High frequency PCB design is too complex? You need to know t

High frequency circuit design is a very complex design process, and its wiring is very important to the whole design! Next, I would like to share some wiring skills of high frequency PCB design. High frequency PCB wiring skills 1. Multilayer board wiring high-frequency circuit is often high integration, wiring density, using multilayer board is not only necessary for wiring, but also an effective means to reduce interference. In the PCB layout stage, reasonable selection of PCB size with a certain number of layers can make full use of the intermediate layer to set the shielding, better realize the nearby grounding, effectively reduce the parasitic inductance and shorten the transmission length of the signal, at the same time, greatly reduce the cross interference of the signal, etc. all these methods are beneficial to the reliability of high-frequency circuit. 2. The shorter the lead, the better the radiation intensity of the signal is proportional to the length of the signal line. The longer the high-frequency signal lead is, the easier it will be coupled to the components close to it. Therefore, for high-frequency signal lines such as signal clock, crystal oscillator, DDR data, LVDS cable, USB cable, HDMI line, etc., the shorter the wiring is, the better. 3. The less the lead is bent, the better. The lead wire of high frequency circuit is best to adopt full straight line, which can be turned by 45 degree broken line or arc. This requirement is only used to improve the fixation strength of copper foil in low-frequency circuit, but in high-frequency circuit, meeting this requirement can reduce the external emission and coupling of high-frequency signals. 4. The less interlaminar alternation, the better. The so-called "less interlaminar alternation of leads, the better" means that the less vias (via) used in the process of component connection, the better. It is measured that a via can bring about 0.5pf distributed capacitance. Reducing the number of vias can significantly improve the speed and reduce the possibility of data error. 5. Pay attention to the "crosstalk" caused by the close parallel routing of signal lines, and pay attention to the "crosstalk" caused by the close parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Because the high frequency signal is transmitted in the form of electromagnetic wave along the transmission line, the signal line will act as an antenna, and the energy of electromagnetic field will be transmitted around the transmission line. The unwanted noise signal generated by the mutual coupling of electromagnetic fields between signals is called crosstalk. The parameters of PCB layer, the spacing of signal lines, the electrical characteristics of driver and receiver, and the termination mode of signal line have certain influence on crosstalk. 6. Add a high frequency decoupling capacitor to the power pin of the IC block, and add a high frequency decoupling capacitor to the power pin of each IC block. Increasing the high-frequency decoupling capacitance of the power supply pin can effectively suppress the high-frequency harmonic interference on the power supply pin. 7. The ground wire of high-frequency digital signal is isolated from the ground wire of analog signal. When the analog ground wire and digital ground wire are connected to the common ground wire, they should be connected with high-frequency choke magnetic beads or directly isolated, and the appropriate single point interconnection should be selected. The ground potential of the ground wire of high frequency digital signal is generally inconsistent, and there is a certain voltage difference between them. Moreover, the ground wire of high-frequency digital signal often has a very rich harmonic component of high-frequency signal. When the ground wire of digital signal is directly connected with the ground wire of analog signal, the harmonic of high-frequency signal will interfere with the analog signal through the coupling of ground wire. Therefore, in general, the ground wire of high-frequency digital signal and the ground wire of analog signal should be isolated. Single point interconnection at appropriate position or high-frequency choke magnetic bead interconnection can be used. 8. Avoid the loop formed by wiring. Try not to form a loop for all kinds of high-frequency signal wiring. If it is unavoidable, make the loop area as small as possible. 9. Good signal impedance matching must be ensured. In the process of transmission, when the impedance does not match, the signal will reflect in the transmission channel, which will cause the synthetic signal to form overshoot and cause the signal to fluctuate near the logic threshold. The fundamental way to eliminate the reflection is to make the impedance of the transmission signal match well. Since the greater the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection will be. Therefore, the characteristic impedance of the signal transmission line should be equal to the load impedance as far as possible. At the same time, it should be noted that the transmission line on the PCB should not have abrupt changes or corners, and try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflection between each section of the transmission line. 10. Keep the integrity of signal transmission, keep the integrity of signal transmission, and prevent "ground bounce" caused by ground wire division.