Misunderstanding 1: a signal above GHz is a high-speed signal?
When "high speed signal" is mentioned in layout design, it is necessary to make clear what is "high speed". Is the signal at the MHz rate considered as high speed or the signal at the GHz rate considered as high speed?The traditional Si theory has a classical definition of "high speed signal".
Si: signal integrity.
In Si theory, for the understanding of signal transmission behavior of PCB board interconnection lines, the signal edge rate almost completely determines the maximum frequency component in the signal. Generally, when the signal edge time is less than 4-6 times of interconnection transmission delay, the signal interconnection path will be treated as a distributed parameter model, and Si behavior needs to be considered.
The so-called "high speed" refers to "interconnection transmission delay with signal edge time less than 4-6 times". It can be seen that whether the signal transmitted by the circuit board is "high speed" depends not only on the edge speed of the signal, but also on the path length of the circuit board. When there is a certain proportion between the two, the signal should be processed according to "high speed signal".
Misunderstanding 2: can we design high-speed PCB layout with simulation software platform?
EDA design software platform integrates the function of high-speed signal simulation, which is very helpful for the formulation and implementation of high-speed PCB design rules, signal quality simulation and evaluation.However, in the actual PCB design process, sometimes the simulation results show that the signal quality is good, but the actual test signal quality is very poor, which does not meet the signal test standard,
In fact, simulation and test are inseparable. Taking IBIS model as an example, we usually call it "behavior level model". This kind of simulation model is also built through the V and I test curves under different working conditions of the chip. There is a problem. If we don't pay attention to which chip model is selected during simulation, the simulation will be inaccurate, such as slow, typical, fast 。
From the above example, it can be seen that "simulation model base" is very important for simulation results. Only through the actual comparison of simulation tests of actual product projects and the modified simulation model can it be regarded as "accurate simulation model".
A good simulation design platform can not solve all the problems, but also needs "accurate simulation model". In addition, the application scenario of the actual product project should also be considered. A simulated signal network will be affected by power noise, other signal crosstalk and other factors, which will also cause the difference between the test results and simulation results.
Misunderstanding 3: Circuit board routing "transmission line model" in layout design simulation software is very accurate?
The PCB routing in the simulation software, whether microstrip or stripline, can be modeled by simulation tools. This model is based on the size of stacking and actual routing, which can generally meet the accuracy requirements. However, if it is "very accurate", there are still some gaps, which need to be analyzed from the following aspects:(1) The roughening / browning processing technology of PCB copper sheet has influence on signal quality
During PCB processing, in order to improve the bonding strength of PCB copper coating and dielectric layer and reduce the risk of PCB delamination, there will be roughening / browning treatment process, that is, the surface of copper coating will become rough by grinding or corrosion.
In the transmission of high-speed signal in the conductor, there is "skin effect", which means that when the high-frequency signal is transmitted, the current flowing in the conductor will move towards the periphery or the "skin" of the conductor.
The rough surface of PCB copper skin, on the one hand, affects the loss, on the other hand, it also affects the signal transmission delay, which is well understood. It is like that when a car is driving on a rugged mountain road, it will definitely take more time than driving on a asphalt road.
(2) The dielectric constant DK and tangent loss angle DF of PCB medium change with frequency
In the simulation tool, the DK and DF of PCB medium are usually constant, but from the angle of signal actual transmission, Dk / DF changes with frequency.
Dk / DF will change with the speed of the transmission signal. If these two parameters are treated as constants in the simulation tool, the simulation accuracy of the transmission line model will be affected. The higher the signal transmission rate, the greater the impact.
(3) Influence of "anisotropy" on PCB
PCB is usually a woven structure of "epoxy resin + glass cloth". The arrangement direction of glass cloth is divided into "warp direction" and "weft direction". At the same time, according to the thickness and spacing of glass fiber, PCB is divided into different types, such as 1080, 2116, etc. When different types of glass cloth are used in PCB board, the composition proportion of glass cloth and resin in the board is different.
The difference between the Dk / DF value of glass cloth and resin material is quite large. When the relative position of PCB normal wiring and glass cloth is different, the Dk / DF value of reference medium and the impedance and loss of signal will be different, as shown in the figure below, which is why some projects require that the wiring direction of the whole PCB should be 10 °.
Error 4: a simulation software platform can solve all signal simulation problems
At present, there is not a unified simulation software platform that can be applied to all signal simulation scenarios. Cadence SPB sigxplorer is used for behavior level signal quality simulation, Synopsys hspie is used for transistor level simulation, Ansoft HFSS is used for 3D electromagnetic field modeling, Ansoft ads is used for time-frequency hybrid simulation.
