3.1 Crosstalk Mitigation
Crosstalk refers to unwanted voltage noise interference that occurs due to electromagnetic coupling between adjacent transmission lines as the signal propagates along the line. Excessive crosstalk can lead to false triggering of circuits and prevent the system from functioning properly.
Since crosstalk is inversely proportional to the spacing between lines, it increases with the parallel length of the lines. Additionally, crosstalk is influenced by the load of the circuit— for the same topology and routing, higher loads result in greater crosstalk. Furthermore, crosstalk is frequency-dependent. In digital circuits, the rate of signal edge transitions has the most significant impact on crosstalk: the faster the edge transitions, the more pronounced the crosstalk.
Based on these characteristics, the following strategies can help reduce crosstalk:
(1) Minimize the transition rate of signal edges wherever possible. When selecting components, choose devices that, while meeting design specifications, offer slower switching speeds. Additionally, avoid mixing different types of signals, as high-speed signals pose a potential crosstalk risk to slower signals.
(2) The crosstalk caused by capacitive and inductive coupling increases as the load impedance of the affected line rises. Therefore, reducing the load impedance can help mitigate coupling interference.
(3) When wiring conditions allow, try to minimize the parallel length between adjacent transmission lines or increase the spacing between capacitively coupled traces. For instance, apply the 3W principle (the spacing between traces should be at least three times the width of a single trace, or the distance between two traces should be greater than twice the width of one trace). A more effective approach is to isolate the traces using ground planes.
(4) Inserting a ground trace between adjacent PCB signal traces can also significantly reduce capacitive crosstalk. This ground trace should be connected to the ground at intervals of 1/4 wavelength.
(5) Inductive coupling is challenging to mitigate, so it is advisable to minimize the number of loops, reduce the loop area, and avoid using the same trace for multiple signal loops.
(6) The signal traces on two adjacent layers should be perpendicular, with parallel traces being avoided as much as possible to minimize crosstalk between layers.
(7) The surface layer has only one reference layer, and its coupling is stronger than that of the inner layers. Therefore, signals more sensitive to crosstalk should preferably be placed on the inner layers.
(8) By implementing termination at both the near and far ends of the transmission line, and matching the terminal impedance to the transmission line impedance, crosstalk and reflection interference can be greatly reduced.
### 3.2 Reflection Analysis
When a signal propagates along a transmission line, any impedance change will cause reflection. The primary solution to the reflection issue is terminal impedance matching.
### 3.2.1 Typical Transmission Line Termination Strategies
In high-speed digital systems, impedance mismatches along the transmission line lead to signal reflection. To reduce or eliminate these reflections, terminal impedance matching should be applied either at the source or the load, ensuring that the reflection coefficient at the source or load is zero. Termination technology should be used if the transmission line length meets the following condition:
**L > tr / (2 * tpd)**
Where L is the transmission line length; tr is the rise time of the source signal; tpd is the load transmission delay per unit length.
There are two common termination strategies for PCB transmission lines:
1. **Parallel Termination**
Parallel termination involves connecting a pull-up or pull-down resistor as close as possible to the load to achieve impedance matching at the terminal. Depending on the application environment, parallel termination can be categorized into several types, as shown in Figure 2.
2. **Serial Termination**
Serial termination is achieved by inserting a resistor near the source end of the transmission line. This technique matches the impedance of the signal source. The resistance of the serial resistor, in combination with the output impedance of the driving source, should be equal to or greater than the transmission line impedance.
This strategy minimizes reflections from the load (especially when the load is high impedance and does not absorb energy) by making the reflection coefficient at the source end zero, preventing further reflection back from the load to the source.
### 3.2.2 Termination Techniques for Different Process Devices
Impedance matching and termination techniques vary depending on the interconnection length and the series of logic devices used in the circuit. The appropriate termination method should be selected based on the specific situation to effectively reduce signal reflections.
Typically, CMOS devices have relatively stable output impedance values close to the transmission line impedance. For CMOS devices, serial termination is generally more effective. In contrast, TTL logic devices have different output impedances depending on whether the output logic is high or low. In this case, a parallel Thevenin termination scheme is more suitable. ECL devices typically have very low output impedance, so a pull-down resistor at the receiver end of the ECL circuit is recommended to absorb energy.
However, these methods are not absolute. Factors such as the specific circuit design, network topology, and the number of loads at the receiving end can influence the termination strategy. Therefore, when designing high-speed circuits, it is crucial to choose the most appropriate termination scheme to achieve optimal signal integrity.
### 4. Signal Integrity Analysis and Modeling
Effective circuit modeling and simulation are commonly used solutions for signal integrity analysis. In high-speed circuit design, simulation analysis has proven to offer numerous advantages. It provides designers with accurate and intuitive results, helping to identify potential issues early and make timely adjustments. This ultimately shortens design time and reduces costs. The three most commonly used models are: SPICE, IBIS, and Verilog-A.
– **SPICE** is a powerful general-purpose analog circuit simulator, comprising two parts: Model Equations and Model Parameters. The SPICE model is highly integrated with the simulator’s algorithms, allowing for efficient analysis and accurate results.
– **IBIS** is specifically designed for analyzing digital signal integrity at the PCB board and system levels. It uses I/V and V/T tables to describe the characteristics of I/O units and pins in digital integrated circuits. The accuracy of IBIS modeling depends on the number of data points and the quality of data in the I/V and V/T tables. Compared to SPICE, IBIS requires less computational power.
### 5. Simulation Verification
An example circuit of an asynchronous transceiver is used to demonstrate the simulation results. In this simulation, the excitation signal is set to 50 ns, the power supply to 5V, and all other settings are default. The simulation of the U3-5 pin of the RTSB network is shown in Figure 3.
Curve a represents the signal waveform before termination, clearly showing significant reflection. Curves b and c depict the waveforms after ground termination with different resistance values. Curve d shows the waveform after Thevenin termination. From the figure, it can be observed that the termination resistor effectively eliminates the reflection. However, the drawback is that the termination resistor causes the ground high-level voltage to drop, and the power low-level voltage to rise.
As microelectronics technology continues to advance, the use of high-speed devices and the design of high-speed digital systems are on the rise. As system data rates, clock rates, and PCB densities increase, the design requirements for PCBs become more demanding. Signal integrity has become a critical concern. To ensure good signal integrity, it is essential to account for various influencing factors, properly plan and route the design, and ultimately improve product performance.
