What is RF PCB Design guidelines

2023-01-05 11:46:51 rf pcb 70 Viewed

SummaryRF PCB is a high-frequency RF PCB board made of PTFE PCB, ceramic-filled fluoropolymer or ceramic-filled hydrocarbon thermoset material with improved dielectric properties. RF PCB material has a low dielectric constant of 2.0-3.8, low dissipation factor and excellent low loss characteristics. RF PCB...

What is RF PCB?

RF PCB is a high-frequency RF PCB board made of PTFE PCB, ceramic-filled fluoropolymer or ceramic-filled hydrocarbon thermoset material with improved dielectric properties. RF PCB material has a low dielectric constant of 2.0-3.8, low dissipation factor and excellent low loss characteristics. RF PCB has good performance, high glass transition temperature, extremely low hydrophilic rate, and excellent thermal stability. RF PCB materials have a coefficient of expansion similar to that of copper, giving the material excellent dimensional stability.

What is RF PCB Design?

RF PCB design has many uncertainties. Usually, for circuits in the frequency band below microwave (including low frequency and low frequency digital circuits), it is necessary to master the design principles of various RF PCBs and plan carefully. For PC-type digital circuits with frequency bands above microwave and high frequency, multiple versions of RF PCB are required to ensure the circuit quality. For RF PCB circuits in the frequency band above microwave, more versions of RF PCB design and continuous improvement are often required, and it is based on the premise of considerable experience. From this we can see the difficulties in RF PCB design.

What are rf pcb design guidelines


RF PCB Design Issues

If the analog circuit (RF) and the digital circuit are working separately, each may work fine. However, once the two are put on the same RF PCB and work together with the same power supply, the whole system is likely to be unstable. This is mainly due to the fact that digital signals frequently swing between ground and the positive supply (>3V), and the periods are extremely short, often on the order of nanoseconds. Due to the large amplitude and short switching time of the RF PCB, these digital signals contain a large number of high-frequency components that are independent of the switching frequency. In the analog section, the signal transmitted from the wireless tuning loop to the receiving section of the wireless device is typically less than 1 μV. So the difference between the digital signal and the RF PCB's signal can reach 120dB. Obviously, if the digital signal is not well separated from the RF PCB signal, the weak RF PCB signal may be damaged, so that the performance of the wireless device will deteriorate, or even fail to work at all.

Digital-analog RF hybrid circuit partition design

RF PCB circuits are quite sensitive to power supply noise, especially to voltage spikes and other high frequency harmonics. Microcontrollers draw most of their current in short bursts during each internal clock cycle because modern microcontrollers are manufactured in CMOS processes. So, assuming a microcontroller is running at an internal clock frequency of 1 MHz, it will draw current from the supply at this frequency. If proper power supply decoupling is not taken, it will cause voltage glitches on the power line. If these voltage glitches reach the power pins of the RF PCB part, it may cause work failure in severe cases.

Pay attention to the rules for the layout of the power supply of the RF chip and the arrangement of the capacitors

The important signal traces of the high-speed clock are routed through the complete reference plane and package protection of the inner layer. If the ground wire of the RF PCB circuit is not handled properly, some strange phenomena may occur. For digital circuit designs, most digital circuit functions perform well even without a ground plane. In the RF PCB frequency band, even a very short ground wire will act like an inductor. Roughly calculated, the inductance per millimeter of length is about 1nH, and the inductance of a 10mm RF PCB line at 433 MHz is about 27Ω. If the ground plane is not used, most of the ground lines will be long, and the RF PCB circuit will not have the designed characteristics.

Unreasonable grounding

In the circuit design of RF PCB, there are usually other analog circuits on the RF PCB. For example, there are analog-to-digital conversions (ADCs) or digital-to-analog converters (DACs) on many circuits. High-frequency signals from the antenna of the RF transmitter may reach the analog input of the ADC. Because any RF PCB line may send or receive RF signals like an antenna. If the handling at the ADC input is not done properly, the RF signal may self-excite within the ESD diodes at the ADC input, causing ADC deviations.

RF PCB Layout

During RF PCB layout, the following points must be met first:

(1) Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, it is to keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. the

(2) The grounding of the filter of the RF PCB should be sufficient.

(3) The circuit and power supply decoupling of the RF PCB are also extremely important.

(4) The RF PCB output usually needs to be far away from the RF input.

(5) Sensitive analog signals should be kept away from high-speed digital signals and RF PCB signals as much as possible.

RF PCB Layout Basics

1. Layout determination: Before RF PCB layout, you should have a detailed understanding of the single board function, working frequency band, current and voltage, main RF device types, EMC, and related RF indicators, and clarify the stacked structure, impedance control, and external structure dimensions of the RF PCB , the size and location of the shielding cavity and cover, and the processing instructions for special devices (such as the size and location of devices that need to be hollowed out and directly dissipated from the chassis), etc. In addition, the main RF device power, heat dissipation, gain, isolation, sensitivity and other indicators and the connection of filtering, bias, and matching circuits should be specified. For the power amplifier circuit, the matching routing requirements recommended by the device manual or RF field analysis software simulation should be obtained. Get the impedance matching circuit guide.

2. Physical partition: The key is to arrange the main components according to the main signal flow of the RF PCB. First, fix the components on the RF PCB path according to the position of the RF PCB port, and adjust their orientation to minimize the length of the RF PCB path. In addition to considering the general layout rules, it is also necessary to consider how to reduce the mutual interference and anti-interference ability between various parts, and ensure that multiple RF PCB circuits have sufficient isolation. For circuit modules with insufficient isolation or sensitivity, and strong radiation sources, consider using metal shields to keep RF energy out of the RF zone.

3. Electrical division: The layout of RF PCB is generally divided into three parts: power supply, digital and analog, which should be separated in space, and the layout and wiring cannot cross regions. RF PCB separates strong and weak current signals as much as possible, and separates digital and analog. Circuits that perform the same function should be arranged within a certain range as much as possible, thereby reducing the area of the signal loop.

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