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RF PCB design is somewhat different from traditional PCB boards, what sets it apart are impedance matching, trace type (preferably coplanar), elimination of via stubs (to avoid reflections), ground planes, vias and Power supply decoupling and other parameters. Other aspects such as stackup and material selection also play a crucial role in these boards. Considering all these factors, the complexity of the RF design process increases due to EMl interference, high frequency signal paths,etc. In this article, we discuss all these issues in detail. So, what are the 7 factors that affect RF PCB design?

1. Impedance matching, in controlled impedance RF circuits, when the impedance of the entire trace remains the same, the maximum power transfer from source to load wil not be distorted. This impedance is called the characteristic impedance (Z0) of the trace. The characteristic impedance depends on the geometry of the trace, such as the trace width, the dielectric constant of the PCB material, the trace thickness, and the height from the reference ground plane. In order to match these impedances, a matching circuit is also designed.

2. RF board material, RF PCB is made of certain materials that meet the requirements of high frequency operation. These materials should have low signal loss, be stable at high frequency operation, and should be able to absorb a lot of heat. Dielectric constant (DK), loss tangent (tan8), and coefficient of thermal expansion (CTE) values also need to be consistent over a wide frequency range. Typical values for the dielectric constant of these plates range from 3 to 3.5. For the frequency range of 10-30 GHz, the loss tangent values are in the range of 0.0022 to 0.0095.

3. RF PCB stacking, RF board stacking needs to pay attention to details such as isolation between traces and components, power supply decoupling, number of layers and arrangement, and component placement. A standard 4-layer RF stackup is shown in the figure below. RF components and traces are placed on the top layer. This layer is by the ground and power planes. All non-RF components and traces are populated on the bottom layer. This arrangement provides minimal interference between RF and non-RF components. A direct ground plane provides the smallest path for ground return currents. So, all in all, this is a good stackup for a small RF board.

4. RF routing design, RF routing propagates high-frequency signals, so it will be affected by transmission loss and interference problems. The characteristic impedance of these traces is a major concern for designers. In RF boards, traces are considered transmission lines. The most common types of transmission lines designed are coplanar waveguide(CPWG), microstrip, and stripline.

5. Ground plane design, any RF trace or component needs a return path for current to propagate through it. The ground plane takes care of this. However, ground planes require some additional design considerations.

6. By design, vias in RF traces should be avoided as much as possible. However, if these cannot be avoided, specific via diameters and lengths must be . Vias induce parasitic capacitance in the board. In the case of RF boards, this capacitance affects high frequency operation.

7. Power supply decoupling, for RF circuit boards, noise reduction is very important. At high operating frequencies, these boards become very sensitive to the effects of noise. Therefore, all possible methods are used for denoising. One such method is called power supply decoupling.