EMI
/ˌiː ɛm ˈaɪ/
n. "Unwanted electromagnetic energy disrupting electronic circuits via conduction, radiation, or coupling unlike clean reference signals."
EMI is any electromagnetic disturbance that interrupts, obstructs, or degrades electronic equipment performance—generated by switching regulators, digital clocks, or wireless transmitters coupling into victim circuits through shared power/ground, PCB traces, or radiated fields. Fast edges from SerDes 56Gbps PAM4 create broadband noise failing FCC Class B radiated limits unless ferrite beads, shielding, or spread-spectrum clocking attenuate peaks.
Key characteristics of EMI include:
- Conducted EMI: Noise currents flowing through power lines/cables below 30MHz; common-mode currents dominate via cable shield.
- Radiated EMI: Electric/magnetic fields above 30MHz; PCB traces >λ/10 act as antennas at FHSS harmonics.
- Coupling Paths: Capacitive (E-field), inductive (H-field), or near-field magnetic between aggressor/victim traces.
- Mitigation Techniques: π-filters, ground planes, shielding cans, clock dithering spreading energy across 0.5-2%.
- Standards Compliance: CISPR 32 Class B 40dBμV/m @3m; peak detectors with 120kHz RBW.
A conceptual example of EMI mitigation flow:
1. Identify switching regulator 48kHz fundamental + harmonics
2. Add π-filter: 10μH inductor + 10μF MLCC + 100nF ceramic
3. Route SerDes traces <1/20λ@56GHz over solid ground plane
4. Spread clock 100MHz ±0.3% reducing 70dB peaks to 45dBμV/m
5. Shield Bluetooth module in mu-metal can >60dB atten
6. Verify 10m semi-anechoic chamber complianceConceptually, EMI is like crosstalk in a crowded room—fast-switching SerDes transmitters shout harmonics drowning Bluetooth receivers unless ground planes, ferrites, and shielding act as soundproof walls isolating conversations.
In essence, EMI challenges high-speed electronics from PAM4 PHYs to DQS DDR5, requiring PCB stackups, filtering, and shielding for FCC/CISPR compliance while enabling dense LED-lit ENIG boards in wireless edge devices.