Brief Discussion on Four Types of Interference in PCB Design

With the continuous improvement of circuit operating speed, high-frequency characteristics become increasingly prominent. Power supply noise interference has gradually become one of the main factors affecting circuit stability. In high-speed and high-frequency circuit systems, engineers often pay attention to the design of clean ground, but easily ignore the importance of clean power supply. The power distribution network of PCB has inherent impedance characteristics. During the operation of high-speed devices, instantaneous current changes will generate voltage ripples and noise superposition on the power supply line. Excessive power supply noise will reduce the stability of chip power supply, cause signal jitter, and deteriorate the overall anti-interference capability of the system.

In PCB design, signal transmission relies on transmission lines, which are mainly divided into microstrip lines and striplines. When the length of the signal trace reaches one-seventh of the signal working wavelength, the transmission line effect cannot be ignored. Impedance mismatch at terminals, vias, bends and branches will cause signal reflection. The reflected signal overlaps with the original signal, resulting in signal distortion, waveform oscillation and return loss degradation. This type of interference generated by transmission line effects is equivalent to additive noise inside the circuit, which seriously affects the normal transmission of high-speed signals.

There are a variety of coupling paths between circuits, and coupling interference is the most widespread interference form on PCBs. The main coupling modes include direct coupling, common impedance coupling, capacitive coupling, electromagnetic induction coupling and space radiation coupling. Unreasonable spacing between adjacent traces, overlapping of power and ground loops, and dense layout of high-frequency components will strengthen the coupling effect. Interference signals will be coupled to sensitive weak current circuits through the above paths, triggering crosstalk and circuit malfunction.

Electromagnetic interference in PCB is divided into conducted interference and radiated interference. Conducted interference propagates along conductive media such as power lines and signal lines; radiated interference spreads outward through space in the form of electromagnetic waves. High-frequency signal traces, high-speed IC pins, connector terminals and unclosed grounding gaps on high-density PCBs are easy to form equivalent transmitting antennas, which continuously radiate high-frequency electromagnetic energy to the outside. This not only causes mutual interference between internal circuits, but also makes the whole equipment fail to meet EMC certification standards.

Reasonably widen the width of power traces to reduce line resistance and loop impedance. The routing direction of power lines and ground lines shall be consistent with the data signal transmission direction, so as to suppress transient voltage fluctuation and noise coupling.

Separate analog ground and digital ground to avoid mutual crosstalk between analog signals and digital noise. For low-frequency circuits, single-point parallel grounding is adopted; for high-frequency circuits, multi-point grounding is applied to reduce grounding impedance. Properly thicken the ground trace width, with the conventional control range of 2 mm to 3 mm. The thickened ground wire can carry three times the rated allowable current and effectively reduce grounding noise. Closed-loop grounding layout is recommended for regional circuits to enhance overall shielding and anti-interference performance.

Connect 10 μF to 100 μF electrolytic capacitors in parallel at the power input terminal to filter low-frequency ripple. Configure 0.01 μF ceramic chip capacitors for each integrated circuit chip for local high-frequency decoupling. When the layout space is limited, 1 μF to 10 μF tantalum capacitors can be arranged for every 4 to 8 chips in a centralized manner. For storage chips and sensitive logic devices, decoupling capacitors shall be directly arranged near the power pins, and the capacitor leads shall be minimized to reduce parasitic inductance.

Minimize the number and area of current return loops on the PCB to cut off the radiation excitation source. Add filter components such as decoupling capacitors, EMI filters and magnetic beads on power lines and key signal lines to suppress conducted noise. Arrange metal shielding structures for high-frequency interference sources and sensitive circuits. Under the premise of meeting functional requirements, properly reduce the switching speed of high-speed devices. Select substrates with high dielectric constant, appropriately increase PCB board thickness and reduce microstrip line thickness, so as to weaken electromagnetic radiation capacity and improve EMC performance.

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