AC Waveform Distortion
Common AC waveform distortions — harmonics, clipping, crossover — and how to measure them.
Reference
Distortion types
| Type | Cause | Symptom |
|---|---|---|
| Harmonic | Non-linear load (rectifier, SMPS, dimmer) | Spikes at integer multiples of fundamental |
| Intermodulation | Non-linearity mixing signals | Sum / difference frequencies appear |
| Clipping (hard) | Input exceeds output range | Flat-topped waveform, rich odd harmonics |
| Clipping (soft) | Gradual saturation (tubes) | Rounded peaks, mild harmonics |
| Crossover | Class-B amp zero-crossing dead zone | Kink at zero crossing |
| Slew-rate limit | Amp can't track signal | Triangular instead of sinusoidal at high f |
| Notch / gap | Thyristor firing, commutation | Missing section at a phase angle |
| Sag / swell | Line disturbance | Brief RMS drop or rise |
Metrics
- THD
- √(Σ V_h² for h ≥ 2) / V_1 — ratio of harmonics to fundamental
- THD+N
- Includes noise in the numerator
- Crest factor
- V_peak / V_RMS — 1.414 for pure sine; higher = more peaky
- Form factor
- V_RMS / V_avg — 1.11 for sine
Mains power quality limits
- IEEE 519 THD-V (medium V)
- ≤ 5% individual harmonic, ≤ 8% total
- EN 50160 THD-V
- ≤ 8% under normal conditions
- Computer PSU harmonics
- Regulated by IEC 61000-3-2
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