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Crosstalk occurs as a result of the interaction of electromagnetic fields. The current trend is toward smaller, faster boards with lower IC supply voltages. However, when the supply voltage is reduced from 3.3 to 1.5 V, the permissible noise margin is also reduced. In addition, the closer the parallel conductor segments are due to the limited space, the more likely they are to interact.
The reflections created by crosstalk violate the immunity to their effects. PCB designers cannot eliminate crosstalk, but they can estimate the expected level of crosstalk and create a topology that minimizes crosstalk.
Their value depends on the gap between the signal conductors, the distance between the conductors and layers, the length of the parallel segments, the load of the transmission line, and the technology used. Crosstalk also differs depending on the multi-layer configuration (stack). We will consider the characteristics of crosstalk in microstrip and stripline transmission lines, as well as ways to suppress them.
Crosstalk in these lines results from the coupling between microstrip and stripline edges, as well as crosstalk between double striplines and the combination of crosslink and edge coupling in double striplines.
As is known, a microstrip transmission line consists of signal conductors on the outer surface of a printed circuit board, and its internal signal layers form a strip line. At the same time, there are victim conductors - signal layers that are affected by aggressor conductors.
a) Microstrip transmission line;
b) Stripline transmission
The first two edge-connected configurations are well known and are commonly used to create differential pairs. Close coupling between conductors is used to improve balance as well as to suppress noise, but it exacerbates the effects of crosstalk.
The other two cross-link configurations have many disadvantages: the main one is the requirement to ensure that the conductors in different layers are identical in the manufacture of boards.
The IPC Class 3 standard specifies that the alignment error of the conductors of a differential pair on two adjacent layers should not exceed ±2 mils (0.0508 mm), but in practice this requirement may not be met, and the spread reaches, for example, ±4 mils (0.0508 mm). 1016 mm), which is reflected in the impedance.
In addition, broadband communication requires the use of a very thick dielectric between adjacent signal layers, which usually makes the substrate too thick. On the other hand, this approach can be a good solution for implementing connector desoldering, when the impedance is kept unchanged without the use of a reference layer.
The combination of transverse and edge coupling requires a thinner dielectric, but in this case, it is very difficult to determine what the impedance will become due to problems arising from the variation in the dimensions of the conductors.
There are two types of crosstalk: forward and backward. They are also referred to as far-end crosstalk (FEXT) and near-end crosstalk (NEXT) depending on whether the crosstalk is measured at the load or receiver, respectively.
The unique property of the stripline configuration is that the ratio of the mutual capacitance Cm to the total capacitance Ctot is equal to the ratio of the mutual inductance Lm to the total inductance Ltot, which eliminates the direct cross noise component Kf according to the equation (1). Since the back crosstalk Kb in equation (2) is the addition of two ratios, it is always present to some extent:
Near and far-end crosstalk in a microstrip configuration in which victim conductors are adjacent to an aggressor conductor (1.5 V at 1 GHz). In this case, the conductors are 4 mils wide, their impedance is 40 ohms, and their spacing is 4 mils.
Crosstalk quickly decreases with the square of the distance, and the degree of its effect depends on the voltage of the signal of the aggressor, the distance between the conductor segments, and their location relative to the layers.
In a microstrip configuration, the mutual capacitive coupling between adjacent conductors is usually weaker than the mutual inductive coupling, due to which the FEXT coefficient is negative, as can be seen from the simulation results. However, direct crosstalk does not exist in a stripline configuration.
The precise balance between inductive and capacitive coupled crosstalk virtually eliminates direct crosstalk. The crosstalk at the near end of a stripline configuration when the conductors are 4 mils wide, the impedance is 40 ohms, and the gap is 4 mils.
Note that there is no FEXT noise component. In addition, the peak crosstalk amplitude has been greatly reduced. Other things being equal, this is another good reason, why high-speed signals should always be routed through the inner layers of a multilayer printed circuit board.
Signals in an edge-coupled stripline can be placed closer together than they would be routed in a microstrip, freeing up more space for other conductors, which is always welcome.
The easiest way to reduce crosstalk from the nearest aggressor signal is to increase the gap between the signal wires. Crosstalk decreases very quickly with the square of the distance. For example, doubling the spacing between conductors can reduce crosstalk to about a quarter of its original level. The gap between them, as a rule, should be three times the width of the conductor.
However, in today's complex designs, it is not always possible to find enough PCB space to meet these requirements. In addition, the use of different technologies should be avoided, since higher voltages create crosstalk of greater amplitude.
Routing with long parallel wire segments should also be avoided. How edge coupling affects crosstalk when microstrip and stripline are used. Note that the amplitude of crosstalk in stripline is about a quarter of the amplitude in a microstrip.
In addition, crosstalk in microstrip is radiated by the outer layers of a multilayer printed circuit board, while in a stripline, the effect of electromagnetic fields between planes is limited.
The amount of crosstalk also depends on the load, which can vary significantly, for example when working with banks of memory modules. It should be remembered that the total crosstalk of each victim conductor is determined by the sum of the crosstalk emitted by several nearby aggressors.
Both forward and backward crosstalk can be reduced by placing the aggressor conductor further away from the victim conductor or by reducing the height of the dielectric layer above/below the reference planes.
On a note
In the second case, it is also necessary to reduce the width of the conductor in order to keep the impedance unchanged. If free space on the PCB is sorely lacking, as is usually the case in dense high-speed configurations, it is recommended to trace in the internal layers and eliminate cross-linking.