• Poor power input configurations: A supply trace has resistance and inductance. A transient load current produces both IR drop and inductive voltage. If the power-distribution network cannot deliver current locally, the rail disturbance spreads through shared impedance and modulates other circuits. The same event appears as power-integrity noise in the time domain and conducted/radiated EMI in the frequency domain. The resulting disturbance contains a resistive component proportional to current and an inductive component driven by how quickly that current changes. At fast edges, the inductive term can dominate. Shared impedance is especially damaging when noisy digital or motor current uses the same return neck as an ADC reference, oscillator, transceiver, or cable shield. The resulting voltage is not 'ground noise' in the abstract, but rather the voltage created by known current through a finite impedance.

Adding capacitors to the trace at both ends helps reduce the amplitude of the RF emissions while lowering the impedance of the trace reduces the voltage drop across the trace and lowers the amplitude. Consider Microstrip or Strip line techniques to control the impedance of a trace that carries clock signals to minimize radiation.

• Poor Grounding: Ground is a network, not an equipotential symbol. At RF, plane spreading inductance, vias, connectors, and chassis bonds create voltage differences. A solid reference plane usually improves both signal integrity and EMC because it supports a compact return path. Splitting the plane under a fast signal forces current around the split, increases loop area, and commonly creates common-mode voltage.
  • Partition circuits by placement and current flow before partitioning copper.
  • Keep noisy current local to its source and load; do not route the current through quiet reference regions.
  • Connect shield current to the intended chassis or boundary path before the current spreads into digital ground.

Improperly designed ground planes or long, high-impedance return paths are primary causes of both conducted and radiated EMI. The current in the PCB circuit must return to the source or power supply at some point. The ground can also exhibit similar characteristics as the supply positive traces and radiate unintentionally. Creating a good ground plane to capacitively couple the EMF from traces helps to prevent them from radiating.

• Large Loop Areas: Every signal forms a loop with its return path; larger loops behave as more efficient antennas, strengthening magnetic fields and emissions. For electrically small loops, magnetic dipole strength increases with loop area and current. Reducing area is therefore a first-order control for switching regulator input loops, gate loops, ESD discharge loops, motor commutation loops, and high-speed signal-return loops. A compact loop also has lower inductance, which reduces overshoot and the energy available to excite resonances.
• PCB Traces: A trace must be treated as a transmission line when propagation delay is no longer negligible compared with edge time. The exact threshold depends on acceptable error, but a common design screen is to compare one-way flight time with roughly one-sixth of rise time. Once distributed behavior matters, impedance mismatch creates reflections regardless of the clock frequency.

The polarity and size of the reflected wave depend on the relationship among load impedance, source impedance, and line impedance. Good termination controls that relationship and prevents repeated energy exchange along the interconnect.

Conductors that carry a current at frequencies at or near one quarter of a wavelength or an odd multiple of a quarter wavelength long that are close to the length of the conductor, that conductor can efficiently radiate electromagnetic energy as far field emissions. These conductors or traces when connected to a data bus, clock lines or communication lines which are often long become transmission lines. When transient currents flow on these transmission lines.

Conductors that carry a current at frequencies at or near one quarter of a wavelength or an odd multiple of a quarter wavelength long that are close to the length of the conductor, that conductor can efficiently radiate electromagnetic energy as far field emissions. These conductors or traces when connected to a data bus, clock lines or communication lines which are often long become transmission lines. When transient currents flow on these transmission lines and traces, these currents act as an efficient antenna, radiating high-frequency electromagnetic waves. If such a transmission line is unterminated, the radiated spectrum shows maximum peak voltages. Impedance mismatching causes a maximum current flow on the transmission line or trace, thus maximizing radiated emissions.