PCB News - Functional module division in high-speed PCB design

Most PCBs contain some functional subsystems or areas, and 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, video/audio processing module, etc. On the one hand, all the components 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 RF energy generated by operations related to digital signal hopping. It is necessary to prevent mutual interference between devices with different operating frequency bands, especially Interference from high-bandwidth devices to other devices.

The solution to the above-mentioned problems is to use functional division, that is, to physically divide subsystems with different functions on the PCB board. According to different products, different division methods are adopted. Usually, multiple PCBs, component isolation, and Layout FE separation methods can be used. Proper segmentation can optimize signal quality, simplify wiring, and reduce interference. The engineer must clarify which functional area a component belongs to, and this information can be obtained from the component supplier.

Functional segmentation can be thought of as separating one functional area from another in order to isolate circuits with different functions, as shown in the example in Figure 1. In PCB design, the goal to be achieved is to limit the electromagnetic field associated with a specific sub-area to the area that requires this part of energy. For example, the designer hopes that electromagnetic energy from the processor area cannot be transferred to the I/O circuit. 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 areas, so the division between them must be well decoupled.

Functional division requires attention to two aspects: dealing with conducted and radiated RF energy. The conducted RF energy will be transmitted between the functional sub-area and the power distribution system through the signal line, and the radiated HZ energy will be coupled through the free space. Reasonable PCB function segmentation is to seek a reasonable solution to transmit useful signals to the place where it is needed, while shutting out the unneeded ones.

The PCB segmentation that realizes the above-mentioned functions includes two aspects: isolation and interconnection.

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

However, the purpose of segmentation is to better arrange layout and wiring and achieve better interconnection. It is not a complete "isolation". It is necessary to provide channels for lines that need to be connected to various sub-functional areas. There are two methods here: one is to use an independent transformer, optical isolator or common mode data line to cross the "ditch", as shown in Figure 2(a); the other is to build a "bridge" on the "ditch", Only those signals with a "bridge pass" can enter (signal current) and exit (return current), as shown in Figure 2(b).

Figure 2 Isolation and bridging

It is impossible to design an optimized split layout. Another way is to metal shield the part that generates undesirable V energy, so as to control the radiation and enhance the anti-interference ability of the PCB.