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, 60Hz 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.
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. We'll now discuss several techniques that can be applied to a number of common problems with system noise and EMI.
Keeping current return paths short
Low-speed signal current returns on the path with the least resistance—the shortest path back to the source. In contrast, high-speed signal current returns on a path with the least inductance: the smallest loop area underneath the signal trace, as indicated in Figure 1.
Figure 1. High-Speed Versus Low-Speed Current Loops
Hence, one goal of high-speed design is to provide least-inductance paths for the signal current. This is accomplished with the power plane and ground plane. A power plane minimizes parasitic inductances by creating natural decoupling capacitors at high frequencies. A ground plane creates a shielding effect, known as an image plane, which provides the shortest current return paths.
An effective PCB layout places the power plane and ground plane close together. This yields higher plane capacitance and lower impedance to help lower noise and radiation. Critical signals are best routed next to the ground plane for shielding. Routing next to the power plane is the next best option.
In a high-speed video system, the goal of keeping return paths short means that video ground should not be isolated. An analog audio ground, which must be isolated, should nevertheless be shorted to a digital ground at the data entry point, as Figure 2 shows.
Figure 2. Audio Ground Isolation
Power isolation and PLLs
Supplying power is one of the most challenging aspects of controlling noise and radiation. The dynamic load switching conditions are complex, and include factors such as
• entering and exiting low-power modes,
• excessive inrush current due to bus contention and capacitor charging,
• large voltage droop due to inadequate decoupling and layout, and
• oscillations that overload the linear regulator output.
Figure 3 illustrates an example of designing for current return using power supply decoupling. The decoupling capacitor in the example circuit is located close to the DSP. Without the decoupling, there would be a larger dynamic current return path. This would increase the power supply voltage droop—and hence electromagnetic radiation.
Figure 3. Power Supply Decoupling
Power isolation is important for powering PLLs, which are highly sensitive to noise and must guarantee low jitter for a stable system. You also need to consider whether the PLL is analog or digital. Analog PLLs tend to be less noise-sensitive than digital PLLs.
As shown in Figure 4, a Pi filter with a low cut-off frequency is often used to isolate the PLLs from the remaining high-speed circuits in the system. A better technique is to use a low drop-out (LDO) voltage regulator and generate the PLL voltage separately, as Figure 5 shows. This technique adds system cost but guarantees low noise and good PLL performance.
Figure 4. PLL Power Supply Isolation
Figure 5. PLL Power Supply Isolation Using LDO
Interference between signals, or crosstalk, can be transmitted through electromagnetic radiation between traces. It can also be created electrically by unwanted signals that propagate on the power and ground planes. Crosstalk is inversely proportional to the square of the distance between traces. Hence, spacing single-ended signals at least twice the trace width apart minimizes interference. For differential signals such as Ethernet and USB, the spacing needs to be the same as the trace width in order to have matched differential impedance. Critical signals can be shielded with ground and power planes or have an added ground wire in parallel for any necessary rework.
Signals also generate high-frequency harmonics that can cause crosstalk. Since radiated energy is directly proportional to the rise and fall times of the signal, slower rise (or fall) times create less interference. Figure 6 shows an example of video interference that can result from internal clock radiation. The third harmonic of an 18.432 MHz audio clock lands in the North American Channel 2 frequency spectrum, creating the interference pattern on the left side. Slowing down the rise and fall times by adding a series resistor on the audio clock trace eliminates the pattern, as shown on the right. The designer needs to understand timing margins in order reduce rising and falling edges within the limits of the system.
Figure 6. Resolving Audio-Video Crosstalk
Transmission line effects
Related to crosstalk are transmission line effects, which occur when high-speed traces become transmitters that create radiation interference. Generally, traces transmit when the rise time of signals is less than twice the propagation delay. One implication of this rule of thumb is to keep traces as short as possible to minimize propagation delay. Another is that signal termination is often required to slow the rise time, minimizing the overshoots and undershoots caused by reflections. Figure 7 illustrates how parallel termination corrects voltage levels and minimizes termination line effects.
Figure 7. Termination to Minimize Transmission Line Effects
Designers may wonder whether it is important to add external termination resistors when these are already integrated internally. In addition to controlling transmission line effects, external termination is useful because it allows fine-tuning for signal integrity. The DSP cannot be perfectly matched for board impedance—and termination resistors reduce source current, as well as rise and fall times.
Like external termination resistors, external pull-up and pull-down resistors are also important. While internal pull-ups and pull-downs are sufficient for unconnected pins, fast switching noise can propagate through and falsely trigger internal logic on connected pins. To prevent this problem, add external pull-ups and pull-downs for critical signals.
Radiation that extends beyond the system is considered EMI and may cause the design to fail FCC certification. Two types of radiation are possible: common mode, where the transmitter is a straight signal trace or cable, and differential mode, where a signal and its return path form a large current loop. Common mode radiation falls off as frequencies rise, whereas differential mode radiation increases with frequency up to a point of saturation. The two modes are illustrated in Figures 8 and 9.
Figure 8. Common Mode Radiation
Figure 9. Differential Mode Radiation
How the EMI is handled depends on its source. For common mode radiation, if EMI comes from an external cable (as shown in Figure 8, for instance), a choke is usually added to the cable. If internal transmission line effects are causing EMI, then line termination is usually employed—although a ground trace between the signal traces also helps reduce radiation. Another possible solution is to reduce the trace length to less than the signal wavelength (or inverse of the signal frequency) divided by 20. For example, to avoid transmission, a trace driven by a 500 MHz signal should be reduced to less than 1.18 inches.
In differential mode transmitters, radiated energy is a function of current, loop area and frequency. Possible methods of minimizing radiation include terminating the signal to lower the source current, providing return paths that reduce the loop areas with proper current return paths, or running the circuit at a lower frequency.
When calculating decoupling transistors, also consider dynamic current. High-speed currents may be changing, with the transients causing radiation. In addition, vary capacitor values to prevent capacitor self-resonance from limiting the frequency range. PCB layers provide a good solution, since power planes create natural decoupling at high frequencies, and ground planes provide the shortest return paths. Isolate high-speed signals and route them away from other signals. Do not isolate ground planes, if possible. Although noise and radiation are caused by complex, unwanted features of system design, straightforward techniques such as the above can bring both under control.
High-speed DSP video systems have many potential sources of noise and EMI that can disrupt system operation or cause the design to fail FCC certification. Fortunately, planning and insight into the sources of noise and radiation can help a system designer minimize these problems. A little work upfront helps save significant debugging time and trouble later on. PCB layout and return path decoupling are two general types of techniques available to the designer for limiting system noise and EMI. Armed with these techniques, DSP video designers can deal with system noise and radiation effectively.
Detailed technical information about the topics discussed in this paper is available in TI's High-Speed DSP Systems Design Reference Guide (PDF). Additional discussion of the issues associated with noise and EMI can be found in TI's DSP application notes and technical training for high-speed DSP design.
Dr. Thanh Tran is a senior member of the technical staff at Texas Instruments. He leads a team developing reference designs and frameworks for high speed DSP systems.