EMI Sources in RF PCB Designs
| Source | Emission Type | Frequency Range |
|---|---|---|
| Switching regulator (buck/boost) | Conducted + radiated | DC to 300 MHz (switching harmonics) |
| Crystal oscillator (10 MHz) | Radiated harmonics | 10, 20, 30... MHz (spurs) |
| MCU clock (48 MHz) | Radiated harmonics | 48, 96, 144... MHz |
| PA transmit harmonic | Radiated | 2×, 3×, 4× operating frequency |
| USB 3.0 differential pair | Radiated | 5 GHz fundamental + harmonics |
EMI Mitigation Techniques
- Bypass capacitors: Place C0G 100pF directly at power pins — SRF at operating frequency for best decoupling
- Ferrite beads: 600Ω at 100 MHz ferrite on all supply lines entering RF area — blocks conducted noise
- Ground stitching: Via fence every λg/20 around RF area boundary
- TX harmonic filter: LPF or BPF between PA and antenna — reduces 2nd harmonic below FCC limit
- Shield can: Solder-down metal shield over entire RF section for 40+ dB additional isolation
Measuring EMI with RF View
Load TX filter .s2p → verify harmonic rejection: S21 at 2×f_TX: must be < (FCC limit − PA harmonic level) Load PCB noise coupling path .s2p (if measured): S21 shows how much switching noise reaches RF input Target: <−40 dB coupling at all RF operating frequencies
RF View: Use S-parameter analysis to verify filter rejection at harmonic frequencies and coupling path attenuation between digital and RF areas. Free on Android.