在设计中 PCB板，随着频率的快速增加，会出现很多不同于低频PCB板的干扰。 而且，随着频率的增加以及PCB板小型化与低成本之间的矛盾，这些干扰会变得越来越复杂。

在实际研究中，可以得出干扰主要有四个方面，包括电源噪声、传输线干扰、耦合和电磁干扰（EMI）。 通过分析高频PCB的各种干扰问题，并结合工作实践，提出了有效的解决方案。

一、电源噪声

在高频电路中，电源噪声对高频信号有明显的影响。 Therefore, the first requirement of the power supply is low noise. 清洁地板与清洁电力一样重要。 为什么？ 功率特性如图 1 所示。 显然，电源具有一定的阻抗，并且阻抗分布在整个电源上，因此，噪声会被添加到电源中。

Then we should minimize the impedance of the power supply, so it is best to have a dedicated power supply layer and grounding layer. 在高频电路设计中，在大多数情况下，将电源设计为层比设计为总线要好得多，这样环路可以始终遵循最小阻抗的路径。

此外，电源板必须为 PCB 上所有生成和接收的信号提供信号回路。 这最大限度地减少了信号环路，从而降低了低频电路设计人员经常忽视的噪声。

高频PCB设计出现干扰解决方案

图 1：功率特性

PCB设计中消除电源噪声的方法有以下几种：

1. Note the through hole on the board: the through hole requires etched openings on the power supply layer to leave space for the through hole to pass through. 如果电源层开度过大，势必会影响信号环路，信号被迫旁路，环路面积增大，噪声增大。 At the same time, if several signal lines are clustered near the opening and share the same loop, the common impedance will cause crosstalk. 见图2。

高频PCB设计出现干扰解决方案

图 2：旁路信号回路的公共路径

2. The connection line needs enough ground: each signal needs to have its own proprietary signal loop, and the loop area of the signal and loop is as small as possible, that is to say, the signal and loop should be parallel.

3.模拟和数字电源要分开：高频设备一般对数字噪声很敏感，所以两者应该分开，在电源入口处连接在一起，如果信号穿过模拟和数字部分的话，可以把信号放在一个环路上，以减少环路面积。 用于信号环路的数模跨度如图 3 所示。

高频PCB设计出现干扰解决方案

Figure 3: Digital – analog span for signal loop

4. Avoid overlapping of separate power supplies between layers: otherwise circuit noise can easily pass through parasitic capacitive coupling.

5. Isolate sensitive components: such as PLL.

6. Place the power cable: To reduce the signal loop, place the power cable on the edge of the signal line to reduce the noise, as shown in Figure 4.

高频PCB设计出现干扰解决方案

图 4：将电源线放在信号线旁边

Two, transmission line

PCB 中只有两条可能的传输线：

带状线和微波线最大的问题是反射。 反射会导致很多问题。 例如，负载信号将是原始信号和回波信号的叠加，这会增加信号分析的难度。 反射会导致回波损耗（return loss），它对信号的影响与加性噪声干扰一样严重：

1、反射回信号源的信号会增加系统的噪声，使接收机更难区分噪声和信号；

2. Any reflected signal will basically degrade the signal quality and change the shape of the input signal. Generally speaking, the solution is mainly impedance matching (for example, the impedance of the interconnection should very match the impedance of the system), but sometimes the calculation of impedance is more troublesome, you can refer to some transmission line impedance calculation software. The methods of eliminating transmission line interference in PCB design are as follows:

(a) 避免传输线的阻抗不连续。 阻抗不连续点是传输线突变点，如直角、通孔等，应尽量避免。 方法：避免直线拐角，尽量走45°角或圆弧，大角也可以； 使用尽可能少的通孔，因为每个通孔都是一个阻抗不连续点，如图 5 所示。 XNUMX; Signals from the outer layer avoid passing through the inner layer and vice versa.

高频PCB设计出现干扰解决方案

Figure 5: Method for eliminating transmission line interference

(b) Do not use stake lines. 因为任何桩线都是噪音源。 若桩线较短，可接在传输线末端； 如果桩线很长，就会以主传输线为源，产生很大的反射，使问题复杂化。 建议不要使用它。

三、联轴器

1. Common impedance coupling: it is a common coupling channel, that is, the interference source and the interfered device often share some conductors (such as loop power supply, bus, and common grounding), as shown in Figure 6.

高频PCB设计出现干扰解决方案

图 6：公共阻抗耦合

In this channel, the drop back of the Ic causes a common-mode voltage in the series current loop, affecting the receiver.

2. The field common-mode coupling will cause the radiation source to cause common-mode voltages in the loop formed by the interfered circuit and on the common reference surface.

If the magnetic field is dominant, the value of the common-mode voltage generated in the series ground circuit is Vcm=-(△B/△t)* area (where △B= change in magnetic induction intensity). If it is an electromagnetic field, when its electric field value is known, its induced voltage: Vcm=(L* H *F*E)/48, the formula is suitable for L(m)=150MHz, beyond this limit, the calculation of the maximum induced voltage can be simplified as: Vcm=2* H *E.

3. Differential mode field coupling: refers to the direct radiation by wire pair or circuit board on the lead and its loop induction received. If you get as close to the two wires as possible. 这种耦合大大减少，因此可以将两根线绞在一起以减少干扰。

4. Inter-line coupling (crosstalk) can cause unwanted coupling between any line or parallel circuit, which will greatly damage the performance of the system. Its type can be divided into capacitive crosstalk and perceptual crosstalk.

The former is because the parasitic capacitance between the lines makes the noise on the noise source coupled to the noise receiving line through current injection. The latter can be thought of as the coupling of signals between the primary stages of an unwanted parasitic transformer. 电感串扰的大小取决于两个回路的接近程度、回路面积的大小以及受影响负载的阻抗。

5、电源线耦合：交流或直流电源线受到电磁干扰

转移到其他设备。

There are several ways to eliminate crosstalk in PCB design:

1、两种串扰都随着负载阻抗的增加而增加，因此对串扰引起的干扰敏感的信号线应适当端接。

2. 尽量拉近信号线之间的距离，有效降低电容串扰。 接地管理，布线之间的间距（例如有源信号线和地线进行隔离，特别是在信号线和地之间跳动的状态下要间隔）并减少引线电感。

3. Capacitive crosstalk can also be effectively reduced by inserting a ground wire between adjacent signal lines, which must be connected to the formation every quarter of a wavelength.

4. 对于合理串扰，应尽量减少环路面积，如果允许，应消除环路。

5. Avoid signal sharing loops.

6、注意信号完整性：设计者应在焊接过程中实现端部解决信号完整性问题。 使用这种方法的设计人员可以关注屏蔽铜箔的微带长度，以获得良好的信号完整性性能。 For systems with dense connectors in the communication structure, the designer can use a PCB as the terminal.

Four, electromagnetic interference

As the speed increases, EMI becomes more and more serious and presents in many aspects (such as electromagnetic interference at interconnects). High-speed devices are particularly sensitive to this and will receive high-speed spurious signals, while low-speed devices will ignore such spurious signals.

PCB设计中消除电磁干扰的方法有以下几种：

1. 减少回路：每个回路相当于一个天线，所以我们需要尽量减少回路的数量、回路的面积和回路的天线效应。 Make sure the signal has only one loop path at any two points, avoid artificial loops and use the power layer whenever possible.

2. Filtering: Filtering can be used to reduce EMI on both the power line and the signal line. There are three methods: decoupling capacitor, EMI filter and magnetic element. EMI filter is shown in Figure 7.

高频PCB设计出现干扰解决方案

图 7：过滤器类型

3. The shielding. 由于本期篇幅加上大量讨论屏蔽文章，不再具体介绍。

4. Reduce the speed of high-frequency devices.

5、增加PCB板的介电常数，可以防止靠近板的传输线等高频部分向外辐射； Increase the thickness of PCB board, minimize the thickness of microstrip line, can prevent electromagnetic line spillover, can also prevent radiation.

At this point, we can conclude that in hf PCB design, we should follow the following principles:

1. Unification and stability of power supply and ground.

2. Carefully considered wiring and proper terminations can eliminate reflections.

3. 仔细考虑布线和适当的端接可以减少电容和电感串扰。

4. 噪声抑制需要满足EMC要求。