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Cellular phones are the ultimate challenge for PCB layout engineers. Modern cellular phones cover almost all portable subsystems, each with conflicting requirements. A well-designed PCB must take advantage of the performance advantages of each interconnected device while avoiding mutual interference between subsystems. Therefore, for each conflicting requirement, each subsystem performance must be compromised. While the audio capabilities of cellular phones continue to increase, little attention has been paid to the layout of audio circuit PCBs.Component layout
The first step in any PCB design is of course to choose the PCB placement of each component. We call this step "layout consideration." Careful component placement reduces signal interconnect, ground segmentation, noise coupling, and board space.
Cellular phones contain digital and analog circuits that must be separated in order to prevent digital noise from interfering with sensitive analog circuits. Dividing the PCB into digital and analog areas helps to improve this type of circuit layout.
Although the RF portion of a cellular telephone is typically treated as an analog circuit, a common problem that needs to be addressed in many designs is RF noise, which is required to prevent RF noise from being coupled to the audio circuit and demodulating to produce audible noise. In order to solve this problem, it is necessary to separate the RF circuit and the audio circuit as much as possible.
After dividing the PCB into analog, digital, and RF regions, you need to consider the component placement of the analog portion. Component layout To minimize the path of the audio signal, the audio amplifier should be placed as close as possible to the headphone jack and speaker, minimizing the EMI emissions of the Class D audio amplifier and minimizing the coupling noise of the headphone signal. The analog audio source must be as close as possible to the input of the audio amplifier to minimize input coupling noise. All input leads are an antenna for the RF signal, and shortening the lead length helps to reduce the antenna radiation effect in the corresponding 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 source, increasing the probability of noise coupling because the leads pass through the noisy digital circuitry and the switching circuitry. Longer leads also enhance the RF antenna effect. Cellular phones use GSM technology, which picks up GSM transmit signals and feeds them into an audio amplifier. Almost all amplifiers can demodulate the 217 Hz envelope to some degree, producing noise at the output. In bad cases, noise can completely drown the audio signal, shortening the length of the input lead can effectively reduce the noise coupled to the audio amplifier.
| Figure 1: Unreasonable component layout. |
Another problem with the component layout shown in Figure 1 is that the op amp is too far away from the speaker and headphone jack. If the audio amplifier is a Class D amplifier, a longer headphone lead will increase the EMI emissions of the amplifier. This type of radiation can cause the device to fail to pass 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 dispersed, the wires between the components will have to pass through other subsystems. This not only increases the difficulty of wiring the audio part, but also increases the difficulty of wiring other subsystems.
Figure 2 shows the arrangement of the same components of Figure 1, with the rearranged components making more efficient use of space and shortening lead lengths. 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, and no audio circuitry is placed in other areas of the PCB. This design reduces overall system noise, reduces RF interference, and is easy to route.
| Figure 2: A reasonable layout of a cellular phone. |
signal path
The effect of the signal path on audio output noise and distortion is very limited, which means that the compromises required to ensure performance are limited.
Speaker amplifiers are typically powered directly from the battery and require considerable current. If you use long, thin power leads, it will increase the power supply ripple. Compared to short and wide leads, the long and thin lead impedance is large, and the current change caused by the lead impedance is converted into a voltage change and fed into the device. To optimize performance, the amplifier power supply should use the shortest possible leads.
Differential signals should be used whenever possible. The differential input has high noise rejection, allowing the differential receiver to reject common mode noise on the positive and negative signal lines. To take full advantage of the differential amplifier, it is important to maintain the same length of the differential signal pair when routing, so that they have the same impedance, as close as possible to each other to make the coupling noise the same. The differential input of the amplifier is very effective in suppressing noise from system digital circuits.
Ground
For audio circuits, grounding is critical to meeting the performance requirements of an audio system. Unreasonable grounding can result in large signal distortion, high noise, strong interference, and reduced RF rejection. It is difficult for designers to invest a lot of time in the ground layout, but careful grounding 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, followed by the reference potential of the digital and analog circuits. It is seemingly simple to ensure that the voltage at any point of the ground line is the same, but it is impossible. All leads have an impedance that produces a corresponding voltage drop as long as current is flowing through the ground. The circuit leads also form an inductance, which means that current flows from the battery to the load and then back to the battery, with some inductance across the current path. When operating at higher frequencies, the inductance will increase the ground impedance.
Designing the optimal ground layout for a particular system is not straightforward. Here are general rules that apply to all systems.
1. Establish a continuous ground plane for the digital circuit:
The digital current of the formation is returned 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, resulting in greater radiation and noise.
2. Guaranteed ground current isolation:
The ground current of the digital circuit and the analog circuit should be kept isolated to prevent the digital current from interfering with the analog circuit. In order to achieve this goal, the components need to be properly aligned. If the analog circuit is placed in one area of ​​the PCB and the digital circuit is placed in another area, the ground current is naturally isolated. It is best to have the analog circuit have a separate PCB layering.
3. The analog circuit uses star ground:
Star grounding is to regard a point of PCB as a common grounding point, and only this point is regarded as ground potential. In a cellular phone, the battery ground is usually used as a star grounding point, and the current flowing into the ground plane does not disappear automatically. Ground current will flow to this ground point.
The audio amplifier absorbs a significant amount of current, which affects the reference ground of the circuit itself and the reference ground of other systems. In order to solve this problem, it is preferable to provide a dedicated return loop bridge amplifier power ground and a ground loop of the headphone jack. Note that these dedicated loops do not traverse the digital signal lines as they block the digital return current.
4. Maximize the effect of bypass capacitors:
Almost all devices require a bypass capacitor to provide transient current that the power supply cannot provide. These capacitors should be placed as close as possible to the power supply pins to reduce parasitic inductance between the capacitor and the device pins, which reduces the effects of the bypass capacitors. In addition, the capacitor must have a low ground impedance to reduce the high frequency impedance of the capacitor. The grounding pin of the capacitor should be directly connected to the layer. Do not ground through a length of lead.
5. Cover all unused PCB areas as a layer:
When the two copper foils are close to each other, a small coupling capacitance is formed between them. A ground wire is placed near the signal line, and high-frequency noise on the signal line is short-circuited to the ground.
Grounding instance
Figure 3 shows an example of a board with a good grounding distribution. First, note that the bottom of the PCB is a digital area and the top is a simulated area. The only signal lines that cross the boundary of the area are the I2C control signals, which have a direct return path, ensuring that the digital signal is only present in the digital area, without the digital ground current caused by the formation split. Also note that most of the ground planes are continuous, even if there are some interruptions in the digital area, but the distance between them is far enough to ensure smooth current channels.
In this example, the star ground point is in the upper left corner of the top layer of the PCB. The breakpoints in the analog formation ensure that the currents of the Class D amplifier and charge pump return directly to the star ground point without interfering with other analog layers. In addition, it is also necessary to note that the headphone jack has a lead that directly returns the headphone current to the star ground point.
| Figure 3: Example of silk screen and formation. |
Summary of this article:
A well-designed PCB is time consuming and challenging, but it's worth the investment. A good PCB layout helps reduce system noise, improve RF signal rejection, and reduce signal distortion. A good PCB design will also improve EMI performance and may require less shielding.
If the PCB is unreasonable, there will be problems that could have been avoided during the testing phase. At this time, if measures are taken, it may be too late, it is difficult to solve the problems faced, it takes more time, more effort, and sometimes additional components are added to increase system cost and complexity.
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