Noise stands as an ever-present challenge in the realm of electronic devices. While eradicating it entirely might be unattainable, employing specific techniques can significantly diminish its impact. In essence, noise only becomes problematic when it impairs the system's functionality. If not adequately addressed, noise can inflict detrimental effects on various printed circuit boards, particularly those operating at higher frequencies—typically surpassing one megahertz. In these scenarios, electromagnetic waves emerge around PCB traces and components themselves, potentially disrupting signals traversing other traces within the same PCB.

Why PCB Makes Noise

Figure 1-png-2.png (1200×630)

Within PCBs, noise frequently emerges as a result of surges in current within specific electrical signals. In digital circuits, these current surges stem from transistor switching instances—those pivotal transitions between on and off states. Analog circuits, conversely, witness current surges shaped by shifts in load current. Excessive noise can also arise from a floating ground or an erroneous ground connection.

For signals with frequencies below 1MHz, a singular ground point typically suffices. However, for high-frequency circuits, the preference leans toward employing multi-point or star ground connections, as depicted in Figure 1. Hybrid configurations also find their place, incorporating both single-point low-frequency and multi-point high-frequency grounding.

The leading sources of noise are segmented into these categories:

  • Ground Bounce: In digital circuits, the swift escalation in switching frequency compresses the time for the electrical signal to revert to the ground reference level. Consequently, the signal can momentarily "bounce" above ground, engendering unforeseen current surges that infiltrate the output signal. Simultaneous multiple switching can intensify noise, potentially causing erroneous or duplicated switching and compromising circuit functionality.
  • Crosstalk: Proximity matters for PCB traces. When traces lack adequate spacing, high-frequency or high-current signals can influence adjacent trace signals. This disruptive electromagnetic coupling, called crosstalk, arises from horizontally proximate traces on the same layer and vertically adjacent traces on different layers.
  • EMI: Electromagnetic interference stems from diverse sources, with improper ground loop design being a prominent culprit. To sidestep such predicaments, a strategic approach involves situating signal traces between power and ground planes. This arrangement ensures an unobstructed return path to the ground plane. Without this alignment, the return signal follows an erratic trajectory in its quest for ground, engendering interference and noise.

PCB Noise Reduction Measures

Integrate Ground and Power Planes

When setting up your PCB arrangement, aim to envelop the board area with ground and power planes as extensively as feasible. If feasible, allocate one layer for grounding and another for power distribution. The ground plane's architecture should prioritize a direct route back to the ground reference, a particularly critical consideration for high-frequency signals. Strive to circumvent disruptions and an overabundance of vias.

In cases where a comprehensive ground plane isn't viable due to spatial or financial constraints, channel all ground traces toward a shared ground junction. This can be achieved through a single-point (low frequency) or star (high frequency) grounding methodology. This strategy curtails shared impedance interaction among diverse subsystems. For traces bearing power signals, maintain alignment as much as possible with the ground traces.

Optimize Trace Dimensions

Minimize the length and width of signal traces traversing the PCB. Incorporate vias when needed to maintain trace brevity. Traces with a width below 8mm and a thickness of 4 to 8 mils are generally effective in curtailing capacitive coupling and noise, particularly at elevated frequencies. To mitigate crosstalk, ensure the spacing between adjacent traces consistently surpasses their width. Opting for multilayer PCBs is often advantageous as they facilitate distinct ground, power, and signal planes.

Conductor inductance scales inversely with the logarithm of its diameter and directly with its length. Hence, adopting short and wide traces is paramount for inductance reduction. Implement single or multi-stage filters to dampen noise for power lines and all incoming PCB signals.

Isolate PCB Sections

The layout of the PCB stackup plays a pivotal role, influencing signal integrity and noise reduction. Signal layers should adopt either microstrip or stripline configurations, contingent on their position within the stackup. This layer arrangement governs the extent of electromagnetic radiation emitted by the board and its resistance to external radiation.

Segregate analog and digital circuits on the PCB. Digital circuits produce high-frequency noise that can introduce anomalies in both digital and analog circuits, particularly if insufficiently isolated. Similar principles extend to frequency divisions, making it prudent to segregate high-frequency circuits from their low-frequency counterparts. Refer to Figure 2 for an illustrative partition example.

Figure 2-jpg-4.jpeg (983×587)

Implement Decoupling Capacitors

The component arrangement on the PCB holds significant sway in noise reduction. To diminish inductance between vias, position power components in close proximity to each other on the same layer. For high-frequency components, prioritize minimal trace length. For curbing current spikes during signal transitions and evading ground bounce, install decoupling (or bypass) capacitors as near as feasible to the power supply pins of active components. Opting for higher capacitance tantalum capacitors, albeit pricier, grants enhanced quality and performance compared to conventional electrolytic capacitors. To isolate integrated circuits from one another, leverage multilayer ceramic capacitors.

Route Traces Carefully

As previously mentioned, shorter and wider traces on a PCB generally contribute to noise reduction by curbing the inductance of the traces themselves. This holds especially true for traces carrying high current or high frequency signals. When dealing with two adjacent signal layers in the PCB stackup, it's crucial to orient the traces on one layer horizontally and those on the other layer vertically. This strategy diminishes the potential for coupling (crosstalk) between traces situated on both layers.

When handling sensitive signals, maintain a distance from the oscillator circuit, regardless of whether the traces are placed on the same layer as the oscillator or on adjacent layers. To minimize noise, refrain from utilizing 90° turns in traces; opt for two 45° angles instead of right angles as a general guideline.