Types of RF Spurious Signals
| Type | Frequency | Source | Level (typical) |
|---|---|---|---|
| 2nd harmonic | 2f_TX | PA nonlinearity | TX_power − 20 to −40 dBc |
| 3rd harmonic | 3f_TX | PA nonlinearity | TX_power − 30 to −50 dBc |
| IM3 (two carriers) | 2f₁−f₂, 2f₂−f₁ | PA/mixer nonlinearity | 2·OIP3 − P_in + const |
| LO leakage | f_LO | Mixer LO feed-through | −20 to −40 dBm typical |
| Image frequency | f_RF − 2·IF | Superheterodyne receiver | Depends on image rejection |
Harmonic Filter Requirements
FCC Part 15 / ETSI EN 300 328 requirements (2.4 GHz WiFi TX): Harmonic emissions must be < −41.3 dBm EIRP (spurious) at harmonics At 20 dBm (100 mW) TX power with 5 dBi antenna (EIRP = 25 dBm): 2nd harmonic (4.8 GHz) raw from PA: 25 dBm − 30 dBc = −5 dBm EIRP Regulation requires < −41.3 dBm → need >36 dB additional rejection at 4.8 GHz Solution: SAW/BAW bandpass filter at PA output rejects harmonics by 30–50 dB LPF as additional harmonic trap: 40+ dB at 2·f₀ S-parameter requirement for TX filter: S21 at f₀ (2.4 GHz): <2 dB IL S21 at 2f₀ (4.8 GHz): <−40 dB (harmonic rejection)
Analyzing Harmonic Rejection with RF View
- Load TX filter .s2p → S21 view
- Extend frequency range to 2× or 3× operating frequency
- Place marker at 2f₀ → read harmonic rejection
- Place marker at 3f₀ → read 3rd harmonic rejection
- Compare with emission limit + PA harmonic level to verify compliance margin
RF View Filter Analysis: Load TX filter .s2p and measure rejection at harmonic frequencies. Extend the frequency sweep beyond the passband to verify harmonic suppression. Free on Android.