Why Is Impedance Matching Essential for High-Speed PCB

When designers delve into high-speed PCB, one crucial concept they encounter is impedance. Impedance refers to the electrical impedance of signal transmission lines within a printed circuit board, serving as a vital parameter that dictates the relationship between current and voltage. So, why is impedance matching essential for high-speed PCB? Which signal lines require impedance control? What impact does this have on circuits? In this article, TechSparks will explore these questions.

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Why Impedance Matching is Essential for High-Speed PCB

Impedance matching refers to the adjustment or design of parameters such as resistance, inductance, capacitance, and other components by circuit designers in order to match the electrical impedance between different parts of the circuit or between the power source and the load. The ultimate goal is to ensure that all high-frequency microwave signals can be transmitted to the load point without signal reflection.

In the modern field of electronics, different types of circuit boards are distinguished to strike a balance between performance and cost. For low-speed PCB circuits, where the signal rise time is relatively long, waveform distortion and signal attenuation in signal transmission lines usually do not pose significant issues, making impedance matching less critical. However, in high-speed PCB circuits, impedance matching is crucial to ensure accurate, interference-free, and noise-free signal transmission. Without proper impedance matching, well-formed signal waveforms can deform, leading to defects such as signal reflections and crosstalk during the signal transmission process. In severe cases, the original functionality of the circuit may be lost. Here, TechSparks lists the purposes of impedance matching in high-speed circuits:

  • Ensure maximum power transfer between the power source and the load to fully utilize the energy source.
  • Help extend the bandwidth of the circuit, allowing it to transmit higher-frequency signals.
  • Avoid waveform distortion caused by signal reflections during the signal transmission process.
  • Mitigate threats such as electromagnetic interference (EMI) and crosstalk in high-speed PCBs through impedance matching.
  • Prevent signal loss or absence in high-speed digital and high-frequency analog circuits.

Factors Affecting Impedance in High-Speed PCB

Dielectric Constant

Different PCB substrate offer varying dielectric constants. Common options include paper-based, epoxy fiberglass, and composite substrates. Among these, cost-effective FR-4 material is often the preferred choice. The dielectric constant of this material can vary with frequency, approximately 4.2 at 1 GHz and decreasing around 1.5 to 2.0 GHz. To mitigate the impact of the dielectric constant on applications, considerations related to the operating frequency should be incorporated into the design.

Dielectric Thickness

Designers should strive for precision in dielectric thickness to avoid impedance issues in high-speed multilayer PCBs. In FR-4 material, the dielectric thickness results from various semi-cured sheets combined, further categorized into core thickness and prepreg thickness. Thinner dielectrics lead to lower impedance, while thicker dielectrics result in higher impedance.

Line Width and Spacing

TechSparks recommends using a PCB trace width calculator during the high-speed PCB design phase to compute critical values for impedance matching. Narrow spacing and width contribute to high impedance, while wider spacing and traces lead to lower impedance.

High Speed PCB Impedance Line Selection

Impedance lines can be divided into single-ended impedance, differential impedance, and coplanar impedance. In multilayer circuits, the reference points for single-ended and differential lines are typically the adjacent copper layers. It is advisable to use a through-layer reference approach during the design to ensure optimal RF antenna performance. For double-layer circuit structures, coplanar impedance can be considered. This means using copper layers to match the impedance of single-ended or differential lines, ensuring that the copper layers on both sides of the signal lines achieve the required impedance matching for high-speed PCB.

High Speed PCB Single-Ended Impedance
Single-Ended Impedance
High-Speed PCB Differential Impedance
Differential Impedance

High-Speed PCB Impedance Matching Requirements

As mentioned earlier, not all lines require impedance matching in high-speed PCB. So, which ones do, and how much?

In high-speed PCB design, the lines that need impedance matching consideration include transmission lines, clock lines, high-speed high-frequency signal lines, power, and ground lines, among others. Different types of signal lines have varying impedance requirements, typically ranging from 90 ohms, 100 ohms, 120 ohms, and more. Here are some common impedance matching requirements:

  • USB 2.0 requires maintaining a 90-ohm impedance.
  • High-speed interfaces like HDMI, USB 3.0, MIPI, Gigabit, and Ethernet often require a 100-ohm impedance.
  • For specific applications like RS422, impedance matching requirements are higher, typically around 120 ohms.
  • Single-ended signal lines generally need to maintain a 50-ohm impedance.
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