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 vulnerable to these interferences. During design, potential noise and interference sources should be identified as soon as possible, and measures should be taken as soon as possible to minimize these interferences. Good planning will reduce a lot of time and repetition of work in the commissioning stage, thus saving the total design time and cost.
Nowadays, the internal clock rate of the fastest DSP is as high as thousands of MHz, and the frequency of transmitting and receiving signals is as high as hundreds of MHz. These high-speed switching signals will produce a lot of noise and interference, which will affect the system performance and produce high-level EMI. The DSP system also becomes more complex, such as audio and video interface, LCD and wireless communication functions, Ethernet and USB controller, power supply, oscillator, drive control and other circuits, all of which will produce noise and be affected by adjacent components. These problems are particularly easy to occur in audio and video systems, because noise will cause the degradation of sensitive simulation performance, but it is not obvious for discrete data.
It is essential to solve the problem of noise and interference from the beginning of the design. Many designs fail to pass the electromagnetic compatibility test of the Federal Communications Commission (FCC) for the first time. If some time is spent on low noise and low interference design methods in the early design, the redesign cost in the subsequent stage and the delay of product launch time will be reduced. Therefore, from the beginning of design, development engineers should focus on:
1. Select the 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 minimum transmission line effect;
If these goals are achieved, development engineers can effectively avoid defects in noise and EMI.
Influence and control of noise
Noise reduction is one of the most important design criteria for high-speed DSP. Excessive noise from any noise source will lead to random logic and phase-locked loop (PLL) failure, thus reducing reliability. It will also lead to radiated interference affecting FCC certification testing. In addition, debugging a noisy system is extremely difficult; Therefore, to eliminate noise – if it can be completely eliminated – requires a lot of effort in circuit board design.
In the audio and video system, even small interference will have a significant impact on the performance of the final product. For example, in the audio capture and playback system, the performance will depend on the quality of the audio codec used, the noise of the power supply, the quality of PCB wiring, and the crosstalk between adjacent circuits. Moreover, the stability of the sampling clock is also required to be very high to avoid unwanted noise, such as “bang” and “click” during playback and capture.
In the video system, the main challenge is to eliminate color distortion, 60Hz “buzzing” sound and audio knocking sound. These are harmful to high-quality video systems, such as security monitoring applications. In fact, these problems are usually related to the poor design of video circuit board. Specifically, the power noise is transmitted to the DAC output of the video; Transient of power supply caused by audio playback; The audio signal is coupled to the signal line of the high impedance video circuit.
These typical video problem sources include: synchronization and pixel clock overshoot and undershoot; Affect the color codec and pixel clock jitter; Image distortion due to lack of termination resistance; Flicker caused by poor audio and video isolation.
Audio and video applications are prone to noise interference, which is also common for all communication systems requiring very low bit error rate. In the communication system, radiation not only causes EMI problems, but also blocks other communication channels, resulting in false channel detection. These challenges can be solved by using appropriate circuit board design technology, shielding technology and isolation of RF and mixed analog / digital signals.
There are many potential switching noise sources in high-speed DSP system, including: crosstalk between signal lines; Reflection caused by transmission line effect; Voltage drop caused by improper decoupling capacitance; High inductance power line, oscillator and PLL circuit; Switching power supply; Large capacitive load caused by instability of linear regulator; disc drive.
These problems are caused by 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 radiation antenna formed by adjacent signal lines. If the radiated interference is strong enough, it will lead to EMI problems that can destroy other systems.
When the noise in high-speed DSP system cannot be eliminated fundamentally, it should be minimized. There is noise inside electronic components, so it is very important to carefully select device characteristics and select appropriate devices. In addition to the correct selection of devices, there are two general technologies, namely PCB wiring and loop decoupling, which can help control system noise. An excellent PCB wiring will reduce the possibility of noise channel generation. In addition, the radiation that can be transmitted to the printed line and current circuit is reduced, and the decoupling avoids the influence of noise generated by adjacent circuits. The best way is to filter the noise from the source, but it can also make adjacent circuits insensitive to noise or eliminate the coupling channel of noise.
Now we discuss several techniques that can solve many common problems caused by system noise and EMI.
Keep the current loop as short as possible
The low-speed signal current returns to the source along the shortest path with the minimum impedance. The high-speed signal returns along the path with the smallest inductance: such a minimum loop area is located below the signal line, as shown in Figure 1.
Figure 1: comparison of high speed signal and low speed signal current.
Therefore, one of the goals of high-speed signal design is to provide the minimum inductance loop for signal current. This can be achieved using a power plane and a ground plane. The power plane minimizes parasitic inductance by forming a natural high-frequency decoupling capacitor. The ground plane forms a shielding surface, that is, the well-known mirror plane, which can provide the shortest current loop.
An effective PCB wiring method is to lean the power plane and the ground plane together. In this way, high plate capacitance and low impedance are formed, which is conducive to reducing noise and radiation. In order to shield, the best choice is that the key signals should be arranged close to the ground plane, while the rest should be close to the power plane.
In high-speed video systems, the purpose of keeping the loop short means that the video ground cannot be isolated. The audio ground that must be isolated must not be short circuited to the digital ground at the data input point, as shown in Figure 2.
Figure 2: Audio ground isolation.
Power isolation and phase locked loop
How to achieve the best power supply is the biggest challenge to control noise and radiation. Dynamic load switching environment is very complex, including: entering and exiting low power mode; Large transient current caused by bus competition and capacitor charging; Large voltage drop caused by decoupling and unreasonable wiring; The oscillator overloads the linear regulator output.
Figure 3 shows an example of designing a current loop using power line decoupling. The decoupling capacitor in this example is as close to the DSP as possible. If there is no decoupling, the dynamic current loop will be large, which will increase the decrease of power supply voltage, resulting in electromagnetic radiation.
Figure 3: power decoupling.
When supplying power to PLL, power isolation is very important because PLL is very sensitive to noise and requires very low jitter for stable system. You also need to choose analog or digital PLL. Analog PLL is less sensitive to noise than digital PLL.
Figure 4: PLL power isolation.
The Π – type filter with low cut-off frequency as shown in Fig. 4 is often used to isolate the PLL from other high-speed circuits in the system. A better method is to use a low dropout (LDO) voltage regulator to independently generate the power supply voltage of PLL, as shown in Figure 5. Although this method increases the cost, it ensures low noise and excellent PLL performance.
Figure 5: isolation of PLL power supply using LDO.
Crosstalk and transmission line effect
Interference between signals, i.e. crosstalk, can propagate between printed lines through electromagnetic radiation. This may also be generated electrically by unwanted signals on the power supply and ground plane. Crosstalk is inversely proportional to the square of the printed line spacing. Therefore, in order to minimize crosstalk, the wiring spacing of single ended signals should be at least twice the width of printed lines. For differential signals such as Ethernet and USB, the printed line spacing needs to be the same as the printed line width in order to match the differential impedance. The key signal can be shielded with the ground and power plane, or the ground wire parallel to the signal can be added when changing the board.
Some signals also produce high-frequency harmonics that cause crosstalk. Since the radiated energy is proportional to the rise and fall time of the signal, the interference caused by the slower rise or fall time will be small. Fig. 6 shows an example of video interference that may be caused by the radiation of the internal clock. In the second channel in North America, the third harmonic of 18.432mhz audio clock will produce interference as shown on the left in the figure. By adding a series resistance to the audio clock printed line to slow down the rise and fall time of the clock and reduce the interference, the result is shown on the right side of Fig. 6. However, designers need to understand the timing margin to reduce the rising and falling edges to the allowable limits of the system.
Figure 6: solving audio and video crosstalk.
Related to crosstalk is the transmission line effect, which occurs when a high-speed printed line becomes a transmitter generating radiated interference. Usually, the printed line transmits the signal when the rise time of the signal is less than twice the propagation delay. This implies an experience that in order to reduce propagation delay, the length of printed wire should be as short as possible. The other is that reasonable signal termination will slow down the rise time of the signal, so as to minimize overshoot and undershoot caused by reflection. Fig. 7 shows how to use parallel termination to correct the level and minimize the transmission line effect.
Figure 7: use termination to minimize transmission line effects.
Designers may question whether it is important to terminate the load resistance externally, since the resistance has been integrated inside the chip. In fact, in addition to controlling the transmission line effect, the external resistance can also realize the precise adjustment of signal integrity. The DSP cannot match the impedance of the circuit board exactly, so terminating the load can reduce the source current and the rise and fall time.
As with external termination load resistors, external pull-up and pull-down resistors are also important. For unconnected pins, although the internal pull-up and pull-down resistors are sufficient, the high-speed switching noise can be transmitted and the internal logic on the connection end will be triggered by mistake.
Radiation that can radiate outside the system is considered EMI, which may make the design unable to pass FCC certification. There are two possible radiations: one is the common mode radiation of a linear signal printed line or cable, and the other is the differential mode radiation whose signal and circuit form a large current loop. The common mode radiation decreases with the increase of frequency, while the differential mode radiation increases with the increase of frequency until its saturation point. The radiation of these two modes is shown in Figures 8 and 9.
Figure 8: common mode radiation.
Figure 9: differential mode radiation.
How EMI is handled depends on the radiation source. For common mode radiation, when EMI comes from an external cable (as shown in Figure 8), a choke can be added to the cable. If the internal transmission line causes EMI, the load is usually terminated, but adding a ground wire between the signal printed lines also helps to reduce the radiation. Another possible solution is to reduce the printed line length of the signal to less than 1 / 20 of the signal wavelength (or the reciprocal of the signal frequency). For example, to avoid transmitting radiation, a 500 MHz printed line should be shorter than 1.18 inches.
For differential mode radiation, the radiated energy is a function of current, loop area and frequency. Methods to reduce radiation include: terminating the load to reduce the source current, providing a circuit that can reduce the circuit area with a suitable current channel, or reducing the frequency.
Dynamic current shall also be considered when calculating decoupling resistance. High speed current may change at any time, and this transient will also cause radiation. In addition, when changing the value of the capacitor, it is necessary to prevent self resonance and limit the frequency range. PCB layering is a good solution because the power layer naturally decouples the high frequency, while the stratum provides the shortest loop. Isolate high-speed signals and keep them away from other signals. If possible, do not separate the strata. Although noise and radiation are caused by complex useless functions in system design, they can be controlled by some simple methods mentioned above.
Summary of this paper
There are many potential noise and radiation sources in the high-speed DSP video system, which can disrupt the work of the system or make the design fail to pass the FCC certification. Fortunately, planning and mastering noise and radiation can help system designers minimize these problems. Early efforts will save a lot of debugging work and later trouble. PCB layout and loop decoupling are two common techniques that designers can limit system noise and EMI. With these technologies, DSP video designers can effectively solve the noise and radiation of the system.