Electronic Design - Some standards of RF PCB board design

1) Low-power RF PCB design mainly adopts standard FR4 material (good insulation performance, uniform material, dielectric constant ε = 4,10%). Mainly use 4-layer to 6-layer board. In the case of very cost-sensitive cases, a double-layer board with a thickness of 1mm can be used to ensure that the opposite side is completely formed. And because the thickness of the double-sided board is above 1mm, it makes the FR4 medium The formation and signal layer is thicker. In order to make the impedance of the RF signal line reach 50 ohms, the width of the signal line is usually about 2 mm, which makes it difficult to control the spatial distribution of the board. For a four-layer board, generally speaking, the top layer only runs the RF signal line, the second layer is complete, the third layer is the power supply, and the bottom is generally used to control the state of the digital signal line of the RF device (such as setting the ADF4360 series PLL CLK, Data, le signal line. The third layer of power is best not to be a continuous plane, but to make the power lines of each RF device distributed in a star shape, and finally the next point.

The power line of the third layer RF device does not intersect the digital line below.

2) The RFPCB part and the analog part of the mixed signal should be far away from the digital digital part (this distance is usually above 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 radio frequency part. The main problem is that the ripple of the switching power supply will modulate the signal in the RF part. This modulation usually severely damages the RF signal, leading to fatal results. Generally speaking, for the output of the switching power supply, it can pass through a large choke, and a Pi filter, and then through a low-noise Ldo linear regulator (Micrel MIC5207, MIC5265 series, used in high-voltage, high-power RF circuits, you can consider Use LM1085, LM1083, etc.)

Obtain power for the RF circuit.

3) 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 and the ADF4360 chip on pins 9 and 10 should be as short as possible to ensure that the distributed series inductance caused by the connection between the inductor and the chip is minimal.

For the ground (GND) pins of each RF device on the circuit board, including resistors, capacitors, inductances, and pins connected to the ground (GND), the holes should be punched as close to the ground (second layer) as possible.

4) When choosing to use components in a high-frequency environment, use desktop stickers as much as possible. This is because the size of the desk sticker component is usually small and the pins of the component are short. This minimizes the influence of additional parameters related to component pins and component internal wiring.

Especially discrete resistors, capacitors, inductance components, using smaller packages

5) When active devices work in a high-frequency environment, there are often multiple power supply pins. At this time, attention must be paid to each pin (about 1mm) near the power supply to set a separate dummy capacitor with a tolerance value of about 100nF. When the board space permits, it is recommended to use two decoupling capacitors for each pin, with capacities of 1nF and 100nF respectively. Ceramic capacitors with x5r or x7r materials are usually used. For the same RF active device, different power pins can supply power to different functional parts of the device (chip), and the functional parts of the chip can work at different frequencies. For example, the ADF4360 has three power pins, which supply power to the VCO, PFD, and digital parts of the chip. These three parts implement completely different functions and have different operating frequencies. Once the digital part of the low-frequency noise reaches the VCO part through the power line, the VCO output frequency may be modulated by this noise, resulting in dispersion that is difficult to eliminate. In order to prevent this from happening, in addition to using separate coupling capacitors, the power pins in each functional part of the active RF device must be connected again through induction solder beads (approximately 10uH).

6) For the RF signal on the PCB feed, be sure to use a special RF coaxial connector when feeding. The most commonly used one is the SMA type connector. For the SMA connector, it is divided into in-line and microstrip type. For signals with a frequency lower than 3GHz, the signal power is very small, and we do not count weaker insertions. The in-line SMA connector is fully usable. If the signal frequency is further increased, we need to carefully select the RF cable and RF connector. At this time, the in-line SMA connector may cause relatively large signal insertion due to its structure (mainly corners).

7) When designing the RF PCB, there are strict regulations on the PCB wiring width of the RF signal. The design should be strictly calculated according to the thickness and dielectric constant of the PCB, and the impedance line at the corresponding frequency point should be simulated to ensure that it is 50 euros (CATV standard is 75 euros). However, we do not always need strict impedance matching. In some cases, a smaller impedance mismatch may be irrelevant (for example, 40 Euros to 60), even if your simulation of the circuit board is based on ideal conditions. When it is actually handed over to the PCB factory for production, the process used by the manufacturer will cause the actual impedance of the circuit board to differ from the simulation result by thousands of miles.

8) RF microstrip circuits used for implementation on PCB, these circuits are simulated in advertisements, HFSS and other simulation tools, especially those with directional couplers, filters (PA narrowband filters), microstrip resonators ( If you are designing a VCO, impedance matching network, etc., you must communicate well with the PCB factory, and use the thickness, dielectric constant and other indicators to be strict and the indicators used in the simulation to be consistent with the board.