Cellular telephone is the ultimate challenge faced by PCB layout engineers. Modern cellular telephone includes almost all portable subsystems, and each subsystem has conflicting requirements. A PCB with perfect design must give full play to the performance advantages of each interconnected equipment and avoid mutual interference between subsystems. Therefore, for conflicting requirements, we have to compromise the performance of each subsystem. Although the audio function of cellular telephone continues to increase, little attention has been paid to the PCB layout of audio circuit.
Of course, the first step in any PCB design is to select the PCB placement of each component. We call this step “layout consideration”. Careful component layout can reduce signal interconnection, ground wire division, noise coupling and occupied area of circuit board.
Cellular telephone includes digital circuit and analog circuit. In order to prevent the interference of digital noise on sensitive analog circuit, they must be separated. Dividing the PCB into digital area and analog area helps to improve the layout of such circuits.
Although the RF part of cellular telephone is usually treated as analog circuit, a common problem that needs to be paid attention to in many designs is RF noise. It is necessary to prevent RF noise from coupling to audio circuit and generating audible noise after demodulation. In order to solve this problem, RF circuit and audio circuit need to be separated as far as possible.
After the PCB is divided into analog, digital and RF areas, the component layout of the analog part needs to be considered. The component layout shall ensure the shortest path of audio signal. The audio amplifier shall be placed as close to the headphone jack and speaker as possible to minimize the EMI radiation of class D audio amplifier and the coupling noise of headphone signal. The analog audio signal source must be as close to the input of the audio amplifier as possible to minimize the input coupling noise. All input leads are an antenna for RF signals. Shortening the lead length helps to reduce the antenna radiation effect of the corresponding frequency band.
Example of element arrangement
Figure 1 shows an unreasonable layout of audio components. The more serious problem is that the audio amplifier is too far away from the audio signal source. Because the lead passes near the noisy digital circuit and switching circuit, the probability of noise coupling is increased. Longer leads also enhance the RF antenna effect. Cellular phones use GSM technology, and these antennas can pick up GSM transmitted signals and feed them into audio amplifiers. Almost all amplifiers can demodulate 217hz envelope to a certain extent and produce noise at the output. When it is bad, the noise may completely drown the audio signal. Shortening the length of the input lead can effectively reduce the noise coupled to the audio amplifier.
There is another problem with the component layout shown in Figure 1: the operational amplifier is too far away from the speaker and headphone socket. If the audio amplifier is a class D amplifier, the longer headphone lead will increase the EMI radiation of the amplifier. This radiation may cause the equipment 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 can be obtained by the load.
Finally, because the components are so dispersed, 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.
Fig. 2 shows the arrangement of the same elements in Fig. 1. The rearranged elements can make more effective use of space and shorten the lead length. Note that all audio circuits are distributed near the headphone jack and speaker, 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. This design can comprehensively reduce system noise, reduce RF interference, and simple wiring.
Figure 2: a reasonable layout scheme of cellular telephone.
The influence of signal path on audio output noise and distortion is very limited, that is, the compromise measures to ensure performance are very limited.
Loudspeaker amplifiers are usually powered directly by batteries and require considerable current. If long and thin power leads are used, the power ripple will increase. Compared with the short and wide lead, the long and thin lead has larger impedance, and the current change generated by the lead impedance will be transformed into voltage change and fed into the device. In order 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, so that the differential receiver can suppress the common mode noise on the positive and negative signal lines. In order to make full use of the advantages of differential amplifier, it is very important to keep the same length of differential signal line pair during wiring to make them have the same impedance, and they should be close to each other as much as possible to make their coupling noise the same. The differential input of the amplifier is very effective in suppressing noise from the digital circuit of the system.
For the audio circuit, grounding is very important to meet the performance requirements of the audio system. Unreasonable ground wire will lead to large signal distortion, high noise and strong interference, and reduce RF suppression ability. It is difficult for designers to invest a lot of time in ground wire layout, but careful ground wire layout can avoid many thorny problems.
Grounding in any system has two important considerations: 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 the same voltage at any point of the ground wire, but it is impossible in practice. All leads have impedance. As long as there is current flowing through the ground wire, there will be a corresponding voltage drop. The circuit leads also form inductance, which means that the current flows from the battery to the load and then returns to the battery. There is a certain inductance on the whole current channel. When working at a higher frequency, the inductance will increase the ground impedance.
It is not easy to design the best ground wire layout for a specific system. Here are the general rules applicable to all systems.
1. Establish a continuous ground plane for digital circuits
The digital current of the stratum returns through the signal path, and the area of the loop shall be kept to a minimum to reduce antenna effect and parasitic inductance. Ensure that all digital signal leads have corresponding grounding 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 vias, will cause the ground current to flow through a larger loop, resulting in greater radiation and noise.
2. Ensure ground current isolation
The ground current of digital circuit and analog circuit shall be kept isolated to prevent the interference of digital current to analog circuit. In order to achieve this goal, the components need to be arranged correctly. If the analog circuit is arranged in one area of the PCB and the digital circuit is arranged in another area, the ground current will be naturally isolated. It is better to make the analog circuit have independent PCB layering.
3. Analog circuit adopts star grounding
Star grounding regards a point of PCB as a common grounding point, and only this point is regarded as the ground potential. In cellular phones, the battery ground terminal is usually regarded as a star grounding point. The current flowing into the ground plane will not disappear automatically, and all ground currents will flow into this grounding point.
The audio amplifier absorbs considerable current, which will affect the reference ground of the circuit itself and other systems. In order to solve this problem, it is best to provide a special return loop to bridge the power ground of the amplifier and the ground loop of the headphone jack. Note that these dedicated circuits do not cross the digital signal line because they will hinder the digital return current.
4. Maximize bypass capacitance
Almost all devices require a bypass capacitor to provide transient current that the power supply cannot provide. These capacitors should be placed as close to the power supply pin as possible to reduce the parasitic inductance between the capacitor and the device pin, which will reduce the role of the bypass capacitor. In addition, the capacitor must have a low grounding impedance, so as to reduce the high-frequency impedance of the capacitor. The grounding pin of the capacitor shall be directly connected to the grounding layer, and shall not be grounded after passing through a section of lead.
5. Cover all unused PCB areas with copper as the stratum
When two copper foils are close to each other, a small coupling capacitance will be formed between them. Ground wire shall be arranged near the signal line, and the high-frequency noise on the signal line will be short circuited to the stratum.
Figure 3 is an example of a circuit board with good grounding distribution. Firstly, it should be noted that the bottom of the PCB is a digital area and the top is an analog area. The only signal lines crossing the boundary of the area are I? C control signals. 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 earth current caused by stratum segmentation. 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 breakpoint of the analog layer ensures that the current of the class D amplifier and charge pump returns directly to the star ground point without interfering with other analog layers. Also note that the headphone jack has a lead that directly returns the headphone ground current to the star ground point.
Figure 3: example of silk screen layer and stratum.
Designing a well-designed PCB is a time-consuming and challenging task, but the investment is really worth it. Good PCB layout helps to reduce system noise, improve RF signal suppression ability and reduce signal distortion. A good PCB design will also improve EMI performance and may require less shielding.
If the PCB is unreasonable, problems that could have been avoided will occur in the test phase. At this time, it may be too late to take measures, it is difficult to solve the problems faced, it needs to invest more time and energy, and sometimes add additional components to increase the cost and complexity of the system.