Characteristics of high-speed printed circuit boards
Fast PCB design has unique characteristics that enable it to handle high-speed frequency signals. Some of these features are as follows:
Low Dk and Df dielectric materials: Dielectric materials are extremely important for the signal integrity of high-speed printed circuit boards. For high-frequency applications, dielectric materials need to have low Dk (dielectric constant) and Df (dissipation factor). Materials with lower Dk will be able to handle higher frequency signals with less attenuation of the signal. Materials with low Df do not generate heat when processing high-frequency signals. The low Dk and Df of the material are crucial for signal integrity. Rogers 4000 series, Isola I-Tera, and Panasonic Megatron-6 dielectric materials are some popular high-speed materials.
Differential pair routing: Communication standards that require high-speed signals typically use differential pairs. Differential pair PCB routing is a design technique that creates balanced transmission systems to transmit differential signals on printed circuit boards.
The definition of difference means equal and opposite. Differential pairs are used for high-speed communication protocols such as Ethernet, RS-485, HDMI, DDR4, etc. Most differential pair signals have a predefined impedance that strictly follows the length of the differential pair signal. On a PCB, they are easily distinguished by two isolated parallel traces of wiring between two ICs or ports.
Impedance control routing: Impedance control is an important component of high-speed signals. High frequency signals have strict impedance tolerances. There are two types of impedance control traces on high-speed PCBs, namely differential pair impedance control traces and microstrip or single impedance control traces.
Length matching trace: The length matching on the PCB ensures that signal traces (especially high-speed signals) have the same length, mainly for time synchronization. Signals like GMII have 8 data lines and are time sensitive at high speeds. The length matched routing is easy to distinguish, and they usually have additional sine wave curves when routing along the channel.
Special stacked layers: High speed PCB boards typically have 4 or more layers. They have special stacking required to handle high-speed PCBs. High speed signals require a reference plane or return path, which is essential for the signal integrity of high-speed signals. This is why a dedicated layer is used as an empty ground reference plane. High speed printed circuit boards have at least one power plane, as high-speed lines are usually not routed on the same plane as the power network. High speed PCBs can have more than 2 core wires to improve insulation and crosstalk. High speed PCB boards require strict dimensional tolerances, as the impedance and track width of high-speed tracks depend on layer stacking.
Tracking geometry: High speed PCB boards have unique routing geometries. High speed lines are usually short and of uniform length. The high-speed signal wiring cannot have a 90 degree bending angle, and the bending is mostly formed by obtuse angles or curves. Some high-speed signals have matching lengths, so they can resemble sine waves.
EMI protection: High speed PCB boards are susceptible to EMI or electromagnetic interference, so special measures need to be taken to protect high-speed signals from EMI. Use through-hole stitching or through-hole pools with grounding networks on high-speed boards. Through hole stitching is typically used in conjunction with dedicated grounding planes to reduce EMI on PCBs. In high-speed communication, weak pull-down resistors are often used to pull down the receiver and transmitter pins, which can protect the PCB from EMI when the communication channel is idle.
Dedicated power supply transmission: High speed printed circuit boards have dedicated power layers used to power the components of the PCB. Usually, one of the inner layers of a PCB is used as a dedicated power layer, placed next to the ground layer. This approach also helps to reduce EMI. The power is transmitted to the IC through a through-hole. Please note that a single PCB can have multiple power layers. The power supply of the IC is provided by decoupling capacitors, which are used to filter high-frequency noise from the power grid.
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