Most of the PCB contains a number of functional subsystems or regions, each functional subsystem consists of a set of devices and their supporting circuitry. For example, a typical motherboard may be divided into the following areas : a processor clock logic, memory , a bus controller , the bus interface , PCT bus, a peripheral device interface and video / audio processing modules. On the one hand , all devices need to be placed close to each other on the PCB together, so you can shorten trace lengths to reduce crosstalk , reflections, as well as electromagnetic radiation , to ensure signal integrity ; on the other hand , RF energy is generated by a different logic of the spectrum is different , particularly in high-speed systems , the higher the frequency of the signal , and the digital signal transitions generated by the operation related to RF energy band is also wider, the operating frequency band different from each other to prevent the PCB partitioning between the device instance interference, especially in high-bandwidth devices interfere with other devices.
Solve the above problem is the use of functional separation , is about to be physically different functional subsystems divided on PCB. Depending on the product and take a different segmentation methods , commonly available multi-block PCB, components and Layout FE isolation from other means. Appropriate segmentation optimize signal quality , simplify wiring to reduce interference . Engineers must be clear which function a component part of the partition , this information can be obtained from the component suppliers.
Partitioning can be considered a functional region out of the functional area is separated from the other , so that the different functions of the circuit to isolate, in the example shown in FIG . In PCB design, in order to achieve the purpose is , the sub-region associated with a particular need for this part of the electromagnetic field is limited to the energy area. For example, the designer wants the electromagnetic energy is not transmitted from the processor to the area I / O circuits. Exists between the processor and I / O potential difference , as long as the presence of potential difference between the two regions in the common mode of energy transfer is generated , the split between them must be well decoupled .
Partitioning requires attention to two aspects: treatment of RF energy conduction and radiation. RF energy is conducted through a signal transmission line between the functional sub-area and power distribution systems , we aimed amount H is coupled through free space radiation . PCB partitioning is reasonable to seek to pass the useful signal to where it is needed , and reasonable solution to unwanted shut out .
Achieve the above functions PCB division includes the meaning of two aspects: isolation and interconnect.
Isolation through the use of " trench " no copper to form an empty area on all layers from the " trenches " of a minimum width of 50 mil. " Moat " like a moat , like the entire PCB split their different functions into one of " islands ." Wherein one functional area ( for example, an isolation transformer in Figure 6-17 - those not connected to the signal line and its path as a PCB is " excluded " area ) . Obviously, " trench " layer-dividing mirror will form an independent power supply and ground for each region , which can prevent RF energy from the power distribution system from one region to another .
But segmentation is to better arrange the placement and routing for better interconnection , not completely " isolated " and must provide access to the various sub-functions need to be connected to the line of those regions . There are two ways: one is to use separate transformers, common mode optical isolator or data lines across the " trenches " , as shown in 2 (a) below; another is in the " trenches " on the ride " bridge" only those with a "bridge pass " signal to feed ( signal current ) and out ( a return current ) , as shown in 2 (b) below.
Design of an optimal partition layout is impossible, there is a way to generate an undesired portion of the metal shield V power , and enhance the PCB to control the radiation noise immunity.
