This is a general guide to the design rules for printed circuit boards (PCB) designed for high frequency or radio frequency (RF) circuits. By following these guidelines, you can avoid some of the most common mistakes in RF design. Poor PCB layout is one of the most common causes of insufficient transmission or reception performance and EMC failures due to spurious emissions. Ideally, the PCB engineer should be familiar with IPC standards, as they provide a rich source of information and best practice regarding the general principles of PCB design.
When planning your RF PCB layout, the first place to start is to contact your preferred board manufacturer and get a set of your recommendations and production capabilities. This will include the minimum track width and clearance, drill sizes and other key parameters. The board manufacturer should also be able to provide you with a standard stack and material data, this will include copper masses, dielectric constant and thickness of core and pre-preg layers in multilayer boards. Without your circuit very simple, I suggest using a 4-layer board that provides a continuous ground plane. When using a double-sided board, it is very difficult to ensure that the ground plane is not broken. Another advantage of the 4-layer PCB is that the dimensions of the microstrip line for a 50 or 75 Ohm design are more manageable. Use the microstrip calculator to determine the desired width of the track for the design impedance and make sure it is within the manufacturing capabilities if you do not need to discuss the layer stack with the manufacturer and choose a custom assembly. Another point to note is that my experience in maintaining separate ground networks for different signal areas usually causes problems and not any benefit, and is a big return to the days before multilayer boards, one plane Low impedance grounding is the safest route.
It is permissible to use FR4 for boards up to 2.4 GHz without the highest level of performance. In certain circumstances, it is worthwhile to indicate the charge as a controlled impedance to ensure consistent radio frequency characteristics.
Having determined the geometry of the track and the bead, the next step is to refer to the placement of the components. Make sure that the RF components are positioned so that all RF tracks can be stored on the top surface with minimal length and direction changes. Start with low signal areas at the antenna or RF input and work back towards the baseband or digital area. Keep digital and power supplies away from the analog RF circuitry and keep all RF components on one side of the board.
If your RF tracks cannot be run in a straight line, use beveled turns, if your CAD system supports them, never use direct corner bends on the RF signal lines. If oblique bends are not supported, use a series of bends or arcs of 45 degrees to minimize impedance mismatches, which will increase losses and spurious emissions.
RF circuits typically use ground flow on the top layer and “stitch” it onto the ground plane using several pass-through devices. If you intend to do this, make sure that the copper is at a suitable distance from the radio frequency tracks and components, otherwise the impedance will be reduced and cause more harm than good. The line spacing can be 5-10 mm from each other, very little benefit in stitching, being closer than 5 mm from each other.
The tracks to the ground of the high-frequency components should be as short as possible and use 2 or 3 across in parallel to minimize resistance.
