Every circuit is both a potential radio transmitter and a potential victim of interference. Electromagnetic Interference (EMI) is the noise your circuit emits or receives; Electromagnetic Compatibility (EMC) is the discipline of ensuring your device works correctly in its electromagnetic environment without disturbing others. If you've ever heard buzzing in a speaker when a phone rings nearby, you've witnessed EMI firsthand.

The two coupling mechanisms you must understand:

• Conducted emissions — noise that travels along wires, traces, and power cables. Your switching regulator's high-frequency current ripple riding out on the DC input cable is a classic example.
• Radiated emissions — noise that propagates through space as electromagnetic waves. Any loop carrying time-varying current is an antenna. The larger the loop area and the higher the frequency, the more it radiates.

Practical rules for reducing EMI at the board level:

• Minimize loop areas. Route the return current path directly beneath or adjacent to the signal trace. A continuous ground plane is your single most powerful EMI tool — it automatically provides the lowest-inductance return path.
• Slow down edges when you can. A 10 ns rise time contains significant spectral content up to about 1/(π × t_r) ≈ 32 MHz. If your design only needs a 1 MHz clock, there's no reason to use a gate with 1 ns edges — add a small series resistor (33–100 Ω) or choose a slower logic family.
• Filter at boundaries. Every cable entering or leaving your enclosure is an antenna. Place ferrite beads or LC filters on power inputs, signal I/O, and especially on cables longer than λ/20 at your highest frequency of concern.
• Don't split the ground plane under high-speed signal traces. A slot in the ground forces return current to detour around it, creating a large radiating loop.

Real-world example: You build a project with a 16 MHz Arduino and a switching buck converter on the same board. Without precautions, the buck converter's 500 kHz switching noise couples into the ADC readings, causing 10–20 LSB of jitter. The fix: keep the buck converter's input capacitor, inductor, and output capacitor in a tight loop with a local ground pour, add a ferrite bead between the buck output and the Arduino's VCC pin, and ensure the ADC signal traces run over unbroken ground.

Key calculation — maximum radiating frequency from rise time:

f_max ≈ 1 / (π × t_rise)

For a 5 ns rise time: f_max ≈ 64 MHz. Your PCB layout must control EMI up to at least this frequency, regardless of your clock's fundamental. This is why EMC engineers care about edge rates, not just clock frequencies.