What is the difference between harmonic distortion and intermodulation distortion?
Harmonics vs Intermodulation
Understanding the distinction between these two types of distortion is fundamental to RF system design and troubleshooting.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
Can I calculate IP3 from harmonic measurements?
Approximately. The third harmonic and IM3 arise from the same third-order nonlinearity coefficient. For a memoryless cubic nonlinearity: OIP3 ≈ P_fundamental + (P_fundamental - P_3rd_harmonic)/2 + 4.77 dB. The 4.77 dB correction accounts for the different coefficient relationships between IM3 and harmonic products. In practice: the two-tone IP3 measurement is more accurate and directly relevant.
Why are even-order harmonics easier to deal with?
Even-order products (2f, 4f, f1+f2, f1-f2): fall far from the fundamental in most narrowband systems. They are easily filtered by the bandpass or low-pass filter that is already in the signal chain. In balanced (push-pull or differential) circuits: even-order products are inherently canceled (the symmetry of the circuit suppresses even-order nonlinearity). This is why balanced mixers and push-pull amplifiers are preferred: they suppress IM2 and the 2nd harmonic.
What is THD+N?
THD+N (Total Harmonic Distortion plus Noise): a combined metric that includes all harmonics and the noise floor. Commonly used in audio and baseband circuits (not typically used in RF). THD+N = sqrt(sum of (harmonic powers) + noise power) / fundamental power. For RF systems: IP3 and SFDR are the relevant distortion metrics.