PCB design is an important stage of electronic product design. High frequency PCB layout technology is also very important.It can realize the connection and function between electronic components, and it is also an important part of power supply circuit design. High frequency circuit has higher integration and higher layout density, so how to make layout more reasonable and more scientific is very important.
PCB can realize the connection and function between electronic components, and it is also an important part of power circuit design. When designing the electrical schematic diagram, multiple function boards should be adopted according to the structure requirements and function division, and the physical size and installation method of each functional board should be determined. The convenience of commissioning and maintenance, as well as shielding, heat dissipation and EMI performance, should also be considered. The engineer is required to determine the layout and wiring plan, such as key circuit, signal line, detailed information of wiring method and wiring principle to be followed. Several stages of PCB design process must be checked, analyzed and modified. Only after the entire layout process has been completed and checked through a comprehensive rule check can it be further processed.
Multilayer High Frequency PCB layout:
High frequency circuits are usually highly integrated and have high wiring density. The use of multilayer board is a necessary and effective means to reduce interference. In the PCB layout stage, reasonable selection of PCB with a certain size and layer number can make full use of the intermediary to set blocks, better ground nearby, and effectively reduce the parasitic inductance, shorten the length of signal transmission, but also can greatly reduce the cross interference of signals, etc.
All these methods are conducive to the reliability of high-frequency circuit. Using the same material, the noise of four-layer board is 20 dB lower than that of double-sided board. However, at the same time, there is still a problem, that is, the more layers of PCB, the more complex the manufacturing process, the higher the unit cost. This requires that in addition to selecting the appropriate PCB layer, reasonable component layout planning should be carried out, and appropriate layout rules should be adopted to complete the design.
- The less alternate leads of pins between each layer of high frequency circuit, the better. This means that the less via is used in the connection, the better. A via can bring about 0.5pf of distributed capacitance. Reducing the number of vias can significantly improve the speed and reduce the possibility of data errors.
- The shorter the high frequency leads, the better. The radiation intensity of the signal is directly proportional to the wiring length of the signal line. The longer the high frequency signal line, the easier it is to couple to its components. Therefore, for signal clock, crystal oscillator, DDR data, LVDS, USB and HDMI, the shorter the wiring length is, the better.
- The smaller the bending between the pins of high frequency electronic equipment, the better.It is better to use full straight line for high frequency wire. If you need to interrupt, you can use a 45 degree broken line or arc. This requirement is only used to improve the bonding strength of copper foil in low-frequency circuit, while in high frequency circuit, it can reduce the coupling between external emission and high frequency signal.
- Pay attention to the “crosstalk” introduced by the signal lines parallel to the short distance. For high frequency circuit wiring, attention should be paid to the “crosstalk” introduced by the signal lines close to parallel. Crosstalk refers to the coupling phenomenon between signal lines without direct connection. Since the high frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line will act as an antenna, and the energy of electromagnetic field will be emitted around the transmission line. Due to the coupling of electromagnetic fields, the harmful noise between signals is called crosstalk. The parameters of PCB layer, the spacing of signal lines, the electrical characteristics of driving terminal and receiving terminal, and the connection mode of signal line have certain influence on crosstalk.
- High frequency digital signal ground wire and analog signal ground wire are isolated. Analog ground wire, digital ground wire, etc. shall be connected to the common ground wire by using high frequency choke bead connection or direct isolation, and appropriate single point connection shall be selected.
- Increase the high frequency decoupling capacitance of IC module power pin. A high frequency demultiplexer capacitor is added near the power pin of each IC module. Increasing the high frequency decoupling capacitance of the power pin of IC module can effectively suppress the interference of high frequency harmonics on the power pin.
- Avoid circuit when wiring.When wiring, all kinds of high frequency signals should not form a loop. If unavoidable, the loop area should be as small as possible.
- Good signal impedance matching must be ensured. In the transmission process, when the impedance does not match, the signal will be reflected in the transmission channel, which will cause the synthetic signal to overshoot and cause the signal to fluctuate near the logic threshold. The basic method to eliminate reflection is to make the impedance of transmission signal match well.
In addition to the above design methods, engineers should avoid high-speed signal route branches or stumps when designing PCB signal wiring. When high frequency signal lines walk on the ground, it is easy to produce large electromagnetic radiation. If the high frequency signal line is connected between the power supply and the ground wire, the radiation generated by absorbing electromagnetic waves through the power supply and the bottom layer will be greatly reduced. To sum up, high frequency circuits are usually highly integrated and have high wiring density.