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Time-domain crosstalk measurement method for quality verification of printed circuit boards
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As the execution speed of digital systems in the fields of communication, video, network and computer technology is accelerating, the quality requirements for printed circuit boards (printed circuit boards) in such systems are also getting higher and higher. The early printed circuit board designs were unable to guarantee the system performance and working requirements in the face of increasing signal frequency and decreasing pulse rise time. In the current printed circuit board design, we must use the transmission line theory to model the printed circuit board and its components (edge connectors, microstrip lines and component sockets). Only by fully understanding the forms, mechanisms and consequences of crosstalk on printed circuit boards and using corresponding technologies to suppress them to the greatest extent can we help us improve the reliability of systems including printed circuit boards. This article mainly focuses on the design of printed circuit boards, but I believe that the content discussed in the article will also help other applications such as the characterization of cables and connectors.
The reason why printed circuit board designers care about crosstalk is that crosstalk can cause performance problems such as increased noise levels; harmful spikes; data edge jitter; and unexpected signal reflections.

Which of these issues will affect the design of the printed circuit board depends on many factors, such as the characteristics of the logic circuit used on the board, the design of the circuit board, the mode of crosstalk (reverse or forward), and interference lines and The termination of both sides of the interfered line. The information provided in this article can help readers deepen their understanding and research on crosstalk and reduce the impact of crosstalk on design.
In order to reduce the crosstalk in the printed circuit board design as much as possible, we must find a balance between capacitive reactance and inductive reactance, and strive to achieve the rated impedance value, because the manufacturability of the printed circuit board requires that the transmission line impedance be well controlled. After the circuit board design is completed, the components, connectors, and termination methods on the board determine which type of crosstalk will have much impact on the circuit performance. Using time-domain measurement methods, by calculating the inflection point frequency and understanding the printed circuit board crosstalk (Crosstalk-on-printed circuit board) model, it can help designers to set the boundary range of crosstalk analysis.
PCB patch processing time domain measurement method
In order to measure and analyze crosstalk, frequency domain technology can be used to observe the relationship between the harmonic components of the frequency in the frequency spectrum and the maximum EMI at these harmonic frequencies. However, the time domain measurement of the digital signal edge (the time it takes to rise from 10% of the signal level to 90%) is also a method of measuring and analyzing crosstalk, and the time domain measurement has the following advantages: the change of the digital signal edge Speed, or rise time, directly shows how high each frequency component in the signal is. Therefore, the signal speed (ie rise time) defined by the signal edge can also help reveal the mechanism of crosstalk. The rise time can be directly used to calculate the inflection point frequency. This article will use the rise time measurement method to explain and measure crosstalk.
In order to ensure that a digital system can work reliably, designers must study and verify the performance of the circuit design below the inflection point frequency. Frequency domain analysis of digital signals shows that signals higher than the inflection point frequency will be attenuated and will not have a substantial effect on crosstalk, while the energy contained in the signals below the inflection point frequency is sufficient to affect the operation of the circuit. The inflection point frequency is calculated by the following formula:
PCBA patch processing printed circuit board crosstalk model
The model provided in this section provides a platform for the study of different forms of crosstalk and clarifies how the mutual impedance between two microstrip lines causes crosstalk on the printed circuit board. Figure 1 is a conceptual transimpedance model. The mutual impedance is uniformly distributed along the two traces. Crosstalk is generated when the digital gate circuit sends a rising edge to the crosstalk line, and it spreads along the trace:
The mutual impedance between two traces on a printed circuit board.
1. Mutual capacitance Cm and mutual inductance Lm will couple to the adjacent interfered line or ‘crosstalk’ a voltage.
2. The crosstalk voltage appears on the interfered line in the form of a narrow pulse whose width is equal to the rise time of the interference line pulse.
3 On the interfered line, the crosstalk pulse splits into two, and then starts to propagate in two opposite directions. This divides the crosstalk into two parts: the forward crosstalk that propagates in the direction of the original interference pulse and the reverse crosstalk that propagates in the opposite direction to the signal source.
PCBA patch processing crosstalk type and coupling mechanism
According to the model discussed above, the coupling mechanism of crosstalk will be introduced below, and the two types of crosstalk, forward and reverse, will be discussed.
capacitive coupling mechanism. It is the interference mechanism caused by the capacitance in the circuit, including: when the pulse of the interference line reaches the capacitor, it will couple a narrow pulse to the interfered line through the capacitor; the amplitude of the coupled pulse is determined by the size of the mutual capacitance; then, the coupled pulse One divides into two, and starts to propagate in two opposite directions along the interfered line.
Inductance or transformer coupling mechanism. It is the interference caused by the inductance in the circuit, including: the pulse propagating on the interference line will charge the next position of the current spike; this current spike generates a magnetic field, and then induces a current spike on the interfered line; transformers Two voltage spikes of opposite polarity will be generated on the interfered line (negative spikes propagate in the forward direction, and positive spikes propagate in the reverse direction).
Reverse crosstalk. The capacitive and inductive coupling crosstalk voltage caused by the above model will produce an additive effect at the crosstalk position of the interfered line. The resulting reverse crosstalk includes the following characteristics: reverse crosstalk is the sum of two pulses of the same polarity; since the position of the crosstalk propagates along the edge of the interference pulse, the reverse interference appears as a low-level, wide pulse signal at the source of the interfered line And there is a corresponding relationship between its width and the length of the trace; the reflected crosstalk amplitude is independent of the pulse rise time of the interference line, but depends on the mutual impedance value.
Forward crosstalk. It needs to be reiterated that the capacitive and inductive coupling crosstalk voltage will accumulate at the crosstalk position of the interfered line. Forward crosstalk includes the following characteristics: Forward crosstalk is the sum of two reverse polarity pulses. Because the polarity is opposite, the result depends on the relative value of capacitance and inductance; forward crosstalk appears at the end of the interfered line as a narrow spike with a width equal to the rise time of the interference pulse; forward crosstalk depends on the rise time of the interference pulse. The faster the rising edge, the higher the amplitude, and the narrower the width; the forward crosstalk amplitude also depends on the length of the pair: as the position of the crosstalk propagates along the edge of the interference pulse, the forward crosstalk pulse on the interfered line will gain more energy .
Characterization of crosstalk inPCBA patch processing
In this section, we will use several measurement examples on single-layer printed circuit boards to study the generation mechanism of crosstalk and the settings of several crosstalk types introduced earlier. In order to effectively measure crosstalk in the laboratory, a wideband oscilloscope with a measurement bandwidth of 20GHz should be used, and a high-quality pulse generator should output a pulse with a rise time equal to the rise time of the oscilloscope to drive the circuit under test. At the same time, high-quality cables, termination resistors and adapters are used to connect the printed circuit board under test.