Design considerations for High-Speed Boards

To achieve high-speed signal operations in low-power PCBs, the layout must meet the following design considerations:  

• Crosstalk 
• Noise 
• Impedance 
• Via stitching 

Crosstalk: Crosstalk is the unintended transfer of a signal due to coupling from adjacent traces. Such unwanted signals are normally generated by current fluctuations in one trace transferring to parallel traces nearby.

Here are some suggestions to reduce PCB crosstalk: 

• Clearance from adjacent high-frequency traces should be 2x to 3x of the height above their ground return plane (restricted by the PCB layer count, stackup, and board size). 
• Avoid routing signals of different frequencies in parallel over medium-to-long distances (such as clock signals in parallel with data and address lines). 
• Maintain close proximity to the ground for all traces by using ground planes directly underneath as a priority. If this is not possible, use grounded coplanar guard traces where applicable (like clock, crystal, RF signals). 
• Ensure adequate amplitude and clearances for tuning to avoid same net coupling.

Noise: Noise is any unwanted voltages produced by different regional circuit elements such as analog, digital, and power circuits. Following are a few sources that will generate noise in a PCB: 

Power distribution network: Power distribution is the regional allocation of electrical energy across the entire board. Typically, this forms a source of low-frequency noise. Following are a few pointers that can reduce power supply noises: 

• Proper component placement in the PCB:
  • Power supply circuits (like Buck and Boost switching regulators) should be kept away from both analog and digital signals.
  • Where this is not possible, ground shielding can be placed over sensitive circuit areas to help isolate them from the airborne noise.
  • Decoupling capacitors should be placed near all IC power pins. 
• The return current path should be of minimum length. Ensure a proper layer stackup and sufficient ground stitching vias. 
• Sufficient copper thickness and width can reduce local inductance and heat dissipation.
• Multiple vias should be used for high current traces and switching outputs.

Ground bounce and simultaneous switching noise: Ground bounce is a type of noise that will occur during MOSFET switching operations. Normally, this will happen when the gate voltage is less than ground potentials.

• Place SMD bypass capacitors near IC pins to suppress these bounces.
• Add current-limiting resistors in series with switching outputs, which will prevent the current flow during MOSFET switching. Place such resistors near MOSFET output pins. 

Via stitching: Via stitching is the technique of placing an array of multiple vias throughout the PCB, which connects between return path layers. This is normally done for ground planes in the PCB. The reason behind this is to create a shorter return path for all currents and generally lower the impedance of the overall circuit. As a result, stitching will improve the total performance of the circuit. Stitching vias also offer an additional benefit for reduced electromagnetic interference (EMI) by reducing the effects of fringing electric fields at the plane edges.  

There may be other considerations that need to be taken into account as per your design requirements. Post your recommendations and design practices for high-speed designs here.