options:
Calculate power spectral density curves
Points per summary
Calculate total noise values
Note: In SPICE, the command that performs a Noise Analysis has the following general form (the complete statement is more complex): .NOISE <OUTPUT_VOLTAGE> <INPUT_SOURCE> <OUTPUT_INTERVAL>
Note that these parameters are similar to those defined in Table 1, however, in Multisim you do not have to worry about the SPICE syntax.
Figure 2. Analysis parameters.
Figure 3. Frequency parameters.
You can reset all the parameters to their default values by clicking the Reset to default button. To copy the settings from the current AC Analysis to this analysis, click Reset to main AC values.
Refer to the AC Analysis tutorial for more details on how to configure the Frequency Parameters tab.
Figure 4. Output variables for the Noise Analysis.
Figure 5. Noise Analysis results.
In the next steps you will plot the power spectral density.
Figure 6. Output variables for the Noise Analysis.
Figure 7. Noise spectral density curves.
As you can see, the graph shows that the noise voltage is constant for lower frequencies. For higher frequencies the noise voltage drops considerably.
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