PCB Tech - ​PCB design: internal noise caused by miniaturized design

In the PCB design of general products, most of the component layouts are determined first, and then the wiring is connected. The interference problem is first improved at the component location, and then the product is improved from the details of the wiring.

With the current design architecture of mobile phones and tablet computers, the volume of products has been continuously compressed and thinner, but the additional functional items have not decreased but increased, and even the core processor component operating clock continues to rise. The current applications of this type of mobile device products The mobile processor has a clock frequency of 1~1.5GHz. If the large number of high-frequency components in the device are not handled well in the layout of the circuit board components, the poor design may affect the presentation of multimedia applications such as video and audio...

Taking mobile phones and smart phones as examples, the available space for internal mechanism design can be said to be extremely narrow. Not only are the components or subsystems such as batteries, panels, backlight modules, camera modules, logic circuit carrier boards...

Extremely stacked and high-density settings, coupled with the high performance and functionality requirements of products, also make the design of such mobile devices more complicated. Developers must not only solve the actual connection of various components and subsystems. /Operation, it is also necessary to solve the problem of possible interference between the systems.

The signal quality of the audio circuit is the key to the operating experience, so special attention should be paid to the circuit layout.

The PCB space available for mobile phones is quite small, and a one-chip design is a cost-saving approach. How to distinguish between different subsystems and reduce noise interference is the key to the design.

Especially in the layout design of printed circuit boards, it can be said to be the most severe challenge in mobile phone design. Various subsystems in the mobile phone may have conflicting design requirements. For example, the wireless module requires the best The antenna field type and the best wireless connection transmission performance, while the digital logic computing core system requires the most stable computing environment. When the two systems are integrated into an extremely compact device at the same time, the wireless radio frequency must be optimized for transmission., Digital logic circuits must operate stably in an environment that can isolate external noise. How to make a single carrier board can exist in two systems during the design, and perform their duties with each other without interfering with each other, has become the key task of product development.

A well-designed printed circuit board must be able to provide optimized operating conditions and environment for each component, functional block or module, and at the same time, it must maintain the mutual interference between the various subsystems! However, the reality is that conflicting design requirements of various subsystems must use engineering means to implement some design compromises, or adopt reinforcement measures such as adding metal barriers. However, such component layout processing or reinforcement measures will also cause PCB The size of the carrier board will inevitably increase, which in turn conflicts with the design goals of light, thin and short products.

This type of radio frequency signal interference problem is actually relatively easy to improve when faced with the impact of digital logic circuits, because the digital logic circuit subsystem is either 0 or 1 in the message processing part. The digital signal processing can ignore slight radio frequency interference. On the contrary, for multimedia applications (such as video playback, MP3 music appreciation), when radio frequency interferes with audio or causes ripple interference in video, the user experience will be quite bad.

With the normal development process, the first thing before PCB board design is to deal with the component layout, that is, to arrange where the components should be placed on the PCB. In this stage of work, it is necessary to consider the best wiring benefits for the components (the shortest distance or the shortest distance). PCB space saving layout), but while simplifying the signal routing, it is also necessary to consider the setting of the ground plane to minimize the possible problems of noise.

As for the layout of components, in most situations, functional subsystems can be divided into different blocks for layout. RF components with interference concerns should be placed as close to the antenna of the device as possible, such as the corners of the carrier board, and the RF function can be shielded by metal. The core system of digital logic, because it also operates at high frequency, is mostly located in the center of the PCB carrier. On the one hand, the heat dissipation module of the processor can simultaneously achieve the effect of dissipating heat for the overall carrier. In other words, placing the core logic circuit in the center of the carrier board also facilitates the functional layout. The audio loop that most affects the user's look and feel has become a key issue for carrier board development. In particular, it is necessary to accumulate a lot of design experience to optimize the layout of the wiring design and avoid interference.

With a hybrid system circuit that combines communication, network, and digital operations like mobile phones, how to effectively separate analog and digital circuits so that the two types of operating systems can be separated without interfering with each other, there are many design methods available. The common practice is to simply divide different circuits into different carrier boards, and use cable to connect the key contact information lines between the carrier boards to achieve an effective cutting and separation design of the functional system.

However, due to cost considerations, the current form of multi-carrier design will focus on the smallest carrier board to achieve maximum functional integration. This will encounter greater challenges for separating digital and analog circuits. The design direction can divide the overall carrier board into The digital block and the analog block are separated from each other by the type of circuit, which can achieve a design effect similar to the separation of the carrier board. In addition, although the radio frequency circuit is also a kind of analog circuit, it is different from the analog processing such as audio after all, because the radio frequency signal will be coupled to affect the audio circuit, causing interference noise in the audio effect, and the radio frequency circuit, wireless network circuit and audio circuit area The clearer the separation and the further the distance between the subsystems, the better, which can reduce the problem of equipment interfering with audio.

The analog circuit is not more complicated than the digital circuit, and its design complexity will be higher, and more design experience is needed to improve the function. For example, the audio amplifier chip can be placed closer to the audio connector, so that the output signal avoids PCB circuit loss as much as possible, and the sound output is more pure. At the same time, most of these mobile devices use high-performance Class D audio amplifier circuits. The amplifier circuit must be able to consider the electromagnetic interference (EMI) problem in the design.

After separating the printed circuit board into analog, digital, and radio frequency zones, the component arrangement of the analog part must be selected. At this time, the principle of the shortest audio signal path must be complied with, and the audio amplifier must be as close as possible to the headphone jack and speaker. It can effectively minimize the electromagnetic interference (EMI) emitted by the class D speaker amplifier, and at the same time, it must deal with the problem of suppressing the additional noise generated by the headphone signal, and then effectively shorten the circuit distance of the audio transmission, so that the audio performance of the product is better. Going to perfection.