Question: I am simulating an audio amplifier. My transient analysis is running fine, and now I
would like to measure the total harmonic distortion of my amplifier output. How do I do this measurement?
Answer: The total harmonic distortion of a waveform can be calculated through the use of the
THD function. The THD function has the following syntax:
THD(S[,F])
where S is the waveform spectrum, and F is an optional parameter that specifies the fundamental
frequency. If the F parameter is not specified, then the fundamental frequency is calculated through
the time range specified in the FFT page of the Analysis Properties dialog box by the following
equation:
F = 1/(Upper Time Limit - Lower Time Limit)
In the Transient Analysis Limits dialog box, the THD function needs to be entered on a new waveform
line. Set the X Expression field to F since the distortion should be plotted versus frequency.
If the THD is to be measured at node Out, the Y Expression field would be defined as:
THD(HARM(V(Out)))
The HARM(V(Out)) portion produces the waveform spectrum needed for the THD calculation. In
this case, the fundamental frequency would be calculated from the FFT page of the Analysis Properties
dialog box. However, if the fundamental frequency needed to be set to a specific value such as
1kHz, the expression above could be modified to:
THD(HARM(V(Out)),1k)
Run the transient simulation. The plot displays a running total of the THD measurement. For example,
if the fundamental frequency is 1kHz, the data point at 4kHz would show the THD value of
all of the harmonics after 1kHz up to and including the harmonic at 4kHz.
There is also an individual harmonic distortion function available in Micro-Cap. Its syntax is:
IHD(S[,F])
This function operates in the same manner as the THD function except that it shows just the individual
distortion of each harmonic.
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