Dealing with noise and electromagnetic interference (EMI) is an inevitable challenge in any high-speed digital design. Digital signal processor (DSP)-based systems that handle audio-video and communication signals can be particularly vulnerable to these disruptions. The designer should know in advance the potential sources of noise and radiation, and design upfront to minimize these disruptions. Smart planning can save considerable time and rework in the debugging stage, thus saving overall time and cost.

Today's fastest DSPs run at internal clock rates in the gigahertz range, while transmitting and receiving signals at frequencies measuring in hundreds of megahertz. These fast-switching signals can generate considerable noise and radiation which degrade system performance and creates high levels of EMI. DSP systems are also becoming more complex, with audio and video interfaces, LCDs, wireless communications, Ethernet and USB controllers, power supplies, oscillators, drive controls and other circuitry—all of which can generate noise or be affected by interference from neighboring components. Audio-video systems are particularly vulnerable to these problems, since noise can cause subtle performance degradation that might not be apparent with discrete data.

It is essential to address noise and radiation problems from the very beginning of the design. Many new designs fail first-time electromagnetic compliance testing for Federal Communication Commission (FCC) certification. Investing a little time in low-noise and low-radiation design methods early in the design can minimize late-stage redesign costs and delays in the product shipment date. From the start of the design, developers should aim for:

• robust power sources with low switching noise under dynamic loading conditions,
• minimum crosstalk between high-speed signal traces,
• high- and low-frequency decoupling, and
• good signal integrity with minimum transmission line effects.

By working to achieve these goals, developers can avoid the pitfalls of noise and EMI.

The impact of noise
Minimizing noise is one of the most important design criteria for high-speed DSP systems. Excessive noise from any source can result in random logic and phase-locked loop (PLL) failures that reduce reliability. It can also result in radiation which may adversely impact FCC compliance. Moreover, debugging systems with excessive noise is extremely difficult; cleaning up the noise—if it can be cleaned up at all—may require multiple spins of the board.

In audio-video systems, even relatively small levels of interference can have a noticeable impact on the performance of the final product. In audio capture and playback, for instance, performance depends on the quality of the audio codec being used, the power supply noise, the PCB layout, and the amount of crosstalk between the neighboring circuitry. Also, the stability of the sampling clock needs to be very good to prevent unwanted sounds such pops and clicks during playback and capture.

In video design, a major challenge is eliminating artifacts such as color distortion, 60-Hz hum, and audio beat. Such artifacts can be detrimental for systems that require the highest video quality, such as security applications. (For more information regarding de-interlacing to reduce artifacts: De-Interlacing and YUV 4:2:2 to 4:2:0 Conversion on DM6446 Using the Resizer) These issues are generally related to improper video board design. Examples include:

• power supply noise propagating to the video DAC output,
• audio playback causing transients in the power supply,
• and the audio section coupling with high-impedance traces in the video section.

Some typical sources of video problems include:

• overshoots and undershoots on synching and pixel clocks,
• codec and pixel clock jitter that affects color,
• image distortion from lack of termination resistors, and
• flicker due to poor isolation of audio and video.

Audio-video applications may also suffer from noise problems that are common to all communications systems that must maintain a very low bit error rate. In such systems, radiation not only generates EMI but can also jam other communication channels, causing false channel detection. Applying proper board design techniques, shielding, and isolation of RF and mixed analog/digital signals can address these challenges.

Controlling noise
High-speed DSP systems have many potential sources of switching noise, including

• crosstalk between signal traces,
• reflections due to transmission line effects,
• voltage droop from inadequate decoupling capacitors,
• high-inductance power supply traces, oscillator and PLL circuits,
• switching power supplies,
• large capacitive loads from linear regulator instability, and
• disk drives.

These problems result from both electrical coupling and electromagnetic coupling. Electrical coupling is due to the parasitic capacitances and mutual inductances of adjacent signals and circuits. Electromagnetic coupling results when signal traces effectively become antennas. If the radiation is strong enough, it can also lead to EMI that may be disruptive to other systems.

While noise in high-speed DSP systems cannot be completely eliminated, it can be minimized. Electronic components have internal sources of noise, so it is important to consider device characteristics carefully and select the right device. In addition to device selection, two general types of techniques—printed circuit board (PCB) layout and return path decoupling—can also help in controlling system noise. A good PCB layout reduces the likelihood of noise paths occurring. It also minimizes radiation, which can propagate to traces and current return loop areas. Decoupling prevents noise from affecting adjacent circuits. It is best accomplished by filtering the noise at the source—though it is also possible to make the adjacent circuits insensitive to noise or to eliminate the noise coupling channel.