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High Frequency PCB Design Rules to Know About

High frequency PCB design in Altium

High-frequency PCB design is demanding and requires careful attention to every design detail. That’s because the higher the frequency, the more sensitive signals become to interference and loss. This article compiles high-frequency PCB design guidelines, from the type of materials used to the trace and stack up or component layout.

What is a High-Frequency PCB?

A high-frequency PCB is a printed circuit board that transmits radio waves or RF signals. To understand how it works, we must first understand what high-frequency circuits are. After that, we’ll see what distinguishes a high-frequency circuit board from other types of PCBs.

High-Frequency Circuits

High-frequency circuits are those that work within the radio frequency spectrum or circuits that operate in the MHz to GHz range. They generally require specific components, materials, and manufacturing processes to help them function properly.

These circuits are generally used in communication technologies, such as satellite systems, mobile phones, Wi-Fi networks, etc. High-frequency circuit design, therefore, requires an understanding of RF design principles and components.

High-frequency Circuit Board

A high-frequency circuit board is a PCB containing components and materials to transmit and receive RF signals specifically. In addition to these components, a high-frequency circuit board includes particular dielectric and conductor materials types.

High-frequency circuits are typically affected by the slightest changes in power, inductance, capacitance, and other electrical properties. As such, ensuring that your board adheres to specific design rules is essential.

High-Frequency PCB Design

High-frequency PCB design includes everything necessary to create a PCB that can accurately and effectively transfer RF signals. This is usually possible while designing a circuit board — and necessary.

For example, the board’s stack up, component placement, and materials must be carefully considered to ensure the signal travels appropriately. Other considerations include the trace width, which will depend on the signal frequencies used, as well as impedance control, which is important for RF signals.

Just as when designing other types of boards, high-frequency PCB design is made possible by combining tools in the board software. The software allows for the proper placement of components, setting up impedance control circuits, and routing traces — all of which are necessary to ensure that the board works properly.

High-frequency circuit diagram
High-frequency circuit diagram
Resource: https://www.edn.com

High-Frequency PCB Design Rules

In the design of a high-frequency PCB, specific design rules apply. These are rules that help designers to eliminate crosstalk, for example, or rules that allow them to introduce impedance control. To ensure your board design is up to par with high-frequency standards, consider the following:

High-Frequency PCB Materials

A high-frequency circuit board requires specific materials that are designed to transmit radio signals efficiently. These include special substrates, laminates, and copper foil, as well as any other material used in the board’s construction.

Each of these high-frequency PCB materials should be carefully chosen for its ability to handle high-frequency signals without interference or signal loss. The substrate or laminate, in particular, should be designed for RF, or there will be signal degradation and other issues.

FR-4 is usually not recommended due to its higher dielectric loss at higher frequencies. Instead, other materials, such as Rogers 4350B, Isola, and Tachyon, are preferred. These show great performance when it comes to high-frequency applications.

High-Frequency PCB Stackup

The stack-up is an essential factor in high-frequency circuit design. The number of layers and their arrangement will directly affect a signal’s performance. A standard high-frequency PCB stack-up will include power planes, several ground planes, and signal layers between those power and ground planes.

To avoid crosstalk in such stack-ups, always use ground planes to isolate signal layers. Similarly, increase the distance between signal layers to minimize crosstalk.

Other high-frequency PCB design rules for stack-up include using or perpendicular orientation of adjacent layers and using micro-vias to decrease inductance and improve signal integrity. Digital signals should also be separated from analog signals for better performance.

Routing High-Frequency PCBs

The key to a successful high-frequency PCB layout is to ensure signal loss is highly minimized. To accomplish this, a designer must pay close attention to routing and component placement.

To avoid coupled interference, consider using orthogonal routing to keep the noise of the signal low. Microstrip and stripline routing techniques can also be used to reduce signal loss.

A fly-by topology of long signal traces, with as much separation between each stack layer as possible, will help reduce reflections and other signal loss.

Pay attention to the length of leads between the pins of your high-frequency circuit. Shorter leads reduce signal loss. If longer leads are necessary, ensure they have a consistent length between pins for optimal performance.

High-Frequency PCB Impedance Control

When designing a high-frequency board circuit board, it’s crucial that you maintain impedance control. To ensure that, several design rules must be followed. First, ensure that you use the correct trace width for your desired impedance. Shorter traces help reduce the inductance, while wider traces can help increase the impedance.

Use the two or 3W rule to space the traces and control their impedance. This rule states that you should space your traces at least 2 or 3 times their width apart for optimal impedance control.

As mentioned earlier, use ground planes to reduce crosstalk and signal loss. Positioning the ground traces directly underneath the signal trace will help to achieve better impedance control.

Pay attention to layer transitions when routing high-frequency circuit signals. Make sure that you transition from one layer to another as smoothly as possible, using micro vias to avoid signal reflections.

Conclusion

High-frequency PCB design requires special considerations to ensure the board’s optimal performance. These include taking into account the rules for material selection, stack up, routing, and impedance control. By following these guidelines and taking the necessary steps, designers can create a high-frequency board that will yield reliable results.

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