1. After helping customers ensure their products comply with EMI standards, a potential problem was discovered: poor PCB design.

2. Based on experience, designers of IoT products often face issues stemming from subpar PCB design.

3. When onboard energy disrupts sensitive receiver circuits, poor design can cause indefinite delays, leading to cellular compliance failures.

4. GPS and Wi-Fi receivers can also experience a loss of sensitivity.

5. How the signal travels through the PCB and how the electromagnetic field behaves influence this movement.

6. The difference between a well-designed and poorly designed PCB stackup can be significant.

7. Several factors contribute to poor EMI design, including:

8. Mixing digital and sensitive analog circuits with noisy circuits, such as power supplies and motor drivers.

9. Placing the clock driver too close to the edge of the circuit board or near sensitive circuits.

10. Poor routing that causes crosstalk.

11. Running clock (or high-speed) traces across gaps or slots in the return plane.

12. Most importantly, incorrect layer stacking.

13. The issue of clock traces crossing return plane gaps has been addressed. However, correcting the last item regarding layer stacking typically resolves numerous other issues, including many listed above.

1. When attending college circuit courses, most of us were mistakenly taught how DC and AC currents work in lumped or distributed (transmission line) circuits.

2. In our “Fields and Waves” course, we are unlikely to be instructed on the practical application of circuit board design or the propagation of signals through the circuit board.

3. In fact, these two concepts—circuits and fields—work together complementarily when propagating a digital signal through a microstrip line or stripline.

4. Before you can delve into how the signal propagates in the PCB, you must first understand some physics.

5. We are all taught that “current” is the flow of electrons in copper. This is close to the truth, except that we often consider positive currents—lack of electrons, known as “holes.”

6. However, electrons and the “holes” (positive charges) they leave behind travel very slowly.

7. Of course, this current is correct for DC circuits (except for the initial battery connection transient). But for the “DC” output (with transients) of AC (or radio frequency) circuits or switch-mode power supplies, we need to understand that all connection lines/wiring must now consider transmission lines.

8. First, let us consider how the capacitor appears to allow electrons to flow. After all, isn’t this the working principle of decoupling capacitors?

9. If we apply a battery to a capacitor, any positive charge applied to the top plate will repel the positive charge on the bottom plate, leaving a negative charge.

10. If we apply AC power to capacitors, you might think that current flows through the dielectric, which is impossible. James Clerk Maxwell called it “displacement current,” where the positive charge merely replaces the positive charge on the opposite plate, leaving a negative charge, and vice versa.

11. This displacement current is defined as dE / dt (time-varying electric field).