PCB design skills of RF circuit and audio circuit for handheld products

PCB是信息产业的基础，从电脑、便携式电子设备，几乎所有的电子电器产品都有PCB。随着通信技术的发展，手持射频电路技术的应用越来越广泛，这些设备（如手机、无线PDA等）的最大特点之一是：第一，几乎囊括了所有的便携式子系统；二是小型化，小型化是指元器件的密度非常大，这使得元器件（包括SMD、SMC、裸片等）的干扰非常突出。因此，设计完善的射频电路和音频电路PCB，以防止和抑制电磁干扰，提高电磁兼容性成为一个非常重要的课题。因为相同的电路，不同的PCB设计 结构，其性能指标会有很大不同。尤其是当今手持产品的音频能力不断提高，更要重视音频电路的PCB布局。基于此，本文分析了手持产品射频电路和音频电路的PCB巧妙设计（包括元件布局、元件布局、布线和接地技巧）。

1.组件布局

首先是布局的一般原则：元器件尽量排列在同一方向，通过选择PCB进入熔锡系统的方向，可以减少甚至避免焊接不良的现象；从实践中我们知道，元件之间的间距至少应为0.5mm，才能满足熔锡元件的要求。如果PCB板的空间允许，元件之间的空间应该尽可能的宽。对于双面板，一侧应设计为SMD和SMC元件，另一侧为分立元件。

* 将PCB分为数字区和模拟区

任何 PCB 设计的第一步当然是为每个组件选择 PCB 布局。我们称这一步为“布局思考”。仔细的组件布局可以减少电路板上的信号互连、地线分段、噪声耦合和占位面积。

电磁兼容要求各电路模块尽量不产生电磁辐射，并具有一定的抗电磁干扰能力。因此，元器件的布局也直接影响到电路本身的抗干扰能力和抗干扰能力，这也直接关系到所设计电路的性能。

因此，在射频电路PCB的设计中，除了普通PCB设计的布局外，还需要考虑如何降低射频电路各部分之间的干扰，如何降低电路本身对其他电路的干扰以及电路本身的抗干扰能力。

由经验可知，射频电路的效果不仅取决于射频电路板本身的性能，还取决于与CPU处理板的交互。由于射频电路包含数字电路和模拟电路，为了防止数字噪声对敏感模拟电路的干扰，必须将两者分开，将PCB分为数字区和模拟区以改进此类电路的布局，尤为重要。

需要防止射频噪声耦合到音频电路

虽然手持产品的射频部分通常被视为模拟电路，但在许多设计中常见的问题是射频噪声，需要防止射频噪声耦合到音频电路，因为射频噪声被解调以产生可听噪声。为了解决这个问题，射频电路和音频电路需要尽可能地分开。将PCB分为模拟元件和数字元件后，需要考虑模拟元件的布局。组件布局应尽量减少音频信号的路径，音频放大器应尽可能靠近耳机插孔和扬声器放置，以尽量减少 D 类音频放大器的 EMI 辐射和耳机信号的耦合噪声。模拟音频信号源应尽可能靠近音频放大器的输入，以尽量减少输入耦合噪声。

2、组件布局应注意的问题及应用实例

2.1 布局需要注意的问题：

* 仔细分析电路结构。阻断电路（如高频放大电路、混频电路和解调电路等），尽可能将大电流信号和弱电流信号分开，在单独的数字信号电路和模拟信号电路中，完成电路的相同功能应注意并尽量安排在一定范围内，从而减少信号环路面积；电路各部分的滤波网络必须就近连接，这样既可以减少辐射，又可以降低干扰的概率，提高电路的抗干扰能力。

* 根据电池电路对使用中的电磁兼容性的敏感性对它们进行分组。电路中易受干扰的元器件也应避开干扰源（如数据处理板上CPU的干扰）。

2.2 元器件布局对音频信号影响举例

* 组件布局不当会影响音频信息质量

Figure 1 shows an unreasonable layout of audio components. There are two serious problems. One is that the audio amplifier is too far from the audio signal source, which increases the chance of noise coupling because the leads pass near the noisy digital and switching circuits. Longer leads also enhance the RF antenna effect. If a mobile phone uses GSM technology, these antennas can pick up the GSM transmitted signal and feed it into an audio amplifier. Almost all amplifiers can demodulate the 217Hz envelope to some extent, producing noise at the output. In bad cases, the noise can drown out the audio signal completely, and shortening the length of the input leads can effectively reduce the noise coupled to the audio amplifier. The second audio amplifier is placed too far from the speaker and headphone socket. If the audio amplifier is a Class D amplifier, a longer headphone lead will increase the EMI radiation of the amplifier. The radiation could cause the device to fail testing standards set by local authorities. Longer headphone and microphone leads also increase lead impedance and reduce the power that can be captured by the load. Finally, because components are so scattered, wires between components will have to pass through other subsystems. This not only increases the difficulty of wiring the audio part, but also the other subsystems.

FIG. 1 Influence of unreasonable component layout on audio signal quality

* Reasonable component layout improves audio signal quality

FIG. 2 shows the arrangement of the same elements in FIG. 1. The rearranged elements make more efficient use of space and shorten the lead length. Note that all audio circuits are allocated near the headphone jack and speakers, the audio input and output leads are much shorter than the above scheme, and no audio circuits are placed in other areas of the PCB. Such a design can reduce overall system noise, reduce RF interference, and simple wiring.

Figure 2. Reasonable component layout shows improvement in audio signal quality

3. Wiring principles and skills

After the components are laid out, wiring can begin.

3.1 Basic principles of wiring

When the assembly density is allowed, low-density wiring design should be selected as far as possible, and the signal wiring should be as thick and thin as possible, which is conducive to impedance matching.

For RF circuit, the unreasonable design of signal line direction, width and line spacing may cause cross interference between signal transmission lines. The signal path has very limited influence on the noise and distortion of the audio output, that is to say, there are very few compromises that need to be provided to ensure performance. Audio amplifiers are usually powered directly by batteries and require considerable current. If long and thin power leads are used, power ripple will be increased. Compared with short and wide leads, the impedance of long and thin leads is larger, and the current changes generated by the lead impedance are converted into voltage changes, which are fed into the device. To optimize performance, the amplifier power supply should use the shortest possible lead. Differential signals should be used whenever possible. Differential input has high noise suppression, so differential receiver can suppress common-mode noise on positive and negative signal lines. In order to make full use of the advantages of the differential amplifier, it is very important to keep the length of the same differential signal line pair during wiring, so that they have the same impedance, and they are as close to each other as possible so that their coupling noise is the same. The differential input of the amplifier is very effective in suppressing noise from the system’s digital circuits. In addition, the system power supply itself also exists noise interference, so in the design of RF circuit PCB must be considered comprehensively, reasonable wiring.

3.2 Cabling Techniques

When wiring, all wiring should be far away from the border of THE PCB board (about 2mm), so as not to cause or have the hidden danger of wire breaking during PCB board production. The power line should be as wide as possible to reduce the resistance of the loop. At the same time, the direction of the power line and the ground line should be consistent with the direction of data transmission to improve the anti-interference ability. The signal lines should be as short as possible and the number of holes should be reduced as far as possible. The shorter the connection between components, the better, to reduce the distribution of parameters and electromagnetic interference between each other; For incompatible signal lines should be far away from each other, and try to avoid parallel lines, and in the positive two sides of the application of mutual vertical signal lines; Wiring in need of corner address should be 135° Angle as appropriate, avoid turning right angles.

4, grounding

In PCB design of rf circuit, the correct wiring of power line and ground wire is particularly important, and reasonable design is the most important means to overcome electromagnetic interference. Quite a lot of interference sources on PCB are generated by power supply and ground wire, among which ground wire causes the most noise interference. The main reason why the ground wire is easy to cause electromagnetic interference is the impedance of the ground wire. When a current flows through the ground, a voltage will be generated on the ground, resulting in the ground loop current, forming the loop interference of the ground. When multiple circuits share a single piece of ground wire, common impedance coupling occurs, resulting in what is known as ground noise. Therefore, when wiring the ground of RF circuit PCB, the following should be done:

* For circuit block processing, rf circuit can be basically divided into high frequency amplification, mixing, demodulation, local vibration and other parts, to provide a common potential reference point for each circuit module circuit grounding, so that the signal can be transmitted between different circuit modules. It is then summarized at the point where the RF circuit PCB is connected to the ground, i.e. summarized at the main ground. Since there is only one reference point, there is no common impedance coupling and thus no mutual interference problem.

* Digital area and analog area as far as possible ground wire isolation, and digital ground and analog ground to separate, finally connected to the power supply ground.

* If space permits, it is better to isolate each module with ground wire to prevent signal coupling effect between each other.

For audio circuits, grounding is critical to achieving the performance requirements of the audio system. Grounding in any system has two important considerations: first, it is the return path of current flowing through the device, and second, the reference potential of the digital and analog circuits. Here are some tips that apply to all systems:

* Establish a continuous ground plane for the digital circuit. The digital current of the formation is returned through a signal path, and the area of this loop should be kept to a minimum to reduce antenna effects and parasitic inductance. Ensure that all digital signal leads have corresponding ground paths. This layer should cover the same area as the digital signal leads and have as few breakpoints as possible. Breakpoints in the formation, including through-holes, allow the earth current to flow through a larger loop, resulting in greater radiation and noise.

* Ensure ground current isolation. The ground current of the digital circuit and the analog circuit should be kept separate to prevent the digital current from interfering with the analog circuit. In order to achieve this goal, the components need to be aligned correctly. If the analog circuit is placed in one area of the PCB and the digital circuit is placed in another area, the ground current will be isolated naturally. It is desirable to have an analog circuit with independent PCB layering.

* Analog circuit uses star grounding. Star grounding refers to a point on the PCB as the common ground point, and only this point is considered as the ground potential. In cellular phones, the ground end of the battery is usually regarded as the star ground point. The current flowing into the ground plane does not disappear automatically, and all the ground current will be merged into this ground point. Audio amplifiers absorb considerable current, which affects the reference ground of the circuit itself and that of other systems. To solve this problem, it is best to provide a dedicated return loop to bridge the amplifier’s power ground and the headphone jack’s ground loop. Note that these dedicated loops do not pass through the digital signal lines, as they will block the digital return current.

* Maximize bypass capacitance effect. Almost all devices require a bypass capacitor to provide transient current that cannot be provided by the power supply. These capacitors should be placed as close to the power supply pins as possible to reduce parasitic inductance between the capacitor and the device pins, which reduces the effect of the bypass capacitor.

An example of a grounded circuit board

Take the evaluation board for ultra-low EMI, 1.5W, filterless Class D audio power amplifier and 80mW DirectDriv headphone amplifier MAX9776 as an example.

FIG. 3 is an example of a circuit board with a good grounding distribution (i.e., silkscreen layer and layer example), FIG. 3 (a) is the substrate layout A(front), FIG. 3 (b) is the substrate layout B(back). The first thing to note is that the PCB has a digital region at the bottom and an analog region at the top. The only signal lines crossing the zone boundary are the I2C control signals, and these signal lines have a direct return path to ensure that the digital signal exists only in the digital zone without digital ground current due to formation segmentation. Note also that most of the ground plane is continuous, and that the digital regions are far enough apart from each other to allow the current channel to flow smoothly, even if there are some interruptions. In this case, the star ground point is in the top left corner of the PCB. Breakpoints in the analog formation ensure that the current from the Class D amplifier and charge pump is returned directly to the star ground point without interfering with other analog layers. Also note that the headphone jack has a lead that returns the earphone ground current directly to the star ground point.

5, conclusion

The above well-designed PCB is time-consuming and challenging, but the investment is well worth it. Good PCB layout helps reduce system noise, improve RF signal suppression and reduce signal distortion. A good PCB design will also improve EMI performance, possibly requiring less shielding. If the PCB is not reasonable, problems can occur during the testing phase that could have been avoided. It may be too late to solve the problem, requiring more time, effort, and sometimes additional components, adding cost and complexity to the system.

如今，PCB技术主要根据电子产品的特性和要求而变化。近年来，电子产品变得越来越多功能、精密和符合环保法规。因此，PCB的精密度高，其软硬结合板的应用也会增加。