Improvement measures of RF circuit PCB design layout

The design of radio frequency (RF)PCB has many uncertainties in the published theory, which is often described as a “black art”. Usually, for circuits in the frequency band below microwave (including low-frequency and low-frequency digital circuits), careful planning on the premise of mastering all kinds of design principles is the guarantee of successful design at one time. For PC digital circuits in the frequency band above microwave and high frequency, 2~3 versions of PCB are needed to ensure the circuit quality. For RF circuits above microwave frequency band, more versions of PCB design are often needed and constantly improved, and it is on the premise of considerable experience. Therefore, the difficulty of RF electrical design can be seen.
Typical RF board
Wireless internet access module
Be familiar with product architecture and signal flow direction before layout.
1 Frequently Asked Questions in RF Circuit Design
1.1 Interference between digital circuit module and analog circuit module
If the analog circuit (RF) and digital circuit work independently, they may each work well. However, once they are put on the same circuit board and work together with the same power supply, the whole system is likely to be unstable. This is mainly because the digital signal frequently swings between the ground and the positive power supply (> 3 V), and the period is extremely short, often in nanoseconds. Because of the large amplitude and short switching time, these digital signals contain a large number of high-frequency components which are independent of the switching frequency. In the analog part, the signal from the wireless tuning loop to the receiving part of the wireless device is generally less than 1 μ V. Therefore, the difference between digital signal and RF signal will reach 120 dB. Obviously, if the digital signal can’t be separated from the radio frequency signal, the weak radio frequency signal may be destroyed, so that the working performance of the wireless device will deteriorate, or even it can’t work at all.
Common interference phenomena
1.2 Noise interference of power supply
RF circuit is very sensitive to power noise, especially to glitch voltage and other high-frequency harmonics. Microcontrollers will suddenly absorb most of the current in a short time in each internal clock cycle, because modern microcontrollers are all manufactured by CMOS process. Therefore, suppose a microcontroller runs at an internal clock frequency of 1 MHz, and it will draw current from the power supply at this frequency. If proper power decoupling is not adopted, voltage glitch on the power line will be caused. If these voltage spikes reach the power supply pins of the RF part of the circuit, they may cause work failure in severe cases.
Pay attention to the rules of RF chip power supply layout and capacitor layout.
High-speed clock important signals are routed with complete internal reference plane and package protection.
1.3 Unreasonable ground wire
If the ground wire of RF circuit is not handled properly, some strange phenomena may occur. For digital circuit design, even if there is no ground plane, most digital circuits perform well. In the RF frequency band, even a short ground wire will act like an inductor. Roughly calculated, the inductance per millimeter length is about 1 nH, and the inductance of 10 mm PCB line at 433 MHz is about 27Ω. If the ground plane is not used, most ground lines will be long, and the circuit will not have the designed characteristics.
1.4 Radiation interference of antenna to other analog circuit parts
In PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have analog-to-digital converters (ADC) or digital-to-analog converters (DAC). The high-frequency signal from the antenna of the RF transmitter may reach the analog input of the ADC. Because any circuit line may send or receive RF signals like an antenna. If the processing at the input of ADC is not reasonable, the RF signal may be self-excited in the ESD diode at the input of ADC, thus causing the deviation of ADC.
2 RF circuit design principle and scheme
2.1 RF layout concept
When designing RF layout, the following general principles must be met first.
(1) isolate the high-power RF amplifier (HPA) from the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit.
Design of Power Amplifier PA Offset Quarter Wavelength
Word arrangement principle
Amplifier bias inductors are arranged vertically.
Wrong amplifier bias layout
(2) Make sure that there is at least a whole piece of ground in the high-power area on the PCB, and it’s best that there are no vias on it. Of course, the larger the copper foil area, the better.
Enough GND vias to improve signal grounding and heat dissipation.
The RF filter shall be fully grounded.
(3) Decoupling between circuit and power supply is also extremely important.
(4)RF output usually needs to be far away from RF input.
(5) Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible.
2.2 Design principles of physical partition and electrical partition
Design partition can be divided into physical partition and electrical partition. Physical zoning mainly involves the layout, direction and shielding of components; Electrical partition can be further divided into power distribution, RF wiring, sensitive circuits and signals, grounding, etc.
2.2.1 principle of physical partition
(1) Principle of component location layout. The layout of components is the key to achieve an excellent RF design. The most effective technology is to fix the components on the RF path and adjust their directions, so as to minimize the length of the RF path, keep the input away from the output, and separate the high-power circuit from the low-power circuit as far as possible.
RX/TX digging or shielding isolation
(2)PCB stacking design principle. The most effective method of stacking circuit boards is to arrange the main ground plane (main ground plane) on the second layer below the surface layer, and to arrange RF lines on the surface layer as much as possible. Minimizing the via size on the RF path can not only reduce the path inductance, but also reduce the virtual solder joints on the main ground, and reduce the chance of RF energy leaking to other areas in the laminate.
Commonly used RF plate models
Reasonable Ro4350 RF lamination and impedance line width design
(3) RF devices and their RF wiring layout principles. In physical space, a linear circuit such as a multistage amplifier is usually enough to isolate RF areas from each other, but there are always multiple RF/IF signals interfering with each other in diplexers, mixers and intermediate frequency amplifiers/mixers, so this influence must be carefully minimized. The RF and IF traces should cross each other as much as possible, and they should be separated by a block as much as possible. The correct RF path is very important for the performance of the whole PCB, which is why the component layout usually takes up most of the time in the PCB design of cellular phones.
The alternative circuit design should pay attention to open microstrip.
2.2.2 principle of electrical zoning
(1) Principle of power transmission. The DC current of most circuits in cellular phones is rather small, so the wiring width is usually not a problem. However, a large current line as wide as possible must be set separately for the power supply of the high power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, it is necessary to adopt multiple through holes to transfer current from one layer to another.
(2) Power supply decoupling of high-power devices. If the power supply pin of the high power amplifier cannot be fully decoupled, the high power noise will radiate to the whole board and bring many problems. The grounding of high power amplifier is very critical, so it is often necessary to design a metal shield for it.
(3)RF input/output isolation principle. In most cases, it is also critical to ensure that the RF output is far away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the outputs of amplifiers and buffers are fed back to their inputs with proper phase and amplitude, they may generate self-excited oscillation. In the best case, they will be able to work stably under any temperature and voltage conditions. Actually, they may become unstable and add noise and intermodulation signals to RF signals.
Input needs to be distanced.
3 Concluding remarks
In a word, the RF circuit has skin effect and coupling effect because it is a distributed parameter circuit, which is different from low frequency circuit and DC circuit. Therefore, special attention should be paid to these problems when designing RF circuit PCB, so as to ensure the effectiveness and accuracy of circuit design. E- Link network data transmitter can also be used to build a new generation of Ethernet measurement and control system, to transform the existing distributed control system composed of fieldbus and to develop and produce network measurement and control equipment. It will promote the intelligentization and networking of household appliances, and make profound changes in people’s lifestyle.