RF circuit board and PCB layout characteristics
2024-12-07
Radio frequency (RF) board design is often described as a "black art" because there are many uncertainties in theory, but this view is only partially true, RF board design also has many guidelines to follow and should not be ignored.
1.RF board layout features
Partition isolation
RF boards require strict partitioning of the RF portion from other digital or analog circuit portions. For example, in a communication circuit board that contains both an RF transceiver module and a baseband digital processing circuit, the RF part will generate high-frequency signals, which can easily interfere with the digital part. Therefore, through physical partitions, isolation strips are set between the two, which can be blank copper foil areas or areas filled with ground lines to reduce crosstalk between each other.
For multi-band RF circuits, different frequency bands of RF circuits should also be partitioned. For example, in a Wi-Fi circuit board that supports 2.4GHz and 5GHz dual bands, the 2.4GHz and 5GHz RF front-end circuits should be arranged in different areas to prevent interference between bands.
Partition isolation
RF boards require strict partitioning of the RF portion from other digital or analog circuit portions. For example, in a communication circuit board that contains both an RF transceiver module and a baseband digital processing circuit, the RF part will generate high-frequency signals, which can easily interfere with the digital part. Therefore, through physical partitions, isolation strips are set between the two, which can be blank copper foil areas or areas filled with ground lines to reduce crosstalk between each other.
For multi-band RF circuits, different frequency bands of RF circuits should also be partitioned. For example, in a Wi-Fi circuit board that supports 2.4GHz and 5GHz dual bands, the 2.4GHz and 5GHz RF front-end circuits should be arranged in different areas to prevent interference between bands.
Transmission line design
Transmission line characteristics in RF boards are critical. Microstrip line and ribbon line are commonly used as transmission lines. The microstrip line has a conductor strip on one side of the dielectric substrate and a floor on the other side. A strip line is a medium and conductor band sandwiched between two floors. In the layout, the length and width of the transmission line are precisely controlled to achieve a specific impedance match.
For example, for a 50Ω microstrip line, the design of its width and length is determined according to the dielectric constant and operating frequency of the board. In general, the higher the operating frequency, the greater the impact of the transmission line length on signal transmission. When designing Bluetooth RF boards, which operate at a frequency of about 2.4GHz, small changes in the length of the transmission line can cause signal reflection and loss.
Grounding design
The grounding of RF circuit is different from that of ordinary circuit in that it requires a large area of ground plane. The ground plane can provide a low-impedance signal return path to reduce electromagnetic radiation. In the layout, make sure that the ground pin of the RF component is directly connected to the ground plane and that the ground holes are sufficient.
Take the layout of an RF power amplifier as an example, the ground pin of the power amplifier is connected to the ground plane through multiple holes, which can effectively dissipate heat, but also provide a good grounding path for high-frequency signals, reducing signal reflection and crosstalk.
Component layout
The layout of RF components should be compact to minimize the length of wires between components. For example, in an RF filter and amplifier layout, the output pin of the filter should be as close as possible to the input pin of the amplifier to reduce loss and interference during signal transmission.
For the layout of active RF components (such as RF transistors) and passive components (such as capacitors, inductors), the interaction between them should be considered. For example, the matching network composed of inductors and capacitors should be close to the RF port that needs to be matched to achieve the best impedance matching effect.
Transmission line characteristics in RF boards are critical. Microstrip line and ribbon line are commonly used as transmission lines. The microstrip line has a conductor strip on one side of the dielectric substrate and a floor on the other side. A strip line is a medium and conductor band sandwiched between two floors. In the layout, the length and width of the transmission line are precisely controlled to achieve a specific impedance match.
For example, for a 50Ω microstrip line, the design of its width and length is determined according to the dielectric constant and operating frequency of the board. In general, the higher the operating frequency, the greater the impact of the transmission line length on signal transmission. When designing Bluetooth RF boards, which operate at a frequency of about 2.4GHz, small changes in the length of the transmission line can cause signal reflection and loss.
Grounding design
The grounding of RF circuit is different from that of ordinary circuit in that it requires a large area of ground plane. The ground plane can provide a low-impedance signal return path to reduce electromagnetic radiation. In the layout, make sure that the ground pin of the RF component is directly connected to the ground plane and that the ground holes are sufficient.
Take the layout of an RF power amplifier as an example, the ground pin of the power amplifier is connected to the ground plane through multiple holes, which can effectively dissipate heat, but also provide a good grounding path for high-frequency signals, reducing signal reflection and crosstalk.
Component layout
The layout of RF components should be compact to minimize the length of wires between components. For example, in an RF filter and amplifier layout, the output pin of the filter should be as close as possible to the input pin of the amplifier to reduce loss and interference during signal transmission.
For the layout of active RF components (such as RF transistors) and passive components (such as capacitors, inductors), the interaction between them should be considered. For example, the matching network composed of inductors and capacitors should be close to the RF port that needs to be matched to achieve the best impedance matching effect.
2.PCB layout features (including but not limited to RF parts)
Overall layout planning
PCB layout must first consider the division of functional modules. For a complex electronic product, such as the PCB of a smart phone, it will contain multiple functional modules such as processor, memory, power management, and communication modules. These modules should be rationally arranged according to the function and signal flow direction, so that the signal transmission path is shortest and electromagnetic interference is reduced.
At the same time, it is also necessary to consider the installation method and outline size of the PCB. For example, for an industrial control board that needs to be installed in a small space, the component layout should be planned according to the location of the mounting hole and the size of the housing to avoid interference between the components and the housing or other mechanical parts.
Wiring rule
When PCB wiring, follow certain line width and spacing rules. In general, power cables and ground wires are wider than signal wires to carry more current. The spacing between signal lines should be determined according to the voltage level and electromagnetic compatibility requirements.
For high-speed digital signal, the differential wiring method should be used to improve the anti-interference ability of the signal. For example, in the PCB routing of high-speed USB interfaces, the differential signal pairs of USB (D + and D -) are routed in parallel, and a fixed spacing is maintained to reduce the common-mode interference of the signal.
Through hole treatment
Through holes play a role in connecting signals from different layers in the PCB. In the layout, the position and size of the hole should be set reasonably. The size of the hole will affect its parasitic capacitance and inductance, which has a certain influence on the transmission of high frequency signal.
For example, in high-frequency RF boards, unnecessary holes should be minimized. If it is necessary to use a pass hole, it should be optimized, such as the use of back drilling technology to reduce the parasitic inductance of the pass hole and improve signal integrity.
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