How to avoid noise and EMI problems in DSP systems-EEWORLD

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Dealing with 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 susceptible to these interferences. Potential noise and interference sources should be identified early in the design, and measures should be taken to minimize these interferences. Good planning will reduce a lot of time and iterations in the debugging phase, thereby saving overall design time and cost.
Today, the fastest DSPs have internal clock rates of up to several thousand gigahertz, and the frequencies of transmitted and received signals are up to hundreds of 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. DSP systems are also becoming more complex, such as audio and video interfaces, LCD and wireless communication functions, Ethernet and USB controllers, power supplies, oscillators, drive controls, and other various circuits, all of which will generate noise and will be affected by adjacent components. Audio and video systems are particularly prone to these problems because noise can cause sensitive analog performance degradation, but it is not obvious for discrete data.
It is crucial to address noise and interference issues from the beginning of the design. Many designs fail the Federal Communications Commission (FCC) electromagnetic compatibility testing the first time. If you spend some time on low-noise and low-interference design methods in the early stages of the design, you will reduce the cost of redesign and time-to-market delays in the later stages. Therefore, from the beginning of the design, the development engineer should focus on:
selecting power supplies with low switching noise under dynamic load conditions;
minimizing crosstalk between high-speed signal lines;
high-frequency and low-frequency decoupling;
good signal integrity with minimal transmission line effects;
if these goals are achieved, the development engineer can effectively avoid noise and EMI defects.
Impact and Control of Noise For high-speed DSP, reducing noise is one of the most important design criteria. Excessive noise from any noise source will cause random logic and phase-locked loop (PLL) failures, thereby reducing reliability. It can also cause radiated interference that affects FCC certification testing. In addition, it is extremely difficult to debug a noisy system; therefore, eliminating noise - if it can be completely eliminated - requires a lot of effort in circuit board design.
In audio and video systems, even relatively small interference can have a significant impact on the performance of the final product. For example, in audio capture and playback systems, 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. In addition, the stability of the sampling clock must be very high to avoid unwanted noise, such as "pops" and "clicks" during playback and capture.
In video systems, the main challenge is to eliminate color distortion, 60Hz "hum" and audio knocks. These are harmful to high-quality video systems, such as security surveillance applications. In fact, the above problems are usually related to poor video circuit board design. Specifically, they include: power supply noise transmitted to the video DAC output; power supply transients caused by audio playback; audio signals coupled to the high-impedance signal lines of the video circuit.
These typical sources of video problems include: overshoot and undershoot of synchronization and pixel clocks; jitter affecting color codecs and pixel clocks; image distortion caused by missing termination resistors; flicker caused by poor audio and video isolation. The
noise interference problems that audio and video applications are prone to are also common to 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 false channel detection. These challenges can be addressed by using appropriate circuit 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; reflections caused by transmission line effects; voltage drops caused by inappropriate decoupling capacitors; high-inductance power lines, oscillators and phase-locked loop circuits; switching power supplies; large capacitive loads caused by linear regulator instability; disk drives. These problems
are caused by both electrical coupling and magnetic coupling. Electrical coupling is caused by parasitic capacitance and mutual inductance of adjacent signals and circuits, while magnetic coupling is caused by adjacent signal lines forming radiating antennas. If the radiated interference is strong enough, it will cause EMI problems that can destroy other systems.
When noise in high-speed DSP systems cannot be eliminated fundamentally, it should be minimized. Electronic components have noise inside, so it is crucial to carefully select device characteristics and choose the appropriate device. In addition to the correct selection of devices, there are two general techniques, namely PCB layout and loop decoupling, which can help control system noise. An excellent PCB layout will reduce the possibility of noise channels. In addition, it also reduces the radiation that can propagate to the printed lines and current loops, and decoupling avoids the influence of noise generated by adjacent circuits. The best way is to filter out the noise from the source, but it is also possible to make adjacent circuits insensitive to noise or eliminate the coupling channel of noise.
Now we discuss several technologies that can solve many common problems caused by system noise and EMI.
Keep the current loop short. The current of slow signals returns to the source along the path with the least impedance, that is, the shortest path. High-speed signals return along the path with the least inductance: such a minimum loop area is located below the signal line, as shown in Figure 1.

Figure 1: Comparison of high-speed and low-speed signal currents.
Therefore, one of the goals of high-speed signal design is to provide the smallest inductance loop for signal current. This can be achieved by using power planes and ground planes. The power plane minimizes parasitic inductance by forming a natural high-frequency decoupling capacitor. The ground plane forms a shielding plane, the well-known mirror plane, which can provide the shortest current loop.
An effective PCB layout method is to place the power plane and ground plane together. This forms a high plate capacitance and low impedance, which helps reduce noise and radiation. For shielding, the best choice is: the key signal is best routed close to the ground plane side, while the rest should be close to the power plane side.
In high-speed video systems, the purpose of keeping the loop short means that the video ground cannot be isolated. The audio ground, which must be isolated, must never be shorted to the digital ground at the data input point, as shown in Figure 2.

Figure 2: Audio ground isolation.
Power supply isolation and phase-locked loops How to achieve the best power supply is the biggest challenge in controlling noise and radiation. The dynamic load switching environment is complex, including factors such as: entering and exiting low power modes; large transient currents caused by bus contention and capacitor charging; large voltage drops due to improper decoupling and wiring; oscillators overloading the output of linear regulators.
Figure 3 shows an example of designing a current loop that utilizes power line decoupling. The decoupling capacitors in this example are as close to the DSP as possible. Without decoupling, the dynamic current loop will be large, which will increase the drop in power supply voltage and generate electromagnetic radiation.

Figure 3: Power supply decoupling.
Power supply isolation is very important when powering a PLL because PLLs are very sensitive to noise and require very low jitter for a stable system. You also need to choose between analog and digital PLLs. Analog PLLs are less sensitive to noise than digital PLLs.

Figure 4: PLL power supply isolation.
A ∏-type filter with a low cutoff frequency as shown in Figure 4 is often used to isolate the PLL from other high-speed circuits in the system. A better approach is to use a low dropout (LDO) voltage regulator to independently generate the PLL supply voltage, as shown in Figure 5. Although this method increases cost, it ensures low noise and excellent PLL performance.

Figure 5: PLL power isolation using LDOs.
Crosstalk and Transmission Line Effects Interference between signals, or crosstalk, can propagate between traces through electromagnetic radiation. It can also be generated electrically by unwanted signals on the power and ground planes. Crosstalk is inversely proportional to the square of the trace spacing. Therefore, to minimize crosstalk, single-ended signals should be routed at least twice the trace width. For differential signals such as Ethernet and USB, the trace spacing needs to be the same as the trace width in order to match the differential impedance. Critical signals can be shielded with ground and power planes, or ground traces can be added in parallel with the signals when the board is modified.
Some signals also generate high-frequency harmonics that cause crosstalk. Since the radiated energy is proportional to the rise and fall times of the signal, slower rise or fall times will cause less interference. Figure 6 shows an example of video interference that can be caused by radiation from an internal clock. The third harmonic of the 18.432MHz audio clock in North American Channel 2 will produce the interference shown on the left. By adding a series resistor to the audio clock trace to slow down the rise and fall times of the clock, the interference is reduced, as shown on the right side of Figure 6. However, the designer needs to understand the timing margins in order to reduce the rising and falling edges to within the limits allowed by the system.

Figure 6: Solving audio and video crosstalk.
Related to crosstalk is the transmission line effect, which occurs when high-speed traces become transmitters that generate radiated interference. Typically, traces transmit signals only when the rise time of the signal is less than twice the propagation delay. This suggests a rule of thumb that the length of the trace should be as short as possible to reduce propagation delay. Another is that reasonable signal termination will slow down the rise time of the signal, thereby minimizing overshoot and undershoot caused by reflections. Figure 7 shows how to use parallel termination to correct the level and minimize transmission line effects.