PCB Blog - Functional segmentation of high-speed PCB board

Most PCB board contain some functional subsystems or areas, each functional subsystem consists of a set of devices and their supporting circuits. For example, a typical motherboard can be divided into the following areas: processor, clock logic, memory, bus controller, bus interface, PCT bus, peripheral device interface, and video/audio processing modules. On the one hand, all devices on the PCB need to be placed close to each other, which can shorten the trace length, reduce crosstalk, reflection, and electromagnetic radiation, and ensure signal integrity; on the other hand, the RF energy generated by different logic devices The frequency spectrum is different, especially in high-speed systems, the higher the frequency of the signal, the wider the frequency band of the RF energy generated by the operation related to the digital signal hopping, it is necessary to prevent the different operating frequency bands. mutual interference, especially the interference of high-bandwidth devices to other devices.

The solution to the above problem is to use functional division, that is, to implement physical division of subsystems with different functions on the PCB. Different segmentation methods are adopted according to different products. Usually, multiple PCB boards, component isolation and Layout FE isolation can be used. Proper segmentation can optimize signal quality, simplify wiring, and reduce interference. Engineers must identify which functional partition a component belongs to, and this information can be obtained from component suppliers.

Functional segmentation can be thought of as separating one functional area from another in order to isolate circuits with different functions, such as the example shown in Figure 1. In PCB board design, the goal to be achieved is to confine the electromagnetic field associated with a particular sub-region to the region that requires this part of the energy. For example, designers want electromagnetic energy from the processor area not to pass into the I/O circuits. There is a potential difference between the processor and the I/O. As long as there is a potential difference, a common mode energy transfer will occur between these two regions, so the division between them must be well decoupled.

Functional segmentation requires attention to two aspects: dealing with conducted and radiated RF energy. Conducted RF energy is transmitted between the functional sub-area and the power distribution system through signal lines, and the radiated energy is coupled through free space. Reasonable functional division of PCB board is to seek a reasonable solution to pass useful signals to the places where they are needed, and keep out the unnecessary ones.

The PCB board division that realizes the above functions has two meanings: isolation and interconnection.

Isolation can be achieved by using "moats" to create voids without copper on all layers, the "moats" being 50 mils wide. The "moat" is like a moat, dividing the entire PCB board into "islands" one by one according to their different functions. One of the functional areas (like an "excluded" area on the PCB for signal lines and paths that are not connected to it). Obviously, the "moat" will split the mirror layer to form independent power and ground for each area, which can prevent RF energy from passing from one area to another through the power distribution system.

But segmentation is to better arrange placement and routing and achieve better interconnection, not complete "isolation", which must provide channels for those lines that need to be connected to various sub-functional areas. There are two methods here: one is to use a separate transformer, opto-isolator or common mode data line to cross the "ditch"; the other is to build a "bridge" over the "ditch", only those who have a "bridge pass" The signal can be in (signal current) and out (return current). It is impossible to design a single split layout, and another way is to metal shield the part that generates the undesired V energy, so as to control the radiation and enhance the anti-interference ability of the PCB board.