Easy understand the PCB layout

PCB layout is laying conductive signal path connecting the various devices, like decorate the  road, connecting the city by car.
Road construction requires a return to a two-line, PCB routing the same principle, you need two lines form a loop ,for the low-frequency circuit point is a circuit; for the electromagnetic field in terms of speed it is  the transmission line, the most common, such as differential signals line. Such as USB, cable, etc.

We can say that the differential signal lines, is the ideal model for connecting devices signals. The higher the signal requirements, the more you want to close the differential signal lines.

When the device is very much in a board, if distribute it as the differential lines: one problem is PCB area is too large, the second problem is should be distribute into 2N lines heavy workload, It is very difficult, so the actual needs of the people for the proposed multi-layer PCB the concept, The typical is the double-sided PCB board. The bottom layer as a common reference circuit wiring ,so only distribute N + 1 lines, PCB layout is also greatly reduced

Common reference circuit, we often say that the reference ground, for most embedded industry, because the signal is digitized ,signal quality requirements is not very high. Using whole Layer of reference ground not only reduce the board size, but also improve efficiency , it saves time. In fact, the board is reduced to shorten the length of the signal line, and some may be offset because the reference to the signal quality degradation caused problems, so in practice, such incorporation by reference to the effect of PCB layout, the difference was similar to the ideal model line

In the dual-panel design, usually there are often cross-line, you need to do cross-line jumper to ground switching, noted that this jumper can not too long, if too long, easy to split reference ground, especially for some signal line high quality requirements, refer to the bottom it can not be split. Otherwise the signal circuit was completely destroyed, reference ground line will be useless.  generally speaking,  reference ground line only suitable for short jumper with signal lines, signal cables distribute on top layer, or use  more layers of PCB board


PCB layout carrier is a PCB board, general reference  ground to  the PCB edge 1mm from nearby, the signal from the reference line to the near edge of 1mm, so the signals are confined within the PCB  board, this way can be reduced EMC radiation

Source:http://www.wonderfulpcb.com/blog/easy-understand-pcb-layout/