PCB Blog - Crosstalk Measurement Method for PCB board quality verification

With the ever-increasing speed of operation of digital systems in the fields of communications, video, networking and computer technology, the demands on the quality of printed circuit boards in such systems are also increasing. The early PCB board design has been unable to guarantee the system performance and working requirements in the face of increasing signal frequency and shortening pulse rise time. In current PCB board design, we need to use transmission line theory to model the PCB board and its components (edge connectors, microstrip lines, and component sockets). Only by fully understanding the forms, mechanisms and consequences of crosstalk on the PCB, and using corresponding technologies to suppress it, can we help us improve the reliability of the system including the PCB. This article focuses on PCB board design, but it is believed that the content discussed in this article will also be useful in other applications such as cable and connector characterization. PCB designers are concerned with the phenomenon of crosstalk because it can cause performance problems such as increased noise levels, unwanted spikes, jitter on data edges, and unexpected signal reflections. Which of these issues will affect the PCB design depends on many factors, such as the characteristics of the logic circuits used on the board, the design of the board, the mode of crosstalk (reverse or forward), and the interference line and the interfered. Termination on both sides of the wire. The information provided below can help readers improve their understanding and research on crosstalk, thereby reducing the impact of crosstalk on designs.

Methods of studying crosstalk
In order to minimize crosstalk in PCB board design, we must find a balance between capacitive and inductive reactance, and strive to achieve the rated impedance value, because the manufacturability of the PCB board requires the transmission line impedance to be well controlled. After a circuit board design is complete, the components, connectors, and terminations on the board determine how much of an impact the type of crosstalk has on circuit performance. Using time domain measurements, by calculating the corner frequency and understanding the PCB board crosstalk (Crosstalk-on-PCB board) model, it can help designers set the boundaries of crosstalk analysis.

Time Domain Measurement Methods
To measure and analyze crosstalk, frequency domain techniques can be used to observe the relationship between the harmonic components of the clock in the frequency spectrum and the EMI values at those harmonic frequencies. However, the time domain measurement of digital signal edges (the time it takes to rise from 10% to 90% of the signal level) is also a means of measuring and analyzing crosstalk, and time domain measurement has the following advantages: Speed, or rise time, is a direct indication of how high each frequency component is in the signal. Therefore, the signal speed (ie, rise time) defined by the signal edges can also help reveal the mechanism of crosstalk. The rise time can be used directly to calculate the corner frequency. This article will describe and measure crosstalk using the rise time measurement method.

Knee frequency
To ensure reliable operation of a digital system, the designer must study and verify the performance of the circuit design below the corner frequency. Frequency domain analysis of digital signals shows that signals above the knee frequency are attenuated so that they do not have a substantial effect on crosstalk, while signals below the knee frequency contain enough energy to affect circuit operation. The knee frequency is calculated by: fknee = 0.5/trise

PCB board crosstalk model
The models presented in this section provide a platform for the study of different forms of crosstalk and illustrate how the mutual impedance between two microstrip lines can cause crosstalk on a PCB. The mutual impedance is evenly distributed along the two traces. Crosstalk occurs when the digital gate hits a rising edge to the crosstalk line and propagates along the trace:
1) Both the mutual capacitance Cm and the mutual inductance Lm will couple or "crosstalk" a voltage to the adjacent disturbed line.
2) The crosstalk voltage appears on the disturbed line in the form of a narrow pulse whose width is equal to the rise time of the pulse on the disturbing line.
3) On the disturbed line, the crosstalk pulse is divided into two and then starts to propagate in two opposite directions. This splits the crosstalk into two parts: forward crosstalk propagating in the direction of propagation of the original interfering pulse and reverse crosstalk propagating in the opposite direction to the signal source.

Types of Crosstalk and Coupling Mechanisms
Based on the model discussed earlier, the coupling mechanism of crosstalk is described below and the two types of crosstalk, forward and reverse, are discussed.
Interference mechanism caused by mutual capacitance in the circuit:
When the pulse on the disturbing line reaches the capacitor, a narrow pulse will be coupled to the disturbed line through the capacitor. The amplitude of the coupled pulse is determined by the magnitude of the mutual capacitance. The coupled pulse then splits in two and begins to propagate in two opposite directions along the disturbed line.

Inductive or Transformer Coupling Mechanisms
Mutual inductance in a circuit can cause disturbances such that a pulse propagating on the disturbance wire will charge the next location where the current spike is present. This current spike generates a magnetic field, which then induces a current spike on the disturbed wire. The transformer produces two voltage spikes of opposite polarity on the disturbed line: the negative spike propagates forward, and the positive spike propagates backward.

reverse crosstalk
The capacitively and inductively coupled crosstalk voltages caused by the above model have an additive effect at the crosstalk location of the disturbed wire. The resulting reverse crosstalk has the following characteristics: The reverse crosstalk is the sum of two pulses of the same polarity. Since the crosstalk position propagates with the edge of the interference pulse, the reverse interference appears as a low-level, wide pulse signal at the source end of the interfered line, and its width corresponds to the length of the trace. The reflected crosstalk magnitude is independent of the interfering line pulse rise time, but depends on the mutual impedance value.

forward crosstalk
To reiterate, capacitively and inductively coupled crosstalk voltages accumulate at the crosstalk location of the victim line. Forward crosstalk includes some of the following characteristics: Forward crosstalk is the sum of two reverse-polarity pulses. Because the polarities are reversed, the results depend on the relative values of capacitance and inductance. Forward crosstalk appears at the end of the victim line as a narrow spike with a width equal to the rise time of the aggressor pulse. Forward crosstalk depends on the rise time of the interfering pulse. The faster the rising edge, the higher the amplitude and the narrower the width. The forward crosstalk magnitude also depends on the pair length: as the crosstalk position propagates along the edge of the aggressor pulse, the forward crosstalk pulse on the victim wire will gain more energy.

Instruments and Setup
To effectively measure crosstalk in the laboratory, a wideband oscilloscope with a measurement bandwidth of 20 GHz should be used, and the circuit under test should be driven by a high-quality pulse generator outputting a pulse with a rise time equal to the rise time of the oscilloscope. At the same time, high-quality cables, terminating resistors and adapters are used to connect the PCB under test. A TDR step voltage generator capable of generating narrow pulses of 250mv with a rise time of 17ps and output with a source impedance of 50 ohms. The tester only needs to connect the PCB to be tested.

Forward Crosstalk Measurement
If only measuring forward crosstalk, terminate all traces to eliminate reflections. Forward crosstalk should be measured at the end of a well-terminated victim wire. The instrument setup is shown in Figure 6.

The effect of circuit design on crosstalk
Although crosstalk can be reduced and its effects attenuated or eliminated through careful PCB board design, there may still be some residual crosstalk on the board. Therefore, when designing the circuit, an appropriate line end load should also be used, because the line end load will affect the magnitude of the crosstalk and the weakening degree of the crosstalk over time. How end-of-line loads at the end of a trace and at the output of a logic gate attenuate crosstalk and reduce its causes.