1 Introduction
With the improvement of the increasing complexity of electronic product features and performance , the frequency -density printed circuit boards and their related devices are rising, maintain and improve the speed and performance of the system has become an important issue in front of the designer . Signal frequency becomes higher , the edge becomes steeper , the printed circuit board size becomes smaller, so that the influence of wiring density increased crosstalk in high-speed PCB design increased significantly. Crosstalk is an objective reality , but beyond a certain boundary error may cause the circuit to trigger , causing the system does not work. Designers must understand the mechanism of crosstalk , and in the application of appropriate design methods , so the negative impact of crosstalk is minimized.
2 . Produce and trend of high-frequency digital signal crosstalk
Crosstalk is the transmission line when the signal propagation , undesirable noise voltage signal due to mutual coupling between the electromagnetic field generated by adjacent signal , i.e., the energy coupled by a line to another line.
As shown, for ease of analysis , we follow discrete equivalent model to describe a model of crosstalk between two adjacent transmission lines , transmission line characteristic impedance of AB and CD Z0, and the termination resistor R = Z0. If the driving source is located at point A the interference source, the A-B line network called interference between the source network (Aggressor line), the line C-D between the network is called a network interference (Victim line), interference ( later also called crosstalk ) network interference sources close to the network drive end crosstalk called near-end crosstalk , and interference sources close to the direction of the receiving end crosstalk network called far-end crosstalk ( also known as forward crosstalk ) . Crosstalk mainly from mutual inductance Lm formed between two adjacent conductors and mutual capacitance Cm.
2.1 inductive coupling
In Figure 1 , we first consider only the inductive coupling caused by mutual inductance Lm . A magnetic field signal to a line in the transmission line B to C to D effects induced voltage , similar to a magnetic coupling of the transformer , since this is a distributed transmission line , the series inductance of the transformer becomes distributed over two adjacent the parallel transmission line. When a voltage step to move from A to B, each distribution transformer interference line sequentially induced a sharp pulse interference is interference in the network . Mutual interference in the network superimposed on the voltage noise is proportional to the drive current changes its size with interference on the network . Noise generated by the mutual inductance is calculated as
Notably , the coupling transformer inductance of each segment is different from the polarity of the coupling , these sensors to be sequentially before the interference to the interference energy network , but of opposite polarity and the rear , respectively, along the transmission line to CD and C D point the road .
2, before the disturbance energy to the direction C , and each is the incident voltage and the mutual inductance Lm is proportional to the component , since all the energy of the forward interference arrive almost simultaneously at point C , so that both the forward and interference energy transmission line inductance proportional to the total amount , the longer the length of the transmission line parallel to the mutual inductance of the total generated the greater the interference energy immediately before also increased ; however, interfere with the energy to go before the disturbance after point D to point C of energy and different , although the coupling between the total area is the same , but each component of the interference is induced by mutual inductance transformer sequentially reached D, to the long 2Tp (Tp is the propagation delay ) , with the lines of the interference energy effective time extend parallel to the length ( ie, increase mutual inductance ) , the magnitude of the crosstalk size will not change , but the duration will increase.
2.2 capacitive coupling
Mutual tolerance is another mechanism to generate crosstalk . Mutual capacitance Cm would be to produce a network of interference induced current which is proportional to the rate of change of the interfering network voltage is calculated from the noise generated by the mutual capacitance Cm :
Distributed inductive coupling mechanism and the coupling capacitance coupling distributed similar , except that the polarity of coupling . As shown in Figure 3 , the forward and polarity capacitively coupled to the mutual interference energy is positive.
2.3 Effect of mutual inductance and mutual capacitance of synthetic
Typically , capacitive and inductive crosstalk crosstalk occur simultaneously . From the literature [ 1 ] , we can calculate the formulas were obtained proximal and distal total crosstalk , which are , respectively, superimposed by the capacitive coupling and inductive coupling made .
NEXT total noise is:
The total far-end crosstalk noise is:
Wherein , Z0, C, l, Cm, Lm, L, V0 respectively, the characteristic impedance of the transmission line , the capacitance per unit length , inductance per unit length , coupling capacitors, coupled inductors , and the length of two parallel transmission lines between the transmission line voltage peak .
From the above two equations , we can see far-end crosstalk noise due to the polarity of the relationship between the total capacitive and inductive coupling with each other abatement , that far-end crosstalk can be eliminated . In the PCB layout , the strip line (Stripline) circuit capable of displaying more inductive and capacitive coupling between a good balance , and its forward coupling energy minimization ; while for microstrip line (Microstfip), a large electric field associated with crosstalk partially through the air , and not some other insulating material , and therefore smaller than the inductive capacitive crosstalk crosstalk , resulting in the forward coupling is a small negative number. This is usually designed , often ignoring far-end crosstalk interference , and more emphasis on the causes of near-end crosstalk improvement.
In the actual design , PCB relevant parameters ( such as the thickness , dielectric constant , etc. ) and the line length , line width, line distance , the transmission line and the ground plane will influence the location and the current flowing c, l, Cm, Lm, L, of the size , while the signal frequency and devices rise / fall time decision.
2.4 crosstalk trends
Mutual inductance and mutual capacitance affects the size of the size of the crosstalk , which is equivalent to changing the characteristic impedance of the transmission line and the propagation speed. Similarly , the geometry of the transmission line to a large extent influence the mutual inductance and mutual capacitance change, so the characteristic impedance of the transmission line itself also have an impact on these parameters . In the same medium , coupling the relatively low impedance of the transmission line and the reference plane ( ground plane ) between the more intense relative coupling adjacent transmission lines will be weaker and thus the impedance change of low -impedance transmission line crosstalk caused smaller.
3 crosstalk caused by the impact of several
In high-speed , high-density PCB design generally provides a complete ground plane , so that substantially only the signal lines and each of its nearest signal lines influence each other, distant from the other cross-coupled signal lines is negligible . However, in an analog system, the signal passes through the low power signal or a signal when the high voltage element ( such as TTL) input signal and a relatively low voltage element ( such as ECL) close , requires a very high resistance to crosstalk capacity. In PCB design, if not handled correctly , crosstalk , principally high-speed PCB signal integrity are two typical effects .
3.1 crosstalk caused false triggering
Crosstalk is a high-speed signal integrity issues faced in the design of an important content, digital circuit function errors caused by the crosstalk is the most common one.
Figure 4 is a typical logic neighboring network transmission errors caused by the crosstalk pulse . Network interference source signal transmitted through a coupling capacitor , the interference caused by the network and receiving a noise pulse , resulting in an unwanted pulse is sent to the receiving end . If the pulse intensity exceeds the trigger value the receiving end , it will generate a trigger pulse can not control , causing the network to the next level logic functions chaos.
3.2 Timing delay caused by crosstalk
In digital design , the timing problem is an important consideration . Figure 5 shows the timing problems caused by the crosstalk noise . Lower half of the figure are two sources of interference noise pulse generated by the network ( Figure 5 glitch Helpful delay caused by crosstalk noise and Unhelpful glitch), when the noise pulse (helpful glitch) superimposed interference network, transmission network signal interference caused by delay reduced; Similarly, when the noise pulse (Unhelpful glitch) superimposed interference network , increases the delay was normal network transmission signal interference . Although this reduces the network transmission delay crosstalk noise to improve PCB timing is helpful , but in the actual PCB design, sources of interference due to the uncertainty of the network, this delay can not be controlled , and thus this crosstalk delay caused by the need to be suppressed.
4 minimize crosstalk
Crosstalk in high-speed high-density PCB design prevalent , the impact of crosstalk on the system are generally negative. To reduce crosstalk, the most basic is to make the network coupled between the source of interference is interference with the network as possible. In the high-density PCB design complexity is impossible to completely avoid crosstalk , but the designers in the system design should consider the case does not affect the performance of other systems , choose the appropriate method to seek to minimize crosstalk . Combined with the above analysis, solving the problem of crosstalk mainly from the following aspects:
Under conditions permitting the wiring conditions as widen the distance between the transmission line ; or reduce as far as possible the adjacent parallel length ( parallel length accumulated ) between the transmission line , preferably between traces in different layers .
Two adjacent signal layers ( non- planar layer isolation ) direction should be perpendicular alignment to avoid parallel lines to reduce crosstalk between layers .
In the case of signal timing to ensure as far as possible choose to convert low- speed devices , so that the rate of change is slow electric and magnetic fields , thereby reducing crosstalk.
Lamination design , to meet the conditions of characteristic impedance , reference should dielectric layer and the wiring layer plane ( power or ground plane ) as thin as possible between , thus increasing the degree of coupling transmission line and the reference plane between adjacent reduction coupled transmission lines.
Since only one field coupling plane surface , stronger than the reference surface of the intermediate wiring layers , and therefore more sensitive to crosstalk signal cables in the inner cloth .
By termination of the transmission line terminal distal and proximal ends of the transmission line impedance matching , can greatly reduce crosstalk amplitude.
5 Conclusion
Digital system design has entered a new stage. Many of the past in a secondary position in high-speed design issues now have critical impact on system performance. Signal integrity issues including crosstalk brought changes , including design concepts, design processes and design methods. Faced with new challenges , for crosstalk noise , the most critical is to find out who the real impact on the normal operation of the network , rather than blindly to all networks crosstalk noise suppression , which is routing resources and limited phase contradictory. Crosstalk issues discussed in this paper for high-speed high-density circuit design to solve the problem of crosstalk has very important significance .
