For PCB layout engineers, today’s mobile phone presents the ultimate challenge. Modern mobile phones contain almost all the subsystems that can be found in portable devices, such as a variety of RF modules (including cellular and short-range wireless transmission); Audio and video subsystem; Special application processor and I / O layout for more and more applications, and each subsystem has conflicting requirements.
In order to integrate so many complex subsystems in such a small space, we need to consider all kinds of practical situations. In addition to the possible interference between the same RF subsystems, the mutual interference and EMI problems between different subsystems, which may be caused by their own operation or wiring, all test the professional ability of mobile phone PCB engineers.
A well-designed circuit board must be able to maximize the performance of each component mounted on it and avoid interference between different systems. Because if there is a conflict between the subsystems, the result will inevitably lead to a decline in performance.
Today, despite the increasing functionality of audio in mobile phones, in circuit board design, a lot of attention is still focused on the RF subsystem, and the audio circuit is often the least concerned. However, the quality of audio, especially the high quality of fax, has become one of the key factors that affect whether a high-end mobile phone can be accepted by the market quickly. This article provides some suggestions to ensure a well laid out circuit board without sacrificing audio quality.
1 recommended practice
*Carefully consider the layout planning. The ideal layout should divide different types of circuits into different areas to minimize the interference.
*Use differential signals whenever possible. The audio module with differential input can suppress noise.
*The grounding current is isolated to avoid the digital current increasing the noise of analog circuit.
*The analog circuit uses star grounding. The current consumption of audio power amplifiers is usually large, which may adversely affect their own grounding or other reference grounding.
*Turn the unused area on the circuit board into a ground plane. By performing ground flood near the signal trace, the unnecessary high-frequency energy in the signal line can be capacitively coupled to the ground.
2. Not recommended
*Use hybrid circuits on circuit boards. Although the RF region of a mobile phone is generally considered to be analog, the noise coupled from the RF region to the audio circuit may be demodulated into audible noise.
*The analog audio signal wiring on the circuit board is too long. Too long analog audio traces may absorb noise from digital and RF circuits.
*Forget the importance of the grounding circuit. The system with poor grounding is likely to have serious distortion, noise, crosstalk and low RF immunity.
*Interrupt the natural circuit of digital current. The loop area of this path is the smallest, which can minimize the influence of antenna and induction.
Figure (text): * when laying out mobile phone circuit boards, careful consideration should be given to layout planning. Ideally, different types of circuits should be divided into different areas to avoid interference.