Analysis of concurrent switch noise in connector design
In high-speed circuit design,Chinese design engineers usually do not particularly understand the role of mutual inductance characteristics of connectors in improving signal integrity design.This paper discusses the most difficult problem in connector design and selection:concurrent switching noise,and reveals the effects of concurrent switching noise on connectors used in high-performance systems.And packaging specifications.
People always think that all the work in the system is done by IC,including the corresponding software.Passive devices such as IC packages,circuit boards,connectors,cables and other discrete components will only reduce system performance,expand system size and increase system costs.Therefore,the interconnection in the system and the selection and design of components are actually to minimize the impact of these components on the system.So most IC designers usually attribute all of the disconnected parts of the system(which is usually what PCB designers refer to)to a general category of parasitic components.
When choosing IC devices,in addition to choosing appropriate components,the subsequent layout and wiring of circuit boards should comply with the following design rules:
1.PCB line with controlled impedance;
The signal delay on the 2.branch line is less than 20%of the fastest signal rise time.
3.the time delay of discontinuity is less than 20%of the fastest signal rise time.
4.adjacent PCB lines have adequate spacing to ensure signal crosstalk is controlled at acceptable levels.
5.reasonable PCB layering design ensures that the dielectric between adjacent power and ground level is very thin.
6.there is a continuous signal return path under each signal line.
Even though the layout and wiring of PCB is very good,things are still not that simple.Every component in a high-performance system needs to be optimized to ensure that it meets the cost,performance,and development schedule requirements of the entire system.High-performance system design is a chain of interlocking,each link must meet the requirements,to ensure that the entire system meets the product performance specifications.
How will other factors in the system affect system performance?Possible problems usually fall into two categories:timing and noise.Signal integrity includes both timing and noise.However,the problem of noise is even more prominent.
Four types of signal integrity problems caused by interconnection and components:
1.signal quality of single network;
2.signal crosstalk between two or more networks;
3.noise in power distribution system;
4.the external electromagnetic radiation of the components in the system.
Unless you pay special attention,and the project starts thinking about these issues,these four types of problems will occur in high-speed products.This paper will discuss the most difficult problem in the design and selection of connectors(including IC packages),concurrent switching noise,and reveal the effect of concurrent switching noise on connectors and package specifications used in high performance systems.Concurrent switching noise
For connectors and IC packages,the high speed performance requirements of switching noise are the most difficult to satisfy.Switching noise belongs to signal crosstalk,which is mainly caused by mutual inductance between connectors and adjacent loops(composed of signals and return paths)in IC packaging.To minimize the switching noise amplitude,it is necessary to ensure that the mutual inductance between adjacent signal path loops is less than a permissible maximum.
When a signal propagates through a connector or IC package,the wavefront of the signal(the jumping component in the waveform)forms a current loop through the signal pin,which couples and returns to the signal return pin.Each signal and the corresponding return path can form a similar loop.There is a loop mutual inductance between the loops of any two signals and their return paths.
When the current in one loop changes,the signal noise is induced in another static loop.When multiple variable signal lines are switched concurrently,the noise coupled to the static loop by mutual inductance accumulates with each other,so it is called"concurrent switching noise".A concurrent switching noise measured on a stationary signal line when five data lines are switched simultaneously.In this example,the noise on the stationary signal line is caused by the mutual inductance between the stationary loop and all five variable loops.
Calculation of mutual inductance
Using the simple model,it is easy to estimate the maximum mutual inductance between the two signal loops.Further discussion is made on how to calculate mutual inductance between adjacent loops in actual connectors.
When the signal passes through a pin pair of the connector,a sudden change in the signal current occurs at the wavefront of the signal in a changing signal path.Variable currents cause voltage noise and are induced in adjacent static signal loops,which are caused by mutual inductance between the two loops.The noise induced by the static signal line is called switching noise because it occurs only when the voltage or current is switched.