Distortion analysis analyzes signal distortion that may not be evident during Transient analysis.

Sample files for use with analyses are installed at C:\Users\Public\Documents\National Instruments\Circuit Design Suite <version>\samples\Analyses\.

Analysis Parameters

Multisim simulates both harmonic distortion and intermodulation (IM) distortion for analog small-signal circuits. For each AC source in the circuit you set the parameters to be used. Multisim will determine the node voltages and branch currents at each point in the circuit. For harmonic distortion, values are determined for the second and third harmonics. For intermodulation distortion, the analysis will calculate values at frequencies f1 + f2, f1 - f2 and 2f1 - f2.

The Analysis parameters tab contains the following:

  Parameter   Description
Start frequency (FSTART) The start frequency for the sweep.
Stop frequency (FSTOP) The stop frequency for the sweep.
Sweep type Defines how points to be calculated are distributed across the frequency range. Choices are Decade, Linear, or Octave.
Number of points per decade

The number of points to be calculated during the analysis. For Linear sweep, use the number of points between start and end.

The greater the number of points calculated, the more accurate the results will be. However, the simulation speed will be slower.
Vertical scale

Controls the y-axis scaling on the output graph. Choices are Linear, Logarithmic, Decibel, or Octave.

To plot the output in decibels (dB), use the Decibel option in the Vertical scale option.

Unless Decibel is selected, the magnitude plot only plots positive numbers. Negative numbers are captured by the phase plot. Therefore, a gain of -3dB appears as +3dB with a phase of -180 degrees.
F2/F1 ratio For intermodulation distortion only. When enabled, if there are signals of two frequencies (F1 and F2), then F2 is set to this ratio multiplied by the start frequency while F1 sweeps. Must be greater than 0.0 and less than 1.0. Also, this number should be an irrational number but because of the finite precision of the computer this is not possible. It is best, therefore, to use a floating point number with a large number of digits.
Reset to default Sets Frequency parameters to the AC Sweep analysis default values.
Reset to main AC values Click to use the Frequency parameter values that are saved for the AC Sweep analysis.

Before you perform the analysis you must decide which sources you will use. The Distortion Analysis parameters are set for each source independently.

Follow the steps below for each AC source that you would like to use in the distortion analysis. To perform harmonic distortion analysis use only steps 1 and 2. If you want to perform an intermodulation distortion analysis follow all three steps.

Complete the following steps to set source options for distortion analysis:

  1. Double-click on the source.
  2. In the Value tab set set 1 mV for the the Distortion Frequency 1 Magnitude parameter.
  3. In the Value tab set set 1 mV for the Distortion Frequency 2 Magnitude parameter.

    Only use this setting if you want to perform an intermodulation distortion analysis.

      The Phase setting is not relevant.

Limitations

Distortion Analysis calculates distortion results with the help of equations built into the semiconductor device models, such as the BJT, diode, and MOSFET. It does not perform any Fourier Analysis on time-domain (transient) results.

As a result, Distortion Analysis may only be useful for circuits that use semiconductor models directly as they would appear in the final design, for example, a transistor amplifier. The analysis should not be used for other circuits, such as those that use op-amp models or any behavioral models. It is recommended that you use Fourier Analysis to study distortion and harmonics.

Distortion Analysis may only provide accurate results for input sources with a magnitude of 1 mV.