Handling noise and electromagnetic interference (EMI) is an inevitable challenge in any high-speed digital circuit design. Digital signal processing (DSP) systems that process audio, video, and communication signals are particularly vulnerable to such interference, and potential noise and sources of interference should be identified early in the design, and early steps should be taken to minimize these interferences. Good planning will reduce the amount of time and effort in the commissioning phase, which will save overall design time and cost.

Today, the fastest DSPs have internal clock rates of up to several gigahertz, and transmit and receive signals at frequencies up to several hundred megahertz. These high-speed switching signals will generate a lot of noise and interference, which will affect system performance and generate high levels of EMI. And DSP systems have become more complex, such as audio and video interfaces, LCD and wireless communication functions, Ethernet and USB controllers, power supplies, oscillators, drive controls, and a variety of other circuits, all of which will generate noise. Affected by adjacent components. These problems are particularly problematic in audio and video systems because noise can cause a drop in sensitive analog performance, but not as discrete data.

It is critical to address noise and interference issues from the outset of the design. Many designs have not passed the Federal Communications Commission (FCC) EMC test for the first time. If you spend some time on low-noise and low-interference design methods in earlier designs, you will reduce the redesign cost and time-to-market delays in subsequent phases. Therefore, from the very beginning of the design, the development engineer should focus on:

1. Select a power supply with low switching noise under dynamic load conditions;

2. Minimize crosstalk between high speed signal lines;

3. High frequency and low frequency decoupling;

4. Excellent signal integrity with minimal transmission line effects;

If these goals are achieved, development engineers can effectively avoid noise and EMI defects.

Noise impact and control

For high speed DSPs, noise reduction is one of the most important design criteria. Excessive noise from any noise source can cause random logic and phase-locked loops (PLLs) to fail, reducing reliability. It can also cause radiated interference that affects FCC certification testing. In addition, debugging a very noisy system is extremely difficult; therefore, eliminating noise - if it can be completely eliminated - requires a lot of effort in board design.

In audio and video systems, even small interference can have a significant impact on the performance of the final product. For example, in an audio capture and playback system, performance will depend on the quality of the audio codec used, the noise of the power supply, the quality of the PCB layout, and the amount of crosstalk between adjacent circuits. Moreover, the stability of the sampling clock is also very high to avoid unwanted noise, such as "beep" and "beep" during playback and capture.

In video systems, the main challenge is to eliminate color distortion, 60Hz "click" sounds and audio percussion sounds. These are harmful to high quality video systems, such as security monitoring applications. In fact, these problems are usually related to poor design of video boards. Specifically, the power noise is transmitted to the DAC output of the video; the audio playback causes a power transient; the audio signal is coupled to the signal line of the high impedance video circuit.

These typical video problem sources include: overshoot and undershoot of sync and pixel clocks; color codec and pixel clock jitter; image distortion due to lack of termination resistors; and flicker caused by poor audio and video isolation.

Noise interference problems that are easily generated by audio and video applications are also common for all communication systems that require very low bit error rates. In communication systems, radiation not only creates EMI problems, but also blocks other communication channels, causing spurious channel detection. These challenges can be addressed with appropriate board design techniques, shielding techniques, and isolation of RF and mixed analog/digital signals.

There are many potential sources of switching noise in high-speed DSP systems, including: crosstalk between signal lines; reflection caused by transmission line effects; voltage drop caused by improper decoupling capacitors; high inductance power lines, oscillators and phase-locked loops Circuit; switching power supply; large capacitive load caused by linear regulator instability; disk drive.

These problems are jointly produced by electrical coupling and magnetic coupling. The generation of electrical coupling is caused by the parasitic capacitance and mutual inductance of adjacent signals and circuits, which are caused by the formation of radiating antennas by adjacent signal lines. If the radiation interference is strong enough, it will lead to EMI problems that can destroy other systems.

When the noise in a high-speed DSP system cannot be completely eliminated, it should be minimized. There is noise inside the electronic components, so it is important to carefully select the device characteristics and select the appropriate device. In addition to the correct choice of devices, there are two general techniques, PCB layout and loop decoupling that can help control system noise. An excellent PCB layout will reduce the possibility of noise channel generation. In addition, radiation that can propagate to the traces and current loops is reduced, and decoupling avoids the effects of noise generated by adjacent circuits. The best way is to filter out noise from the source, but it can also make adjacent circuits noise-insensitive or noise-canceling coupling channels.

Now we discuss several techniques that can solve many common problems caused by system noise and EMI.

Keep the current loop shortest

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