PCB Design Considerations for Improving Mobile Phone Audio Performance

For PCB layout engineers, mobile phones present the ultimate challenge. Modern mobile phones contain almost all subsystems found in portable devices, and each subsystem has conflicting requirements. A well-designed circuit board must maximize the performance of each device connected to it and avoid interference between multiple systems. Inconsistent requirements of each subsystem will inevitably lead to performance degradation.

Despite the increasing amount of audio functionality in mobile phones, audio circuits often receive the least attention during the circuit board design process. The following are some suggestions to help ensure a well-laid out board without sacrificing audio quality.

should

- Carefully consider the floor plan. An ideal floor plan should divide different types of circuits into different areas. Figure 1 shows a good floor plan.

-Use differential signals as much as possible. Audio devices with differential inputs can suppress noise. Generally, a ground wire cannot be added in the middle of a differential signal. Because the most important point in the application principle of differential signals is to utilize the benefits brought by the mutual coupling between differential signals, such as magnetic flux elimination and noise immunity. If a ground wire is added in the middle, the coupling effect will be destroyed. There are two points to note when wiring differential pairs. One is that the lengths of the two lines should be as equal as possible, and the other is that the spacing between the two lines (this spacing is determined by the differential impedance) should always remain unchanged, that is, they should remain parallel. There are two ways to be parallel, one is that the two lines run on the same routing layer (side-by-side), and the other is that the two lines run on the upper and lower adjacent layers (over-under). Generally, the former side-by-side is more commonly implemented.

-Isolate the ground current to prevent the digital current from increasing the noise of the analog circuit. Basically, it is right to separate the analog/digital ground. It should be noted that the signal line should not cross the divided area as much as possible, and the return current path of the power supply and signal should not change too much. The requirement that the digital and analog signal lines cannot cross is because the return current path of the slightly faster digital signal will try to flow back to the source of the digital signal along the ground near the bottom of the line. If the digital and analog signal lines cross, the noise generated by the return current will appear in the analog circuit area.

- Use star ground for analog circuits. Audio power amplifiers typically have high current draws, which may have adverse effects on their own ground or other reference grounds.

·Convert all unused areas on the circuit board into ground planes. Implement ground coverage near signal traces to shunt excess high-frequency energy in the signal line to the ground through capacitive coupling.

Should not

-Use hybrid circuits on the board. Although the RF area of ​​a cell phone is generally considered analog, noise coupled from the RF area into the audio circuits can be demodulated into audible noise.

-The analog audio signal routing is too long. Too long analog audio traces may be interfered by noise from digital and RF circuits.

-Forgetting the importance of ground loops. Poorly grounded systems can suffer from severe distortion, noise, crosstalk, and poor RF immunity.

-Interrupts the natural return path of digital current. This path creates the smallest loop area, reducing antenna influence and inductance effects.

- Neglecting to place bypass capacitors as close as possible to the supply pins they are bypassing.