Power, Linearity, and Distortion Intermodulation and Spurious Informational

What is the difference between harmonic distortion and intermodulation distortion?

Harmonic distortion and intermodulation distortion are both caused by device nonlinearity, but they involve different signal conditions and produce different spurious outputs: (1) Harmonic distortion: occurs with a single input signal at frequency f. The nonlinearity generates harmonics at 2f, 3f, 4f, etc. The second harmonic (2f) is the strongest, followed by the third harmonic (3f). Harmonic levels: 2nd harmonic rises at 2 dB per 1 dB input increase. 3rd harmonic rises at 3 dB per 1 dB. Harmonics are at specific, predictable multiples of the input frequency. They are easy to filter (a low-pass or bandpass filter removes harmonics without affecting the fundamental). Measurement: single-tone test with a spectrum analyzer. The difference between the fundamental and the highest harmonic is the harmonic distortion (in dBc). (2) Intermodulation distortion: occurs with two or more input signals at different frequencies (f1 and f2). The nonlinearity generates mixing products at frequencies: m×f1 ± n×f2 (where m and n are integers). The most problematic products are the third-order intermodulation products (IM3): 2f1 - f2 and 2f2 - f1. These IM3 products fall close to the original signals (within the passband) and CANNOT be filtered without also filtering the desired signals. This is why IM3 is much more problematic than harmonics: IM3 causes in-band interference. (3) Key differences: signal condition: harmonics = single signal; IMD = multiple signals. Frequencies: harmonics at multiples of f; IMD at combinations of f1, f2. Filterability: harmonics are easily filtered (out of band); IM3 is NOT filterable (in band). Primary specification: THD (total harmonic distortion) for audio/baseband; IP3 (intercept point) for RF. Relevance: harmonics matter for transmitter spectral purity; IMD matters for receiver dynamic range and multi-carrier systems.

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Amplifiers, Filters, Connectors

Harmonics vs Intermodulation

Understanding the distinction between these two types of distortion is fundamental to RF system design and troubleshooting.

When Each Matters

(1) Harmonics matter for: transmitter spectral compliance (FCC, ETSI regulations limit harmonic emissions), frequency multiplier design (harmonics are intentionally generated and selected), and mixer spurious analysis (LO harmonics mixing with RF produce spurious responses). (2) IMD matters for: receiver dynamic range (IM3 from strong signals desensitizes the receiver), multi-carrier transmitters (IM3 between carriers causes adjacent channel interference), and shared amplifier systems (cellular, CATV, satellite transponders). (3) Relationship: both harmonic distortion and intermodulation arise from the same nonlinear coefficients in the device transfer function. A device with high IP3 also has low harmonic distortion (and vice versa). The theoretical relationship: OIP3 ≈ output power of 3rd harmonic at the same two-tone IM3 level + some constant offset (depends on the nonlinearity model). In practice: measuring IP3 (from a two-tone test) is more relevant for RF systems than measuring THD (from a single-tone test).

Distortion Comparison

Harmonics: at 2f, 3f (easily filtered)
IM3: at 2f₁-f₂, 2f₂-f₁ (in-band)
2nd harmonic: 2 dB/dB slope
3rd harmonic/IM3: 3 dB/dB slope
IM3 is NOT filterable (critical difference)

Common Questions

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.

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