Clever circuit board wiring improves the sound quality of cellular phones

Abstract: This article discusses the key factors that affect audio characteristics in cellular phone PCB design. The article gives a problematic cellular phone PCB design and a good PCB layout plan. The comparison of the two layouts emphasizes the design considerations for improving audio performance.




Introduction Cellular phones are the ultimate challenge facing PCB layout engineers. Modern cellular phones include almost all portable subsystems, and each subsystem has conflicting needs. A well-designed PCB must give full play to the performance advantages of each connected device while avoiding mutual interference between subsystems. Therefore, for conflicting requirements, the performance of each subsystem must be compromised. Although the audio functions of cellular phones continue to increase, little attention has been paid to the PCB layout of audio circuits.

Component layout The first step in any PCB design is of course the choice of PCB placement for each component. We call this step "layout consideration". Careful component layout can reduce signal interconnection, ground splitting, noise coupling, and occupy board area.

Cellular phones contain digital and analog circuits. In order to prevent digital noise from interfering with sensitive analog circuits, the two must be separated. Dividing the PCB into a digital area and an analog area helps improve the layout of such circuits.

Although the RF part of a cellular phone is usually treated as an analog circuit, a common issue that needs attention in many designs is RF noise. It is necessary to prevent RF noise from coupling into the audio circuit and generating audible noise after demodulation. In order to solve this problem, the RF circuit and the audio circuit need to be separated as much as possible.

After dividing the PCB into analog, digital, and RF regions, the component placement of the analog part needs to be considered. The component layout should make the path of the audio signal shortest, the audio amplifier should be placed as close as possible to the headphone jack and speaker, so that the EMI radiation of the class D audio amplifier is minimized, and the coupling noise of the headphone signal is minimized. The analog audio signal source must be as close as possible to the input end of the audio amplifier to minimize input coupling noise. All input leads are an antenna for RF signals, and shortening the lead length helps reduce antenna radiation effects in the corresponding frequency band.

Component layout example Figure 1 shows an unreasonable audio component layout. The more serious problem is that the audio amplifier is too far away from the audio signal source. The lead wire passes through the noisy digital circuit and the switch circuit, thereby increasing the probability of noise coupling. Longer leads also enhance the RF antenna effect. Cellular phones use GSM technology. These antennas are able to pick up GSM transmission signals and feed them into audio amplifiers. Almost all amplifiers can demodulate the 217Hz envelope to some extent, producing noise at the output. When it is bad, the noise may completely submerge the audio signal, and shortening the length of the input lead can effectively reduce the noise coupled to the audio amplifier.

The component layout shown in Figure 1 has another problem: the op amp is too far from the speaker and headphone jack. If the audio amplifier is a Class D amplifier, a longer headphone lead will increase the EMI radiation of the amplifier. This radiation may cause the device to fail the test standards set by the local government. Longer headphone and microphone leads also increase lead impedance and reduce the power that the load can draw.

Finally, because the components are arranged so scattered, the wiring between the components will have to pass through other subsystems. This will not only increase the wiring difficulty of the audio part, but also increase the wiring difficulty of other subsystems.

Figure 1. Unreasonable component layout
Figure 1. Unreasonable component layout

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

Figure 2. A reasonable layout for cellular phones
Figure 2. A reasonable layout for cellular phones

Signal path The signal path has a very limited impact on audio output noise and distortion, which means that the trade-offs that need to be provided to ensure performance are limited.

Loudspeaker amplifiers are usually directly powered by batteries and require considerable current. If you use long and thin power leads, it will increase the power supply ripple. Compared with short and wide leads, long and thin leads have a larger impedance. The current changes caused by the lead impedance will be transformed into voltage changes and fed into the device. To optimize performance, the amplifier power supply should use the shortest possible lead.

Differential signals should be used whenever possible. The differential input has high noise suppression, which allows the differential receiver to suppress common mode noise on the positive and negative signal lines. In order to make full use of the advantages of differential amplifiers, it is very important to maintain the same length of the differential signal line pair during wiring, so that they have the same impedance, and the two are as close as possible to make the coupling noise the same. The amplifier's differential input is very effective in suppressing noise from the system's digital circuits.

Grounding is essential for audio circuits, and whether it can meet the performance requirements of audio systems. Unreasonable grounding leads to large signal distortion, high noise, strong interference, and reduced RF suppression capability. It is difficult for designers to invest a lot of time in the ground layout, but careful ground layout can avoid many difficult problems.

There are two important considerations for grounding in any system: first, it is the current return path through the device, and second, the reference potential of digital and analog circuits. It seems simple to ensure that the voltage at any point on the ground is the same, but it is actually impossible. All leads have impedance, as long as there is current flowing through the ground wire, there will be a corresponding voltage drop. Circuit leads also form an inductance, which means that current flows from the battery to the load, and then back to the battery, there is a certain inductance in the entire current channel. When working at a higher frequency, the inductance will increase the ground impedance.

Designing the best ground layout for a particular system is not simple. Here are the general rules that apply to all systems. To create a continuous ground plane for digital circuits, digital current returns through the signal path, and the area of ​​the 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 with as few breakpoints as possible. Breakpoints in the formation, including vias, cause ground currents to flow through larger loops, thus producing greater radiation and noise. Ensure the ground current isolation. The ground current of digital circuits and analog circuits should be kept isolated to prevent the interference of digital currents on analog circuits. In order to achieve this goal, the components need to be arranged 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 naturally isolated. It is best to have separate PCB layers for analog circuits. The analog circuit uses a star ground. A star ground considers one point of the PCB as a common ground point, and only this point is regarded as the ground potential. In cellular phones, the battery ground terminal is usually used as a star ground point, and the current flowing into the ground plane It will not disappear automatically, and all ground current will be imported to this ground point.

Audio amplifiers absorb considerable current, which affects the reference ground of the circuit itself and other systems. In order to solve this problem, it is best to provide a dedicated return loop to bridge the power ground of the amplifier and the ground loop of the headphone jack. Note that these dedicated loops do not cross digital signal lines because they will hinder the digital return current. Maximizing the role of bypass capacitors Almost all devices require a bypass capacitor to provide transient current that the power supply cannot. These capacitors need to be placed as close as possible to the power supply pin to reduce the parasitic inductance between the capacitor and the device pin. The inductance will reduce the role of the bypass capacitor. In addition, the capacitor must have a low ground impedance, thereby reducing the high-frequency impedance of the capacitor. The ground pin of the capacitor should be directly connected to the ground plane, and do not go to ground after passing through a lead. When all unused PCB areas are covered with copper, and two copper foils are close to each other as a ground layer, a small coupling capacitor will be formed between them. Place a ground wire near the signal line, and high-frequency noise on the signal line will be short-circuited to the ground layer. Grounding example Figure 3 is an example of a circuit board with a good grounding distribution. First, note that the bottom of the PCB is the digital area, and the top is the analog area. The only signal line that crosses the boundary of the area is the I²C control signal. These signal lines have a direct return path to ensure that the digital signal only exists in the digital area and there is no digital ground current caused by the division of the stratum. It should also be noted that most ground planes are continuous. Even if there are some interruptions in the digital area, the distance between them is far enough to ensure the smoothness of the current channel.

In this example, the star ground point is in the upper left corner of the top layer of the PCB. The breakpoints of the analog ground ensure that the current of the Class D amplifier and charge pump returns directly to the star ground and does not interfere with other analog ground. In addition, it should be noted that the headphone jack has a lead to directly return the headphone ground current to the star ground point.

Figure 3. Examples of silkscreen and strata
Figure 3. Examples of silkscreen and strata

Conclusion A well-designed PCB is a time-consuming and challenging task, but this investment is indeed worthwhile. A good PCB layout helps reduce system noise, improve RF signal suppression, and reduce signal distortion. A good PCB design will also improve EMI performance and may require less shielding.

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