RF PCB Design and the Factors to Consider

An RF PCB design can be quite different from a traditional board. It is distinguished by parameters such as impedance matching, type traces (preferably coplanar), elimination via (to avoid reflecting), ground planes and vias, and power supply decoupling. These boards also have important aspects like stack-up and material selection.

These factors make RF design more complex due to elements such as EMI interference and high-frequency signal channeling. 911EDA PCB design services team are experts in RF PCB design. We will be discussing all of these issues in detail in this article. Let us start with impedance matching.

Impedance Matching

Impedance matching is a fundamental concept in radio frequency (RF) circuit design. It ensures that the power transfer between source and load is maximized while minimizing signal distortion. The essence of impedance matching is to make sure the impedance remains consistent throughout the circuit trace. This constant impedance value is termed the “characteristic impedance.”

• Characteristic Impedance (Z):
  • The characteristic impedance is unique for every circuit trace and is denoted as Z. It serves as a standard against which the actual impedance of a circuit trace can be compared.
  • The achievement of a consistent characteristic impedance ensures minimal reflection and optimal power transfer within the RF circuits.
• Determinants of Characteristic Impedance:
  Several factors dictate the characteristic impedance of a trace:
  • Trace Width: The width of the trace plays a direct role in determining its impedance. Wider traces generally have lower impedance, whereas narrower traces have higher impedance.
  • Dielectric Constant of PCB Material: The material used in the printed circuit board (PCB) has an intrinsic property called the dielectric constant. This property can affect the speed and efficiency of the signal transmission.
  • Trace Thickness: The vertical dimension of the trace, or its thickness, also affects impedance. Thicker traces can handle more current but may also exhibit varying impedance levels.
  • Height from the Reference Plane: The distance of the trace from the PCB’s reference plane can also influence the characteristic impedance. This is especially important in multi-layer PCB designs.
• Designing Matching Circuits:
  • PCB designers have the task of ensuring that the impedances match throughout the circuit to ensure efficient performance.
  • By using matching circuits, designers can manipulate and match the varying impedances in different parts of the RF circuit. This process helps in maintaining consistent signal integrity and maximizing power transfer.

Radio-Frequency Board Materials

In the domain of electronic circuitry, radio-frequency (RF) printed circuit boards (PCBs) hold a distinct place due to their high-frequency operational needs. Manufacturing these boards necessitates the use of specialized materials to cater to these stringent requirements.

• Characteristics of RF PCB Materials:
  • Low Signal Losses: For a PCB to operate efficiently at high frequencies, it’s imperative that its constituent materials exhibit minimal signal losses.
  • Thermal Stability: High-frequency operations can generate significant amounts of heat. Thus, RF PCB materials should have a high thermal absorption capacity.
  • Consistent Dielectric Properties: Consistency in the dielectric constant (DK), loss tangent (tan-d), and coefficient of thermal expansion (CTE) across broad frequency ranges is essential. Typically, RF boards have dielectric constant values of around 3 to 3.5. Within the frequency spectrum of 10-30GHz, they display loss tangent values ranging from 0.0022 to 0.0095.
• Considerations Beyond Technical Specifications: While the aforementioned factors are vital, it’s equally important to weigh in the cost implications and the ease of manufacturing these boards. These considerations help strike a balance between performance and feasibility.
• Popular RF PCB Materials:
  • Polytetrafluoroethylene (PTFE): Known for its exceptional dielectric properties, PTFE is a prime choice.
  • Ceramics: Owing to their stability at high frequencies, ceramics are often incorporated into RF PCBs.
  • Hydrocarbons & Glass: A combination of hydrocarbons, either mixed or in conjunction with a form of glass, offers the desired consistency and performance.
• Rogers Materials: Among the popular materials used in RF circuit board design, Rogers materials stand out. They are offered in a myriad of variants to cater to different needs. Some of the well-known Rogers materials include:
  • RT/duroid: A versatile laminate known for its excellent dielectric constant.
  • RO3000: Tailored for high-performance needs, especially in wireless and broadband applications.
  • RO4000: Recognized for its blend of performance, cost-effectiveness, and ease of use.
  • Rogers TMM: A series that amalgamates thermoset polymer and ceramic, giving it unique characteristics.

RF PCB Stack-Up

The stacking sequence or “layer stack-up” of a radio-frequency (RF) printed circuit board (PCB) plays a crucial role in the board’s performance. Given the high-frequency nature of RF signals, there’s an inherent need to meticulously design the board layout, ensuring optimal signal integrity, reduced interference, and enhanced overall performance.

• Key Considerations for RF PCB Stack-Up:
  • Isolation: Maintaining isolation between components and traces is essential to reduce potential interference and crosstalk between adjacent circuits.
  • Power Supply Decoupling: Decoupling the power supply ensures that transient voltage spikes do not disrupt the functioning of RF components.
  • Layer Count & Arrangement: Determining the number of layers and their sequence in the stack-up is pivotal. Each layer serves a specific purpose, and its placement impacts the overall RF performance.
  • Component Placement: Strategic placement of components on different layers, based on their function, ensures minimal interference and optimal signal propagation.
• Standard 4-Layer RF Stack-Up Breakdown:
  • Top Layer: This is the primary layer where:
    • Radio-frequency components are mounted.
    • RF traces that carry high-frequency signals are routed.
  • Ground & Power Plane: Positioned directly beneath the top layer, this plane serves multiple functions:
    • Provides a reference plane for RF signals.
    • Acts as a shield, minimizing interference.
    • Facilitates efficient power distribution and grounding for the components.
  • Bottom Layer: This layer is designated for:
    • Housing non-RF components.
    • Routing non-RF trace information.
  • Benefits of this Arrangement: The described stack-up offers several advantages:
    • It minimizes interference by segregating RF and non-RF components.
    • The ground plane’s close proximity to the top layer ensures the shortest path for the ground return current, crucial for maintaining signal integrity.
    • This configuration is particularly beneficial for compact RF boards, maximizing performance while economizing space.

RF Trace Design

High-frequency signals propagate RF traces and are, therefore, susceptible to interference and transmission losses. The traces are transmission lines in radio-frequency boards. Coplanar waveguide, microstrip, and stripline are the most commonly used transmission lines. Designers are concerned about the characteristic impedance of these lines. Here are some radio frequency trace design considerations that ensure proper operation and minimal losses.

• It is crucial to keep the trace as short as possible. This reduces attenuation.
• Never place RF and standard traces parallel in a layout. Interference will result if the two are placed in this manner.
• Ground planes are needed to provide signal return routes.
• It is not recommended that test points be placed on the trace. It will disrupt the trace’s impedance matching value.
• For trace performance, it is better to gradually curve bends than to keep sharp right turns.

When right-hand bends are impossible, PCB designers can use the metering process to reduce their effects. Below is an illustration of how to measure a trace.

The following formula gives M:

Design ground planes

A return path is required for any radio-frequency trace or component or to allow current to propagate through it. This is done by a ground plane. Ground planes require additional design considerations. Let us take a look at them.

• Each RF layer should have a dedicated ground plane. To make the current flow path as short as possible, PCB designers should place the ground plane directly below each layer.
• The ground plane must be continuous. Breaks are not allowed. These breaks could open the way to shorter routes for the current to return.
• Two grounding vias must be installed for every shunt component placed in an RF transmission cable.

Via Design

In the domain of printed circuit board (PCB) design, particularly for radio-frequency (RF) applications, the design and placement of vias play a pivotal role. Vias, essentially tiny interconnections that run through the PCB layers, have the potential to introduce parasitic capacitance. This can, in turn, impact the high-frequency operations of the board. Given this, let’s delve deeper into the nuances of via design for RF PCBs.

• The Challenge with Vias in RF Design:
  • Parasitic Capacitance: The introduction of a via into the circuit board leads to unintended capacitance, which is termed parasitic capacitance. In high-frequency RF applications, this capacitance can introduce signal distortions and interfere with the desired operation.
  • High-Frequency Operations: RF circuits are inherently sensitive to even minute disturbances, making the parasitic effects of vias particularly critical.
• Guidelines for Optimized Via Design in RF PCBs:
  • Minimize Vias: The first rule of thumb is to avoid vias in RF traces as much as possible. This minimizes potential disturbances to the RF signals.
  • Dimensional Consistency: When vias are inevitable, ensuring specific dimensions and lengths can help minimize their adverse effects.
  • Multiple Parallel Vias: Introducing more parallel vias can distribute the parasitic capacitance, thereby reducing its impact on a single RF trace.
  • Dedicated Via for Each Pin/Pad: To ensure minimal interference, each pin or pad of a component should have its dedicated via.
  • Ground Plane Stitching: Using ground plane stitching can significantly minimize interference. This technique offers a shorter and more efficient return route for the ground current.
  • Layer-to-Layer RF Traces: Vias are vital when routing RF traces between different layers. However, this must be executed with caution to minimize signal integrity issues.
  • Maximize Vias between Inner & Top Ground Planes: For optimal performance, deploy as many vias as feasible between the inner and top layer ground planes. Moreover, these vias should maintain a specific distance from the signal trace, ideally at least 1/20th of the signal wavelength.

In essence, while the inherent challenges of vias in RF design cannot be entirely eliminated, with judicious design practices, their adverse impacts can be considerably mitigated. This ensures that the RF circuit retains its signal integrity and performs optimally across its desired frequency range.

Power Supply Decoupling

Radio-frequency boards require noise reduction to be effective. These boards are susceptible to noise at high frequencies. Noise removal is, therefore, a complex task. One of these methods is power supply decoupling.

Selecting decoupling capacitors

Decoupling capacitors removes any noise introduced to the circuit by the power supply. These capacitors are known as decoupling capacitors. These capacitors are connected to the power supply.

Impedance matching should be an integral part of every RF circuit board. The impedance of the circuit should remain constant after connecting the decoupling capacitors. To avoid impedance changes, follow the following design considerations:

1. For decoupling, always connect capacitors with the minimum impedance
2. To achieve minimum impedance, operate the capacitors at the self-resonant frequency (SRF). The SRF value of a capacitor will be inversely proportional to its capacitance.
3. Look for capacitors with a close SRF to the noise frequency.

Placing decoupling capacitors

It is vital to place decoupling capacitors correctly. Below is a circuit that shows two decoupling capacitors being placed in parallel with an IC.

The higher capacitor is used to filter low-frequency noise and store energy. The lower capacitor filters out the high-frequency noise. These are other placement guidelines:

• The components and the decoupling capacitors should be placed on the same surface.
• Place the capacitors parallel to the signal flow path.
• Each capacitor should have its own ground via.
• Place the capacitors in ascending order according to their capacitance. The capacitor with the lowest capacitance is the closest to the power supply.

Both manufacturers and designers need to pay more attention to RF boards’ design and fabrication process. The DFM team should follow the design checklist. These boards are susceptible to interference and high-frequency noises, so even the slightest mistake can significantly impact the operation. These aspects and other methods will assist us in improving our designs.

Conclusion and Key Takeaways

Radio-frequency (RF) PCB design is a sophisticated area of electronics that demands meticulous attention to various parameters, distinct from traditional PCB design:

1. Impedance Matching: Central to RF PCB design is the concept of impedance matching, ensuring optimal power transfer between source and load. The goal is to achieve a consistent “characteristic impedance” throughout the circuit trace.
2. RF PCB Materials: Material selection is pivotal. The chosen materials should have low signal losses, high thermal stability, and consistent dielectric properties. Popular choices include Polytetrafluoroethylene (PTFE), ceramics, hydrocarbons mixed with glass, and Rogers materials.
3. Stack-Up: The RF PCB stack-up plays a crucial role in overall performance. Proper layering can ensure signal integrity, reduce interference, and offer a balanced design that maximizes both space and functionality.
4. RF Trace Design: High-frequency signals demand particular attention to trace design. Critical considerations include minimizing trace length to reduce attenuation, ensuring no parallel placement of RF and standard traces, and optimal bending of traces.
5. Via Design: Vias, though essential for interconnections, can introduce parasitic capacitance. Minimizing vias, ensuring dimensional consistency, and deploying techniques like ground plane stitching can mitigate potential disruptions.
6. Power Supply Decoupling: Noise reduction is paramount for RF PCBs. Power supply decoupling, involving the use of decoupling capacitors, acts as a filter to eliminate noise from the power supply. Proper capacitor selection and placement are crucial to maintain impedance stability.
7. Design Prudence: The sensitivity of RF boards to interference and high-frequency noise underscores the importance of attention to detail. Both designers and manufacturers must be rigorous in their design and fabrication processes to avoid compromising the functionality of the board.

In essence, RF PCB design is a domain where precision, informed material selection, and thoughtful design strategies intersect. Adhering to best practices will ensure that these boards function optimally and reliably across their intended frequency ranges.