The key point of High Frequency PCBs design

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 are often highly integrated and have high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means of reducing 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. Therefore, for data such as signal clock, crystal, DDR, LVDS cable, USB cable, HDMI cable and other high-frequency signal lines are required to be as short as possible.

3. The fewer lead bends, 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. In high-frequency circuits, this is met. One requirement can reduce the external emission and coupling of high-frequency signals.

4. The less alternating between the lead layers, the better

The so-called "the fewer the alternate layers of the leads, 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 parallel traces of signal lines at close range. 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 termination method of the signal line have certain effects on the crosstalk.

6. Add high frequency decoupling capacitors to the power pins of the integrated circuit block

A high frequency decoupling capacitor is added 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. The ground of the high-frequency digital signal is isolated from the ground of the analog signal

When the analog ground wire and digital ground wire are connected to the common ground wire, high-frequency choke magnetic beads should be used to connect or directly isolate and select a suitable 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

All kinds of 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, resulting in the signal fluctuating near the logic threshold.

The fundamental method to eliminate reflection is to make the impedance of the transmission signal well matched. As the difference between the load impedance and the characteristic impedance of the transmission line increases, the reflection also increases, so the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible. 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 signal transmission and prevent the "ground bounce phenomenon" caused by the division of the ground wire.