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I've always used the notch filter method in EC to suppress the fundamental frequency and measure distortion in my simulated amps, but lately I've been using this method instead, because other than initial set-up, it doesn't have any settling time, whereas the notch method takes up to 5-10 minutes to get an accurate reading.
The idea behind this method is the following:
1. Make a bode plot of the output. Take note of the phase shift at the selected frequency. In this case, it's 0.201 degrees.
2. Measure the output voltage - in our case it's 6.81V
3. Add a new source in antiphase with the output and set the parameters as above (appropriate phase shift and voltage level).
4. Create a resistive bridge between the output and the new AC source. Tweak the source's voltage until you get lowest fundamental frequency amplitude at the mid-point of the resistive bridge. As they are in anti-phase, only the fundamental frequency should cancel out in the bridge and the noise should remain.
For example in this example I have a 6.81V output at 0.201 degrees phase shift from the simple amplifier. I have added an AC source with 180 degrees plus 0.201 degrees phase shift, with the same amplitude to cancel out the fundamental. After canceling out, I'm getting 48mV of distortion.
Measuring for the distortion we get:
THD = (Vdistortion / Vout) * 100 = (0.048V / 6.81V) * 100 = 0.007 * 100 = 0.7%
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