PCB News - How to deal with PCB signal across the splitter line

In the process of PCB design, the partition of power plane or ground plane will lead to the incomplete plane. In this way, when the signal is routed, its reference plane will cross from one power plane to another power plane. This phenomenon is called signal cross partition.

Schematic diagram of cross segmentation phenomenon

Cross segmentation may not be relevant for low speed signals, but in high speed digital signal system, high speed signals take the reference plane as the return path, that is, the backflow path. When the reference plane is incomplete, the following adverse effects may occur:

Leads to the impedance discontinuity of the wiring;

Easy to cause crosstalk between signals;

Causes reflections between signals;

Increase current loop area, increase loop inductance, so that the output waveform is easy to oscillate;

It increases radiation interference to space and is susceptible to space magnetic field;

Increase the possibility of magnetic field coupling with other circuits on the board;

The high frequency voltage drop on the loop inductor constitutes the common-mode radiation source, which is generated through the external cable.

Therefore, PCB wiring should be as close to a plane as possible and avoid cross-cutting. If it is necessary to cross the partition or cannot be near the power supply ground plane, these conditions are only allowed in low speed signal lines.

The processing of cross segmentation in design

If the inevitable occurrence of cross - division in PCB design, how to deal with it? In this case, the segmentation needs to be stitched to provide a shorter backflow path for the signal. Common treatment methods include adding sewing capacitance and bridge.

Stiching Capacitor

Usually, a 0402 or 0603 ceramic capacitor is placed across the signal partition, and the capacitance is 0.01uF or 0.1uF. If space permits, more such capacitors can be added.

At the same time, try to ensure that the signal line within the sewing capacitance of 200mil, the smaller the distance, the better; The networks at both ends of the capacitor correspond to the networks of the reference plane through which the signal passes. See the networks connected at both ends of the capacitor in the figure below, and the two different networks highlighted in two colors:

The pick up of flyover

It is common to "wrap the ground processing" of the cross-segmented signals at the signal layer, or to wrap the signal lines of other networks. This "wrap the ground" line should be as thick as possible. Please refer to the following figure for this processing method.

High speed signal wiring skills

Multilayer wiring

High-speed signal wiring circuit is often high integration, wiring density, the use of multi-layer board is not only necessary for wiring, but also an effective means to reduce interference.

Reasonable selection of layers can greatly reduce the size of the board, make full use of the intermediate layer to set the shield, realize the nearest grounding, effectively reduce the parasitic inductance, effectively shorten the transmission length of the signal, and greatly reduce the cross interference between signals.

The less you bend the lead, the better

The less lead bending between pins of high-speed circuit devices, the better.

High-speed signal wiring circuit wiring leads adopt full line, need to turn, can be 45° broken line or arc turning, this requirement in low-frequency circuit is only used to improve the fixation strength of steel foil.

However, in high-speed circuits, meeting this requirement can reduce the external transmission and coupling of high-speed signals, and reduce the radiation and reflection of signals.

The shorter the lead, the better

The shorter the lead between pins of high speed signal wiring circuit components, the better.

The longer the lead, the greater the value of distributed inductance and capacitance, the high frequency signal of the system through a lot of influence, but also change the characteristic impedance of the circuit, resulting in system reflection, oscillation and so on.

The less alternating between lead layers, the better

The less alternating lead layers between pins of high-speed circuit devices, the better.

The so-called "as few layers of lead alternating as possible" refers to the connection of components with as few holes as possible.

It has been measured that a single hole can bring about 0.5 pF of distributed capacitance, resulting in a significant increase in circuit delay, reducing the number of holes can significantly improve the speed.

pay attention to parallel cross interference

In high-speed signal wiring, attention should be paid to the "cross interference" introduced by the close parallel routing of signal lines. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of parallel signal lines to greatly reduce interference.

Avoid branches and stumps

High-speed signal wiring should avoid branching or Stub formation as far as possible.

Stumps have a large effect on impedance and can lead to signal reflection and overshoot, so we should generally avoid stumps and branches in our design.

Daisy chain wiring will reduce the impact on the signal.

Signal cables should be routed through the inner layer as far as possible

High frequency signal lines walking on the surface are prone to produce large electromagnetic radiation, and are also prone to interference by external electromagnetic radiation or factors.

If the high frequency signal cable is routed between the power supply and ground wire, the radiation generated by the power supply and the bottom layer will be reduced by absorbing electromagnetic waves