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Crosstalk Solutions Employed in PCBs and VLSI Circuits

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • When energy gets coupled between two or more lines that are placed adjacent to each other, it creates crosstalk.

  • Stub alternated and parallel serpentine microstrip lines in PCBs alter capacitive coupling and equalize it to inductive coupling, which reduces crosstalk. 

  • An error correction technique can be a single error correction or multiple error corrections, depending on factors such as complexity, efficiency, delay, and power consumption.

VLSI circuits

Crosstalk is a serious limitation in VLSI circuits and  printed circuit boards (PCBs)

In the electronics market, reducing the size of an electronic circuit or device with the same or improved operational features compared to its present model can save money. As the size is minimized with advanced features, it leads to a high-density internal system. In these devices, subcircuits, wires, and components are in close proximity to achieve high packing density. However, miniaturization in electronics faces a problem of unwanted coupling of signals in adjacent lines, otherwise called crosstalk.

In high-performance circuits or system design, crosstalk is a major issue. Crosstalk is a serious limitation in VLSI circuits, printed circuit boards (PCB), optical networks, communication channels, etc. Crosstalk solutions are necessary for any system that is affected by crosstalk to maintain the reliability, signal integrity, and output quality of the system. In this article, we will explore crosstalk and some crosstalk solutions employed in PCBs and VLSI circuits.

What Is Crosstalk?

When energy gets coupled between two or more lines that are placed adjacent to each other, it creates crosstalk. The reasons for crosstalk vary with the system. Some common causes include:

  1. Non-linear system behavior

  2. Coupling between signal transmission lines

  3. Impedance imbalance in transmission lines or wires

  4. Poor performance of filters or filter defects

  5. Poor control of frequency responses in analog systems

Crosstalk causes an undesired influence on the signal in the adjacent pair or channel. When several transmission lines or interconnections are packed into a congested space, the signals propagating through them radiate out and interfere with the signals in the neighborhood. The transmission line that is the source of noise is called the aggressor and the adjacent line affected by the aggressor is called the victim. Crosstalk can be initiated from any end of the victim such as forward crosstalk and backward crosstalk. There are several different types of crosstalk, including:

  • Near-end crosstalk (NEXT)

  • Far-end crosstalk (FEXT)

  • Alien crosstalk (AXT)

Irrespective of type, crosstalk degrades signal transmissions, signal-to-noise ratios (SNR), and challenges the signal integrity and reliability of the system.

Crosstalk can affect one entire system, a part of it, or adjacent systems. Various mitigation techniques and methods can be used to mitigate crosstalk, and these methods vary from one system to another. The crosstalk reduction techniques employed in PCBs are different from ICs and so on. 

Crosstalk Solutions in PCBs

Closely packed striplines and microstrip lines in high-density PCBs induce crosstalk. They transmit high-speed signals and provide chip-to-chip interconnections. Signal routing with only an acceptable crosstalk level is essential for preventing integrity problems and performance degradation.

Increasing the distance between adjacent lines is the most potent solution for crosstalk problems in PCBs. However, this increases the real estate of the PCB and challenges the electronic market’s demand for miniaturization. Providing solid return paths and using differential signaling schemes can alleviate crosstalk in PCBs. 

Inserting guard trace between the signal transmission lines is a crosstalk solution that forms a shield between the aggressors and victims. Impedance imbalance is a shortcoming that prevents the effectiveness of guard trace and the via-stitched guard trace is introduced to offset this limitation. A via-stiched guard trace helps to maintain a constant potential throughout the signal transmission line and prevent crosstalk.

Stub alternated (tabbed) micro and parallel serpentine microstrip lines are reported to reduce FEXT crosstalk. Stub alternated microstrip lines utilize stubs that are uniformly distributed over one of the parallel lines. Single stub as well double stub arrangements are employed. In parallel serpentine microstrip lines, parallel lines are made in a serpentine form. Both the stub alternated and parallel serpentine microstrip lines alter the capacitive coupling and equalize it to the inductive coupling, thus reducing crosstalk. 

Crosstalk Solutions in VLSI  Circuits

In VLSI circuits, switching is one of the major contributors to crosstalk. The crosstalk in VLSI circuits can be minimized by encoding bits such that switching activities are reduced. Various error correction and detection codes are employed in VLSI circuits to reduce crosstalk-induced errors.  An error correction technique can be a single error correction or multiple error correction, depending on factors such as complexity, efficiency, delay, and power consumption. Examples of the single error correction codes employed in VLSI circuits include:

  • Boundary shift codes (BSC)

  • Duplicate add parity (DAP)

  • Modified dual rail (MDR)

  • An extended version of the hamming code, popularly known as single error correction double error detection (SECDED)

The crosstalk avoidance double error correcting (CADEC) code and the joint crosstalk avoidance and triple error correction (JTEC) code are the multiple error correction codes that offer high throughput and energy savings.

Crosstalk solutions are employed in PCBs and VLSI circuits to improve system-level crosstalk suppression. In PCBs, the user has the freedom to design within the given space constraints and can utilize routing-based crosstalk mitigation methods. VLSI circuits such as ICs are off-the-shelf products for the user, and the choice is to use error correction and detection codes for crosstalk reduction. Cadence offers a suite of design and analysis tools that help improve the signal integrity of a system’s design by eliminating crosstalk.

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