Several Points of RF Circuit Board Design (2)

3. Physical Partition

Component placement is the key to a good RF design, and the most effective technique is to first fix the components on the RF path and adjust their orientation to minimize the length of the RF path. And keep the RF input away from the RF output and as far away as possible from high power circuits and low noise circuits.

The most efficient board stacking method is to arrange the main ground on the second layer below the surface layer, and run the RF traces on the surface layer as much as possible. Minimizing the size of vias on the RF path not only reduces path inductance, but also reduces void solder joints on the main ground and reduces the chance of RF energy leaking to other areas within the stack.

In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF zones from each other, but duplexers, mixers, and IF amplifiers always have multiple RF/IF signals interfering with each other. Care must therefore be taken to minimize this effect. The RF and IF traces should be crossed as much as possible, with a ground area between them as much as possible. Proper RF routing is very important to the performance of the entire PCB, which is why component placement usually takes up most of the time in mobile phone PCB design.

On a mobile phone PCB, it is usually possible to place the LNA circuit on one side of the PCB proofing board and the high power amplifier on the other side and finally connect them to the RF antenna on the same side via a duplexer one end and the other end of the baseband processor. This requires some tricks to ensure that RF energy does not pass through the vias from one side of the board to the other. A common technique is to use blind vias on both sides. The adverse effects of vias can be minimized by arranging blind vias in areas where both sides of the PCB are free from RF interference.

4. Metal Shield
Sometimes it is not possible to maintain sufficient separation between multiple circuit blocks, in which case metal shields must be considered to shield RF energy in the RF area, but metal shields also have side effects, such as: manufacturing costs and assembly costs are high.

Metal shields with irregular shapes are difficult to ensure high precision during manufacture, and rectangular or square metal shields limit the layout of components. Metal shields are not conducive to component replacement and fault displacement. Soldered on the ground plane, and must maintain a proper distance from the components, it takes up valuable PCB board space.

It is very important to ensure the integrity of the metal shield as much as possible, so the digital signal line entering the metal shield should go to the inner layer as much as possible, and it is better to set the next layer of the signal line layer as the ground layer. The RF signal line can be routed from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but the gap should be surrounded by a large ground area as much as possible, and the ground on different signal layers can be connected through multiple vias. linked together. Despite the above drawbacks, metal shields are still very effective and are often the only solution for isolating critical circuits.