With the device operating frequency is higher and higher, the signal integrity of high-speed PCB design problems become a bottleneck of traditional design, the engineer in the design of a complete solution on more and more challenges. Despite the high speed simulation tools and interconnection tools can help designers to solve some problems, but also need more experience in high-speed PCB design to accumulate and in-depth exchanges between the industry.
This is a list of some of the popular concern.
The wiring topology impact on signal integrity
When the signal transmission along the line on the high-speed PCB board may lead to signal integrity issues. Stmicroelectronics of tongyang q: for a set of bus (address, data, command) drive up to 4, and 5 devices (FLASH, SDRAM, etc.), the PCB wiring, is the bus arrive at each device, such as the first to SDRAM, and FLASH... Or bus radial distribution, namely the separation from somewhere, connected to each device respectively. These two approaches on signal integrity, which is better?
Li Baolong pointed out that the wiring topology impact on signal integrity, mainly reflected in the moment, each node signal reaches reflection signals do not match the moment arrived at a same node, so cause the signal quality deterioration. Generally speaking, a star topology structure, can control the same long several branches, make the signal transmission and reflection time delay, achieve a better signal quality. Between the use of topology, want to consider to signal topology node situation, the working principle and the difficulty of wiring. Reflection of different Buffer, to signal the impact is not consistent, so the star topology is not very good solve the data address bus is connected to the FLASH and SDRAM delay, which can not ensure the quality of the signal; High speed signal, on the other hand, generally in the communication between DSP and SDRAM, FLASH when loading rate is not high, so just make sure the actual high speed when high-speed simulation waveform signal effective work node, without having to pay attention to the FLASH waveform; The star topology is Daisy chain topology, such as wiring is difficult, especially a large amount of data address signals using a star topology.
The influence of bonding pad for high speed signal
In PCB, from the design point of view a through hole is mainly composed of two parts: in the middle of the drilling and welding plate around the borehole. Famous for fulonm engineers consult guest solder have any impact on high speed signal, the Li Baolong said: bonding pad has influence to the high speed signal, the impact of similar packaging of components of the device. After detailed analysis, the signal from the IC inside out, after binding wire, pin, encapsulation shell, solder, solder to the transmission lines, all joints in the process will affect the quality of the signal. But the actual analysis, it is difficult to get solder, solder and specific parameters of the pin. So IBIS model is used in the packaging parameters they are summed up, and, of course, such an analysis on the low frequency can receive, but for the higher frequency signal higher precision of simulation is not precise enough. Now it is a trend of using IBIS V - I, V - T describe Buffer characteristics curve, using the SPICE model describes encapsulation parameters.
How to restrain electromagnetic interference
PCB is the source of electromagnetic interference (EMI), so the PCB design is directly related to the electronic products of electromagnetic compatibility (EMC). If in high-speed PCB design for EMC/EMI attach importance to it, will help to shorten product development cycle faster time-to-market. As a result, many engineers in the BBS very attention to suppress electromagnetic interference problem. Auspicious birth medical imaging, for example, wuxi ShuJian co., LTD., said in the EMC test found in the harmonic levels of the clock signal is very serious, could you tell me whether to use to the clock signal IC power supply pins for special processing, now just connect the decoupling capacitor on the power supply pins. Also need to pay attention to what aspects in the PCB design to reduce electromagnetic radiation? The Li Baolong pointed out that the three elements of EMC for radiation source, transmission and victims. Route of transmission is divided into space radiation transmission and transmission cable. So to suppress harmonic, the first way to look at it. Power decoupling is spread conduction solution, in addition, the necessary matching and shielding is needed.
Li Baolong when answer questions on WHITE net friend also pointed out that filtering is to solve EMC conduction radiation by a good way, in addition, can also from the interference sources and suffer body aspects to consider. Interference sources, try to use oscilloscope to check whether signal rise along too quickly, is reflective or Overshoot and undershoot ringing, if you have, can consider to match; Also try to avoid doing 50% duty ratio of the signal, because the signal is not accidentally harmonic, more high frequency components. The victims body, can consider to measures such as package.
RF wiring is to choose a hole or DaWan wiring
This BBS, also don't have a little friends were asked about high-speed analog circuit design. Fine constant electronic a net friend asked: in the high speed PCB, can also reduce a lot of return path, but some people say that you're willing to bend it don't played, that should be how to choose?
The Li Baolong pointed out that the analysis of the flow path of the RF circuit, and signal in high-speed digital circuit reflux is different. Both have something in common, are distributed parameter circuit, Maxwell equations are used to calculate the characteristics of the circuit. But radio frequency circuit is the analog circuits, circuit voltage V = V (t), current (I) = I (t) two variables need to be controlled, and the digital circuit only pay attention to the change of the signal voltage V = V (t). Therefore, in the RF wiring, besides considering signal flow, also need to consider the current layout. Namely DaWan wiring and via signal current have impact. In addition, most RF board are single or double sided PCB, not the complete plane layer, flow distribution in the signal path around each ground and power supply, the simulation needs to use the 3 d field extraction tool to analysis, then DaWan wiring and hole reflow needs concrete analysis; High speed digital circuit analysis generally deal only with a complete surface layer of multilayer PCB, use 2 d field extraction and analysis, considers only in adjacent flat signal flow, via only as a lumped parameter R - L - C processing.
