PCB Technical - Ten standards of RF PCB

Ten standards of RF PCB
1) In the low-power RF PCB design, standard FR4 materials are mainly used (good insulation properties, uniform materials, dielectric constant ε=4, 10%). Mainly use 4-layer to 6-layer board. In the case of very sensitive costs, double-sided boards with a thickness of less than 1mm can be used. Ensure that the reverse side is a complete stratum. At the same time, the thickness of the double-sided board is above 1mm, making the stratum and signal layer The FR4 medium between them is thick. In order to make the impedance of the RF signal line reach 50 ohms, the width of the signal trace is usually about 2mm, which makes it difficult to control the spatial distribution of the board. For a four-layer board, generally the top layer only uses RF signal lines, the second layer is a complete ground, and the third layer is a power supply. The bottom layer generally uses digital signal lines that control the status of the RF device (such as setting the clk, Data and LE signal lines.) It is better not to make the power supply of the third layer into a continuous plane, but to make the power lines of each RF device distributed in a star shape, and finally connect to one point. Do not cross the power traces of the third-layer RF devices with the digital lines on the bottom layer.

2) For a mixed-signal PCB, the RF part and the analog part should be far away from the digital part (the distance is usually more than 2cm, at least 1cm), and the ground of the digital part should be separated from the RF part. It is strictly forbidden to use a switching power supply to directly supply power to the RF part. The main reason is that the ripple of the switching power supply modulates the signal of the RF part. This kind of modulation often severely damages the radio frequency signal, leading to fatal results. Under normal circumstances, the output of the switching power supply can be passed through a large choke coil, a π filter, and then a low-noise LDO (Micrel's MIC5207, MIC5265 series). For high-voltage, high-power RF circuits, You can consider using LM1085, LM1083, etc.) to get the power supply to the RF circuit.

3) In the RF PCB, each component should be closely arranged to ensure the shortest connection between each component. For the ADF4360-7 circuit, the distance between the VCO inductor on the pin-9 and pin-10 pins and the ADF4360 chip should be as short as possible to ensure that the distributed series inductance caused by the connection between the inductor and the chip is minimized. For the ground (GND) pins of each RF device on the board, including the pins connecting resistors, capacitors, inductances and ground (GND), holes and ground planes should be drilled as close as possible to the pins (second Layer) connectivity.

4) When selecting components to work in a high-frequency environment, use surface mount components as much as possible. This is because surface mount components are generally small in size and the component leads are very short. In this way, the influence of additional parameters caused by component pins and internal wiring of the component can be reduced as much as possible. Especially for discrete resistors, capacitors, and inductance components, using a smaller package (0603\0402) is very helpful to improve the stability and consistency of the circuit;

5) Active devices working in a high frequency environment often have more than one power supply pin. At this time, we must pay attention to setting a separate decoupling capacitor near each power supply pin (about 1mm), with a capacitance value of 100nF about. When the board space permits, it is recommended to use two decoupling capacitors for each pin, the capacitance values are 1nF and 100nF respectively. Generally, ceramic capacitors made of X5R or X7R are used. For the same RF active device, different power pins may power different functional parts in the device (chip), and each functional part in the chip may work at different frequencies. For example, the ADF4360 has three power pins, which supply power to the on-chip VCO, PFD, and digital parts. These three parts realize completely different functions, and the operating frequency is also different. Once the low-frequency noise of the digital part is transmitted to the VCO part through the power trace, the output frequency of the VCO may be modulated by this noise, causing spurs that are difficult to eliminate. In order to prevent this from happening, in addition to using separate decoupling capacitors, the power supply pins of each functional part of the active RF device must be connected together through an inductive magnetic bead (about 10uH). This design is very beneficial to the improvement of the isolation performance of the active mixers LO-RF and LO-IF that include LO buffer amplification and RF buffer amplification.

6) For the feeding and feeding of RF signals on the PCB, special RF coaxial connectors must be used. One of the most commonly used is the SMA type connector. For SMA connectors, it is divided into in-line type and microstrip type. For signals with a frequency below 3GHz, and the power of the signal is not large, and we don't care about the weak insertion loss, you can use the in-line SMA connector. If the frequency of the signal is further increased, we need to carefully choose the RF connection wire and RF connector. At this time, the in-line SMA connector may cause relatively large signal insertion loss due to its structure (mainly turning). At this time, a better quality microstrip SMA connector can be used (the key lies in the PTFE insulator material used in the connector) to solve the problem. Similarly, if your frequency is not high, but you are demanding indicators such as insertion loss and power, you can also consider a microstrip SMA connector. In addition, small RF connectors include SMB, SMC and other types. For SMB connectors, this type of connector generally only supports signal transmission below 2GHz, and the snap structure used in SMB connectors will appear in high vibration situations. The "flash" situation. So carefully consider when choosing SMB connectors. Most RF connectors have a limit of 500 plugs and unplugs. Too frequent plugs and unplugs may permanently damage the connector, so don't use the RF connector as a screw when debugging the RF circuit. Since the PCB socket of the SMB is a pin structure (male), the connector that is frequently plugged and butt welded at one end of the PCB has relatively little loss, which reduces the difficulty of maintenance. Therefore, in this case, the SMB connector is also a kind of Good choice. In addition, for those occasions with extremely high space requirements, there are also miniature connectors such as GDR for selection. For those whose impedance is not 50 ohms, low frequency, small signal, precision DC and other analog signals or digital signals such as high-frequency clocks, low jitter clocks, high-speed serial signals and other digital signals, SMA can be used as a feed-out connector. .