Several Points of RF Circuit Board Design (3)

5. Power Decoupling Circuit

Proper and efficient chip power decoupling circuits are also very important. Many RF chips that integrate linear lines are very sensitive to noise from the power supply, typically requiring up to four capacitors and an isolating inductor per chip to filter out all noise from the power supply.

The minimum capacitance value usually depends on the resonant frequency of the capacitor itself and the inductance of the pin, and the value of C4 is selected accordingly. The values ​​of C3 and C2 are relatively large due to their own pin inductance, so the RF decoupling effect is less, but they are more suitable for filtering lower frequency noise signals. RF decoupling is accomplished by inductor L1, which prevents RF signals from coupling from the power line into the chip. Since all traces are potentially an antenna that can both receive and transmit RF signals, it is necessary to isolate RF signals from critical lines and components.

The physical location of these decoupling components is also often critical. The layout principles of these important components are: C4 should be as close as possible to IC pins and grounded, C3 must be closest to C4, C2 must be closest to C3, and the connection traces between IC pins and C4 should be as short as possible. The ground terminal of each component (especially C4) should usually be connected to the ground pin of the chip through the first ground plane under the board. The vias connecting the components to the ground plane should be as close as possible to the component pads on the PCB. It is best to use blind vias punched on the pads to minimize the inductance of the connection lines. The inductance L1 should be close to C1.

An integrated circuit or amplifier often has an open collector output, so a pullup inductor is required to provide a high impedance RF load and a low impedance DC source. The same principle applies to this The power supply side of the inductor is decoupled. Some chips require multiple power supplies to operate, so two or three sets of capacitors and inductors may be required to decouple them individually, and if there isn’t enough space around the chip, the decoupling may not be effective. In particular, special attention should be paid to the fact that the inductors are rarely parallel together, because this will form an air-core transformer and induce interference signals with each other, so the distance between them should be at least the height of one of them, or at right angles arranged to minimize mutual inductance.