This analysis produces a sweep of distortion (THD or IMD) over frequency. The data is valid only for small signals. For example, it does not account for x-over distortion. Large signal distortion can be analyzed using the FFT feature of SuperSpice
Method
1 Using the distortion setup dialog page, set either THD or IMD and enable the run.
2 Select a voltage source that is to be used as the signal generator. For this source, set the value of its IMD/THD values to that of the signal generating the distortion. These values are the peak values of Vp.Sin(wt). Usually its a good idea to set the normal AC value to the same value so that S/D ratios can be computed.
The distortion output values are absolute values, not ratios to the fundamental. A simultaneous plot of AC and distortion will allow the output value of the fundamental to be determined. This fundamental output value can be added/subtracted (if in db) to the distortion value to obtain the final ratio.
Limitations
Not all active device models support small signal distortion analysis.