How to do signal of PCB of good high speed to flow to processing?
[introduction] signal integrality is the quality that points to signal to go up in transmission method, namely signal can make response ability with correct sequential and voltage n in circuit. If circuit design can achieve a signal to be in with formulary sequential, duration and voltage amplitude,interrelate transmit in the system, show this circuit has good signal integrality. Signal integrality problem is reflected in a lot of respects, reduce fixed rate when signal rise time, when the parasitism capacitance on circuit board and parasitism inductance begin to bring about a few noise signal that affect circuit performance possibly and transient state signal, consider the integrality issue of signal with respect to need, it may cause the happening of the following problem.
1. defer: Defer is to show signal is on the transmission line of PCB board transmit with finite speed, signal from send end to give out arrive at sink, meantime is put in to transmit defer. Defer meets signal produce influence; to transmit defer to basically depend on to systematic sequential all round the length of lead and lead the dielectric constant of medium. In system of high speed number, length of signal transmission line is influence clock pulse the directest element of phasic difference, clock pulse is phasic difference is the time that shows the two clock signal that generates at the same time arrives at sink different situation. Phasic difference reduced clock pulse signal can forecast a gender along what arrive, if clock pulse is phasic difference is too big, wrong signal can generate in sink.
2.Reflection: Reflection is signal the echo on line. When signal defer time ambitious jump at signal change when time, line must regard as transmission line. When the characteristic impedance when transmission line and laden impedance do not match, signal power (voltage or electric current) one part transmits a line to go up and reach laden point, but one part is reflexed. If laden impedance is less than former impedance, reflection is negative; conversely, reflection is. The geometrical appearance of wiring, incorrect line termination, transmission that passes connector and power source plane are discontinuous the change that waits for an element all can bring about this kind of reflex.
3. strings together faze: Stringing together faze is the noise that the mutual inductance between the coupling between two line, line and each other look cause line to go up. The coupling that allow a gender causes coupling electric current, and perceptual coupling causes coupling tension. The noise that string together faze results from between line network, between distributinging system of signal system and power source, the between electromagnetism coupling that crosses aperture. String circle cause mistake of data of false clock, intermittence to wait likely, cause an effect to the transmission quality of adjacent signal. In reality, cannot remove string of trouble completely, but can control its in the system can in susceptive limits. Span of the parameter of PCB ply, line, drive is carried and means of termination of the electric character of sink, base line is opposite the influence with string together faze to have certain.
4. rushs too and next developing: Rush too even if the first peak value or Gu Zhichao cross set voltage, to rising along, it is to point to highest voltage; to dropping along, it is to point to lowermost voltage. Rushing below is to show next cereal is worth or peak value exceeds set voltage. Exorbitant is too strong can cause protective diode work, bring about its premature invalidation. The clock that can cause a holiday or data mistake rush below exorbitant (operate) by accident.
5. oscillation and surround oscillation: Oscillation seems what appear repeatedly to cross rush and next developing now. The oscillation of signal is the oscillation that causes by the inductance that transfers on the line and capacitance namely, belong to owe damp state, and surround oscillation, had attributed damp position. Oscillation and surround oscillation also is caused by a variety of elements together like reflection, oscillation can give through proper termination reduce, but eliminate completely impossibly.
To improve quality of high speed signal, need to understand 3 following important electronics principles:
(1) electric current is electronic flow;
(2) electric current can flow in closed loop only;
(3) electric current is in closing is constant in loop.
Generally speaking, PCB architect should cost many time and energy to undertake designing planning to the method of going from place to place of line, and processing returns signal, make signal can search method to return. A of line integrality problem main reason is the control of pair of signal circumfluence method, decorating nice element while form loop.
Some netizen is modular loop of flying line signal pursues as follows
