Maintaining the integrity and low impedance of signal lines in EMC designs is equally important for systems

Any current injected into the system will eventually return to the source. Therefore, the signal not only propagates on the signal line but also propagates on the reference plane as shown in the figure below. Therefore, maintaining the integrity and low impedance of the reference plane is as important as maintaining the integrity and low impedance of the signal line.

1. The signal is transmitted on the signal line and the reference plane

In traditional low-speed designs, the loop current in the system recirculates along the smallest resistive path, while in high-speed systems, the current flows back along the smallest impedance loop. At high frequencies, the inductance of the loop exhibits much greater inductive reactance than its own, so the minimum impedance path is the path of the smallest inductor. Under normal circumstances, the path of the minimum inductance is just below the signal line, as shown below:

2. High-frequency return current is returned along the minimum inductance path

We refer to the medium provided to the loop current of the signal line as the reference plane. In an actual system, the reference plane can be VCC or GND. The important point is to ensure the continuity of the reference plane. A pair of differential signals can be referenced to each other, and their dependence on the reference plane is not so strong.

If there is a large discontinuity in the reference plane of the signal on the PCB, such as a trench, the loop current of the signal cannot be transmitted through the path close to the signal trace plane at this trench, but must be bypassed. Gully. This increases the inductive reactance to the return path, causing the high-frequency component of the receiver signal to attenuate severely, even with steps. As shown below:

3. The loop is not continuous

In the design, as far as possible, there should be no gaps in the loop of the signal. If the gap is unavoidable, some decoupling capacitors can be placed at both ends of the trench to form an AC path across the trench to provide high-speed loop current.

Another common circuit discontinuity is when the signal switches between different reference planes, which also introduces noise into the power system. The following figure shows a 4-layer PCB structure. The signal is transmitted first in the first layer, then goes through a via to the fourth layer to continue transmission. The second and third layers are reference planes. When the signal is in the first layer, the loop current propagates in the second layer of the reference layer opposite to the signal path. When the signal is in the fourth layer, the loop current is also transmitted in the third layer of the reference layer opposite to the signal path. So when the signal passes through the signal vias, how does the loop current pass from the third layer to the second layer?

4. Signal switching reference plane

If there is no DC path between the reference planes, the loop current can only be transmitted through the capacitive coupling between the two planes. In the following figure, we can clearly see how the loop current is coupled from the third layer to the second layer.

5. Loop current is transmitted through capacitive coupling between planes

Due to the limited coupling between the planes, the loop current will encounter greater impedance during the transition. Therefore, the loop current will generate an inductive noise on two planes and propagate to other parts of the system. Since this noise is very similar to ground noise, it is also called a loop ground bomb. So, how to reduce the loop ground noise?

If these two planes have the same potential, for example, they are all ground planes, then the most direct and effective method is to add several ground vias near the signal vias to directly connect the two planes as shown in the figure below. The current passes through these vias and maintains the continuity of the circuit.

6. Use ground vias to reduce loop inductance

If the potentials of the two planes are not the same, for example, one is VCC and the other is GND, the impact of ground loop noise on the power system must be reduced. Some decoupling capacitors are added between the two planes to provide loop current. A low impedance transient AC loop.