How to Design RF PCB with 7 Primary Elements for Designers?
Routing your RF board to ensure signal integrity is just as much about designing the right layer stack as it is about laying traces. You can suppress transmission line effects in your signal lines with the right layer stack in your PCB.
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While normally discussed in terms of digital signals, signal reflection at an impedance discontinuity affects analog signals when traces operate as transmission lines. When the propagation delay along an interconnect is greater than one-quarter the oscillation period of the analog signal, then you will need to worry about transmission line effects and ensure that your traces are impedance matched.
If you are searching for the basic information of RF PCB design, please check and read the content below in this passage for professional knowledge.
How to select right material for RF PCB?
Materials commonly used in PCB fabrication, such as FR-4 (flame retardant level 4), are very cheap but are generally not the most suitable choice for high frequency RF applications, especially considering the non-uniformity of the dielectric constant and a worse tangent angle. For RF PCBs, specific materials are used, such as FEP, PTFE, ceramic, hydrocarbons, and various types of glass fiber. The PFE and PTFE materials, belonging to the fluoropolymer family, improve the chemical resistance of the base material, have anti-adhesion and smoothness properties, as well as an extraordinary heat resistance. If budget is not a problem and quality is more important than price, the best solution is PTFE with fiberglass, eventually woven glass fiber.
FR4 materials are acceptable for RF transmission lines and interconnect operating up to WiFi frequencies (~6 GHz). Beyond these frequencies, RF engineers recommend using alternative materials to support RF signal propagation and printed RF circuit designs. Standard FR4 laminates use resin-filled fiberglass weaves to hold components, but these fiber weave effects in certain materials could create signal and power integrity problems if fabrication procedures are not specified properly.
Alternative material systems use PTFE-based laminates and bondply materials to bond a PTFE layer with the next layer in your PCB stackup. These materials have lower loss tangent than FR4 materials, so signals can travel farther without attenuating and still fall within acceptable margins. These laminates should form the substrate that supports RF transmission lines at very high frequencies, such as 77 GHz radar, or for very long interconnects at lower frequencies, such as 6 GHz WiFi. The table below summarizes some important material properties for common RF PCB materials.
What is RF reflection in PCB design?
RF reflection functions similarly to how sound is bounced back and creates an echo. It’s also analogous to waves of water crashing and bouncing back. The reflection occurs because the wave encounters a discontinuity of the medium that it’s traveling, and that’s the same for reflected RF waves.
RF reflection must be considered when discussing transmission lines. One of the basic principles of transmission lines is to ensure that the characteristic impedance of the driver, traces, and load are matched. Characteristic impedance is not defined by the resistance or the trace length but rather determined by the dielectric, trace width, and the separation between the trace and the plane.
How to process RF PCB design?
1) Surely use a multilayer PCB. If your PCB design includes only two layer, the top layer should include the power stage, RF signal lines and RF components. Then the bottom layer must be the ground plane.
2) The length of the lines that carry RF/Microwave signals is a very important issue. They should be at most 1/20 length of the wavelength. So there will be no loss. For instance, when we calculate for 433 MHz;
λ (Wavelength) = c (The Speed of Light) / f (Frequency)
λ = 300000000/433000000 = 69,28 cm
Max. Line Length : λ/20 = 3,46 cm
If the line must be necessarily longer, then impedance matching with L and C components must be applied at the end of the line.
3) If you use multilayer PCB, draw the short RF lines on the top layer. To reduce the noise, draw the power lines between two ground layers. There must be absolutely a ground layer under the layer that includes the RF signal lines.
4) Draw the RF signal lines quite separately. If they are adjacent to eachother, then crosstalk may occur. (Crosstalk : Undesired transfer of signals between or among two lines such as telephone lines, data lines, or system components. )
5) Use least number of vias in RF stage.
What are the applications of RF PCB design?
RF boards have a multitude of different applications, including wireless technologies, smart phones, sensors, robotics and security. With the advent of new technologies that are pushing the limits of electronics, the demand for RF boards is on the rise.
Finding a capable RF PCB manufacturer is critical to make sure the boards are fabricated to high quality standards and on-time. Our reputation speaks for itself. We pride ourselves on bringing the most demanding layout concepts to reality.
PCB Knowledge ⋅ 04/21/2022 10:03
