What You Need to Know About RF Circuit Boards
A rf circuit board is an electro-magnetic printed circuit board that is used for communication of RF signals. It needs to meet stringent impedance targets and geometric tolerances. It also requires a specific stackup and a continuous ground plane under all traces that carry RF signals, as high frequencies cannot tolerate discontinuities in the ground planes.
Material
When it comes to RF PCBs, the material used to build the boards needs to fulfill several requirements. It must have low signal losses, be stable over high-frequency operation, and provide consistent loss tangent values and coefficients of thermal expansion (CTE). It also needs to be moisture resistant.
For these reasons, many RF PCBs use polytetrafluoroethylene (PTFE)-based materials. PTFE has a lower dielectric constant than other common materials, and its loss tangent remains stable over a wide range of frequencies. Its moisture absorption rate is also relatively low.
Other material options include FEP and LCP, which offer lower lamination temperatures and re-melt temperatures. These are ideal for applications that don’t require soldering or thermal stressing.
It is important to choose a material that can handle the physical stresses of drilling and assembly. The material must also have a good CTE to handle the thermal stresses of soldering. A poor CTE can cause the board to crack and fail during the assembly stage. The best RF PCB materials have a CTE that is low enough to prevent these problems.
Layout
RF circuit boards are typically multi-layered. The top layer carries RF components and traces, while the ground and power planes are located on adjacent layers. The choice of material for each layer must take into account thermal properties and cost. It is important to choose a manufacturer who understands the subtle nuances of these types of PCBs. For example, a RF design may require unusually-shaped metal areas for specialized traces.
Keeping signal interference to a minimum rf circuit board requires careful consideration of the layout of the PCB components and the trace widths. It is also crucial to follow standard high-frequency design rules and use good CAD tools. These tools must be able to interface with the RF simulation software to accurately calculate impedance targets and geometric tolerances.
Traces carrying RF signals should be as short as possible and routed away from high-speed digital traces. They should also be kept away from power supply lines, which can cause interference. Additionally, it is a good idea to include decoupling capacitors in the design. One method is to use a star configuration with large decoupling capacitors at the center, and smaller ones at each branch of the power supply.
Vias
RF circuit boards use vias to connect the layers of the PCB together. These vias need to be properly designed to prevent ground current loops that can increase parasitic ground inductance and cross-coupling of RF signal lines. It is also important to choose the right bonding material for these vias. For example, FEP and LCP have lower lamination temperatures than ceramic filled PTFE. They are also less sensitive to thermal stressing during assembly and operation.
During the manufacturing process, via holes are filled with copper. They are then plated and etched. There are several different types of vias used in a multi-layer board. The most common is a through-hole via, which is drilled right through the PCB layers. Another type of via is a blind hole, which connects internal layers of the PCB but cannot be seen from the outside. This is called a buried via.
RF circuits often need to pass power and return currents from the component layer to the assigned ground plane on the bottom of the board. The ground paddle’s secondary function is to dissipate heat, and it should contain the maximum number of via holes that other layout considerations allow.
Power
The power supply for the RF circuit board must be placed in an appropriate location. It should be away from high-speed digital signal lines to avoid interference. This is because digital noise from clocks and PLLs can link into the RF RF Circuit Board Supplier carrier and interfere with it. Also, it is a good idea to use smaller decoupling capacitors for each power connection to reduce losses.
When routing long power lines, a high-frequency filter should be installed at each end of the line to prevent loops. This will reduce the power supply current radiation and the common ground impedance.
RF circuits are more sensitive to interference than conventional circuit boards, so they require tighter tolerances. The smallest details of an RF PCB can affect performance, including crosstalk, skin effect, and coupling between signal lines. The best way to ensure these details don’t cause problems is to work with a reputable RF PCB manufacturer. They understand the subtle nuances of RF PCB design and are capable of providing fast turnaround times. This helps to avoid delays in the manufacturing process and minimizes potential quality issues.
Ground
The RF circuit board uses the ground connection to prevent interference with other circuits and components. It should be placed as close to the signal lines as possible and connected to the main ground. The ground connections should be made as thick as possible to reduce the ground impedance and prevent interference. RF signal traces should also be rounded and chamfered, as they can cause significant reflection when bent at right angles.
The characteristic impedance of an RF transmission line depends on its width and the thickness of its dielectric layer. Using fixed-width traces on the inner layers and solid ground areas above and below can help to achieve this.
Grounded coplanar waveguides can provide excellent line-to-line isolation. However, it is unrealistic to achieve isolation better than -45 dB on a small PCB. Consequently, it is recommended to use a multilayer board with several plane layers for RF applications. A solid (continuous) ground area on Layer 2 is required for RF components and transmission lines, as well as additional ground areas above and below the intermediate conductor. These areas must be unique to the ground and must not be shared or assigned to signal or power networks.