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High frequency PCB design is too complicated? You need to know these wiring tips

2020/05/14 17:30
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High frequency circuit design is a very complicated design process, and its wiring is crucial to the entire design! Next, I will share with you some wiring techniques for high-frequency PCB design.
High frequency PCB wiring skills
1. Multilayer board wiring
High-frequency circuits tend to have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. In the PCB Layout stage, the size of a certain number of printed boards is reasonably selected, the middle layer can be fully used to set the shield, and the near ground can be better achieved, and the parasitic inductance and signal transmission length can be effectively reduced. Amplifying the cross-interference of the signal, etc. All these methods are beneficial to the reliability of high-frequency circuits.
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, the easier it is to couple to the components close to it. LVDS cable, USB cable, HDMI cable and other high-frequency signal lines are required to be as short as possible.
3. The less the lead bending, the better
The high-frequency circuit wiring lead is best to use a full straight line, which needs to be turned. A 45-degree fold line or an arc can be used to turn. This requirement is only used to improve the fixing strength of copper foil in low-frequency circuits. One requirement can reduce the external emission and coupling of high-frequency signals.
4. The less alternate the lead layers, the better
The so-called "the less the interlayer lead is, the better" means that the fewer vias (Via) used in the component connection process, the better. It is measured that 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.
5. Pay attention to the "crosstalk" introduced by the signal lines in close parallel
High-frequency circuit wiring should pay attention to the "crosstalk" introduced by the short-distance parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since the high-frequency signal is transmitted in the form of electromagnetic waves along the transmission line, the signal line will act as an antenna, the energy of the electromagnetic field will be emitted around the transmission line, and the undesired noise signal between the signals due to the mutual coupling of the electromagnetic field Called Crosstalk.
The parameters of the PCB layer, the spacing of the signal lines, the electrical characteristics of the driving end and the receiving end, and the signal line termination method have a certain effect on crosstalk.
6. Add high frequency decoupling capacitors to the power pins of the integrated circuit block
Add a high-frequency decoupling capacitor to the power supply pin of each integrated circuit block. Increasing the high-frequency decoupling capacitor of the power supply pin can effectively suppress the interference of high-frequency harmonics on the power supply pin.
7. Isolate the ground of high-frequency digital signal and the ground of analog signal
When the analog ground wire, digital ground wire, etc. are connected to the common ground wire, use high-frequency choke magnetic beads to connect or directly isolate and select the appropriate place for single-point interconnection. The ground potential of the high-frequency digital signal ground is generally inconsistent, and there is often a certain voltage difference between the two. Moreover, the ground of the high-frequency digital signal often also has a very rich harmonic component of the high-frequency signal. When the digital signal ground and the analog signal ground are directly connected, the harmonics of the high-frequency signal will be coupled through the ground To interfere with analog signals. Therefore, under normal circumstances, the ground of the high-frequency digital signal and the ground of the analog signal are to be isolated. You can use the single-point interconnection method at a suitable location, or the high-frequency choke bead interconnection method.
8. Avoid loops formed by traces
Various high-frequency signal traces should not form a loop as much as possible. If it cannot be avoided, the loop area should be as small as possible.
9. Must ensure good signal impedance matching
In the process of signal transmission, when the impedance does not match, the signal will be reflected in the transmission channel. The reflection will cause the composite signal to overshoot, causing the signal to fluctuate near the logic threshold.
The fundamental method of eliminating reflection is to make the impedance of the transmission signal well matched. Since the larger the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection, so the characteristic impedance of the signal transmission line should be as equal as possible to the load impedance. At the same time, it should also be noted that the transmission line on the PCB must not have sudden changes or corners, and try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflections between the various sections of the transmission line.
10. Maintain the integrity of signal transmission
Maintain the integrity of the signal transmission and prevent the "ground bounce phenomenon" caused by the ground segmentation.