PCB Tech - Learn PCB Layout, need to master high-speed knowledge

With the continuous miniaturization, precision, and high-speed of electronic products, PCB design not only needs to complete the circuit connection of various components, but also consider the various challenges brought by high-speed and high-density. At present, high-speed PCB design has increasingly become a design skill that practitioners in the entire PCB design industry must master.

Transmission line

The definition of a transmission line is a signal line with signal return (composed of two wires of a certain length, one is the signal propagation path and the other is the signal return path.), the most common transmission line is also the trace on the PCB board.

The two most common types of transmission lines in digital design are microstrip and stripline.

Microstrip lines usually refer to the traces on the outer layer of the PCB, and there is only one reference plane. There are two types of microstrip lines: buried or non-buried. Buried (sometimes called submerged) microstrip line simply embeds a transmission line in a dielectric, but it still has only one reference plane.

Stripline refers to the inner layer trace between two reference planes.

Schematic diagram of a typical PCB transmission line

Note: In high-speed PCB design, if it is a 4-layer PCB or more, try to use the strip line, which means that the high-speed signal line should be routed to the inner layer, which can reduce EMI, EMC and enhance the anti-interference ability. .

Delay

Delay means that the signal is transmitted at a limited speed on the wires of the PCB, and the signal is sent from the sending end to the receiving end, during which there is a transmission delay. The delay of the signal will affect the timing of the system, and the transmission delay mainly depends on the length of the wire and the dielectric constant of the medium around the wire.

In high-speed digital systems, the length of the signal transmission line is the most direct factor that affects the phase difference of the clock pulse. The phase difference of the clock pulse refers to the two clock signals that are generated at the same time, and the time when they arrive at the receiving end is not synchronized.

The clock pulse phase difference reduces the predictability of the signal edge arrival. If the clock pulse phase difference is too large, an error signal will be generated at the receiving end. As shown in the following figure, the transmission line delay has become an important part of the clock pulse cycle.

Serial bus/parallel bus

High-speed signals are common in various serial buses and parallel buses. Only when you know what the bus is and how fast it runs can you start wiring.

What is a serial bus? What is a parallel bus? You can get a rough idea from the literal meaning. Serial means the data is sent bit by bit, and parallel means the data is sent in groups.

The best example of parallel transmission is the memory chip DDR. It has a set of data lines D0-D7, plus DQS, DQM. This set of lines is transmitted together. No matter which bit has an error, the data will not be transmitted correctly. Only retransmit.

Parallel data is transmitted together in groups, and each bit must be transmitted together. It cannot be said that one bit can be a little late. Therefore, a group of lines must be of equal length when PCB wiring .

Serial data is different. Data is transmitted bit by bit, and there is no connection between bits. However, although serial data is transmitted one by one, there is no need to be equal in length, but it is not a single line. High-speed lines are generally differential lines, that is, one positive and one negative two lines. This is to improve the anti-interference performance.