With the increasing complexity of electronic products and the improvement of performance, the density of printed circuit board and the frequency of its related devices are rising. Maintaining and improving the speed and performance of the system has become an important topic for designers. The influence of crosstalk in high-speed PCB design increases significantly with the increase of signal frequency, steep edge, smaller size of printed circuit board and higher wiring density. Crosstalk is an objective problem, but exceeding a certain limit may cause false triggering of the circuit, resulting in the failure of the system to work normally. Designers must understand the mechanism of crosstalk and apply appropriate methods in the design to minimize the negative impact of crosstalk.
2. Generation and change trend of high frequency digital signal crosstalk
Crosstalk refers to the undesired noise voltage signal generated by the mutual coupling of electromagnetic fields between adjacent signals when the signal propagates on the transmission line, that is, the energy is coupled from one line to another.
As shown in Figure 1, for the convenience of analysis, we describe the crosstalk model of two adjacent transmission lines according to the discrete equivalent model. The characteristic impedance of transmission lines AB and CD is Z0, and the terminal matching resistance R = Z0. If the driving source at point a is the interference source, the network between A-B is called the aggressor line, the network between C-D is called the victim line, the crosstalk near the driving end of the interference source network is called the near end crosstalk (also known as backward crosstalk), and the crosstalk near the receiving end of the interference source network is called the far end crosstalk (also known as forward crosstalk) 。 Crosstalk mainly comes from mutual inductance LM and mutual capacitance cm formed between two adjacent conductors.
2.1 inductive coupling
In Figure 1, only the inductive coupling caused by mutual inductance LM is considered first. The magnetic field of the signal transmitted on lines a to B induces voltage on lines C to D. the effect of magnetic coupling is similar to that of a transformer. Because this is a distributed transmission line, mutual inductance also becomes a series of transformers distributed on two adjacent parallel transmission lines. When a voltage step signal moves from a to B, each transformer distributed on the interference line will sequentially induce an interference spike pulse to appear on the disturbed network. The voltage noise superimposed by mutual inductance on the disturbed network is directly proportional to the change of driving current on the disturbed network. The calculation formula of noise generated by mutual inductance is
It should be noted that the polarity of mutual inductance coupling of each section of the coupling transformer is different. The interference energy induced by the disturbed network is forward and backward in order, but the polarity is opposite, and travels to points c and d along the transmission line CD respectively.
As shown in Figure 2, the forward interference energy in the direction of C is directly proportional to the incident voltage and each mutual inductance component LM. Because all forward interference energy reaches point C almost at the same time, the forward interference energy is directly proportional to the total mutual inductance of the two transmission lines. The longer the parallel length of the transmission line, the greater the total mutual inductance generated, and the forward interference energy increases immediately; However, the difference between the backward interference energy to point D and the forward interference energy to point C is that although the total coupling area of the two is the same, the interference component induced by each mutual inductance transformer reaches D in sequence, and the effective time of the backward interference energy is up to 2tp (TP is the propagation delay). With the extension of the parallel length of the line (i.e. the mutual inductance increases), The amplitude of backward crosstalk will not change, but the duration will increase.
2.2 capacitive coupling
Mutual tolerance is another mechanism for crosstalk. Mutual capacitance cm will generate an induced current to the disturbed network, which is proportional to the change rate of voltage on the interference network. The calculation formula of noise generated by mutual capacitance cm is:
The coupling mechanism of distributed coupling capacitance is similar to that of distributed inductive coupling, but the difference lies in the polarity of coupling. As shown in Fig. 3, the polarity of forward and backward interference energy of mutual capacitance coupling is positive.
2.3 combined effect of mutual inductance and mutual capacitance
Generally, capacitive crosstalk and inductive crosstalk occur at the same time. From reference , we can obtain the calculation formulas of total crosstalk at the near end and far end respectively, which are superimposed by capacitive coupling and inductive coupling respectively.
The total near end crosstalk noise is:
The total far end crosstalk noise is:
Wherein, Z0, C, l, cm, LM, l and V0 are the characteristic impedance, capacitance per unit length and inductance per unit length of the transmission line, the coupling capacitance and inductance between the two transmission lines, the parallel length and voltage peak of the two transmission lines.
From the above two formulas, we can see that the total noise of far end crosstalk can be eliminated due to the polarity relationship between capacitive and inductive coupling, that is, far end crosstalk can be eliminated. In PCB wiring, stripline circuit can better show a good balance between inductive and capacitive coupling, and its forward coupling energy is very small; For microstrip lines (microstfip), most of the electric field related to crosstalk passes through air rather than other insulating materials. Therefore, capacitive crosstalk is smaller than inductive crosstalk, resulting in a small negative forward coupling. This is why the interference of far end crosstalk is often ignored and the improvement of near end crosstalk is more emphasized in the general design.
In the actual design, the relevant parameters of PCB (such as thickness, dielectric constant, etc.), line length, line width, line distance, the position of transmission line and ground plane and current flow direction will affect the size of C, l, cm, LM, l, which is determined by the signal frequency and the rise / fall time of the device.
Here, we do not make quantitative analysis of the influence of these parameters on crosstalk. For the relationship between these parameters and the degree of influence on crosstalk, see other relevant references.
2.4 change trend of crosstalk
The magnitude of mutual inductance and mutual capacitance affects the magnitude of crosstalk, which equivalently changes the characteristic impedance and propagation speed of transmission line. Similarly, the geometry of the transmission line largely affects the changes of mutual inductance and mutual capacitance, so the characteristic impedance of the transmission line itself also affects these parameters. In the same medium, the coupling between the relatively low impedance transmission line and the reference plane (ground plane) is stronger, and the coupling with the adjacent transmission line is weaker, so the impedance change caused by crosstalk is smaller.
3 several effects caused by crosstalk
In the design of high-speed and high-density PCB, a complete grounding plane is generally provided, so that each signal line basically only interacts with its nearest signal line, and the cross coupling from other distant signal lines can be ignored. Nevertheless, in an analog system, a very high anti crosstalk capability is required when a high-power signal passes through a low-level input signal or when an element with a higher signal voltage (such as TTL) is close to an element with a lower signal voltage (such as ECL). In PCB design, if not handled correctly, crosstalk has the following two typical effects on the signal integrity of high-speed PCB.
3.1 false triggering caused by crosstalk
Signal crosstalk is an important part of the signal integrity problem faced by high-speed design. The function error of digital circuit caused by crosstalk is the most common one.
Fig. 4 is a typical transmission of adjacent network error logic caused by crosstalk pulses. The signal transmitted on the interference source network causes a noise pulse at the interfered network and the receiving end through the coupling capacitor, resulting in an unwanted pulse sent to the receiving end. If the pulse strength exceeds the trigger value at the receiving end, an uncontrollable trigger pulse will be generated, resulting in the confusion of the logic function of the next level network.
3.2 timing delay caused by crosstalk
In digital design, timing problem is an important consideration. Fig. 5 shows the timing problem caused by crosstalk noise. The lower part of the figure shows two kinds of noise pulses generated by the interference source network (Fig. 5 delay glitch and unhelpful glitch caused by crosstalk noise). When the noise pulse (helpful glitch) is superimposed on the interfered network, the signal transmission delay of the interfered network is reduced; Similarly, when the noise pulse (unhelpful glitch) is superimposed on the interfered network, the delay of the normal transmission signal of the interfered network is increased. Although this crosstalk noise to reduce the network transmission delay is helpful to improve the PCB timing, in the actual PCB design, this delay can not be controlled due to the uncertainty of the interference source network, so the delay caused by this crosstalk must be suppressed.
4. Crosstalk minimization
Crosstalk is common in high-speed and high-density PCB design, and the impact of crosstalk on the system is generally negative. In order to reduce crosstalk, the most basic thing is to make the coupling between the interference source network and the disturbed network as small as possible. It is impossible to completely avoid crosstalk in high-density Complex PCB design, but in system design, designers should choose appropriate methods to minimize crosstalk without affecting other performance of the system. Combined with the above analysis, the crosstalk problem is mainly considered from the following aspects:
If wiring conditions permit, the distance between transmission lines shall be increased as much as possible; Or reduce the parallel length (cumulative parallel length) between adjacent transmission lines as much as possible, preferably routing between different layers.
The routing direction of signal layers (without plane layer isolation) of two adjacent layers shall be vertical, and parallel routing shall be avoided as far as possible to reduce crosstalk between layers.
Under the condition of ensuring the signal timing, select devices with low conversion speed as far as possible to slow down the change rate of electric field and magnetic field, so as to reduce crosstalk.
When designing the lamination, under the condition of meeting the characteristic impedance, the dielectric layer between the wiring layer and the reference plane (power supply or ground plane) should be as thin as possible, so as to increase the coupling between the transmission line and the reference plane and reduce the coupling of adjacent transmission lines.
Since the surface layer has only one reference plane, the electric field coupling of the surface layer wiring is stronger than that of the middle layer, so the signal lines sensitive to crosstalk shall be arranged in the inner layer as much as possible.
Through termination, the far end and near end terminal impedance of the transmission line can be matched with the transmission line, which can greatly reduce the amplitude of crosstalk.
Digital system design has entered a new stage. Many high-speed design problems that used to be secondary have a key impact on system performance. Signal integrity problems including crosstalk have brought about changes in design concepts, design processes and design methods. Facing the new challenges, for crosstalk noise, the key is to find out those networks that really affect the normal operation of the system, rather than blindly suppress crosstalk noise for all networks, which is also in contradiction with limited wiring resources. The crosstalk problem discussed in this paper is of great significance to solve the crosstalk problem in high-speed and high-density circuit design.