With the rise of the Internet of Things technology, it is becoming more and more common for electronic products to carry wireless communication functions. The wireless communication technology relies on the radio frequency circuit on the PCB to achieve and requires professional design and simulation analysis tools. Now I will share the module RF circuit PCB design with you.
Wiring principle
For the module without its own connector, it needs to be connected to the antenna feed point or connector through the RF trace. Therefore, it is recommended to use the microstrip line for the RF line. The shorter the better, the difference loss should be controlled within 0.2dB, and the impedance should be controlled within 50Ω.
Reserve a π-type circuit between the module and the antenna connector (or feed point) (the ground pins of the two parallel devices should be directly connected to the main ground) for antenna debugging.
When PCB routing, this signal routing controls 50Ω. The RF performance of the product is closely related to this trace. The factors affecting the impedance of this trace on the PCB are as follows:
The width and thickness of the trace
The dielectric constant and thickness
The thickness of the pad
The distance from the ground
Near the trace
Impedance design
The RF signal line impedance of the two antenna interfaces needs to be controlled to 50Ω. In practical applications, other parameters of the PCB such as reference layer thickness, number of layers, and lamination will affect the RF routing method. The reference GND layer is different in different situations, and the routing gap will also be large.
3W Principle When
designing a multi-layer board to route the RF signal of an antenna on the PCB, the first consideration is to satisfy the basic "3W principle". In order to reduce crosstalk between lines, the line spacing should be large enough. If the line center distance is not less than 3 times the line width, 70% of the electric field between lines can be kept without mutual interference, which is called the "3W principle".
3W principle schematic diagram
RF PCB Layout section
Try to make 50 ohm impedance for the RF trace. If it is irrelevant, keep the width of the RF trace at 0.5mm 1mm. The safety distance from the ground should also be maintained at 0.5mm 1mm, with ground holes around
it. Try to be as clear
as possible The via pad is recommended to end with a teardrop
RF traces should be as far away as possible from power supplies, SIM cards, clocks, and high-speed digital signals; to ensure that they do not affect surrounding components, it is recommended that customers not place other components around the antenna, and traces on the PCB should be as far away as possible from the RF part.
The RF trace should be as short as possible, and a serpentine line or arc should be taken when it is bent, and ground holes should be drilled around it. The
impedance design of the two-layer board is designed
because most customers use double-sided boards, so the typical 1.6mm, 1.0 for two-layer boards mm thickness PCB design as an example:
Case 1: PCB thickness 1.6mm.
Considering the influence of PCB thickness, it is difficult to fully comply with the 3W principle. It is necessary to ensure 50Ω wiring and ensure
When the RF trace is too long, it is recommended to reserve matching circuits for module testing and antenna testing.
If the board space is abundant, the layout should be prioritized to achieve short and straight RF traces. If the layout space is not allowed, corner traces are required. For wiring at 45° corners, circular arc wiring is required. If it is really going to be at right angles, you can replace the wiring by placing components and place the components to make a 90-degree turning angle, so as to avoid sudden impedance changes. The signal reflection caused by the influence.
Two-layer board impedance design
Because most customers use double-sided boards for design, an example is given for the typical 1.6mm, 1.0mm thickness PCB design for two-layer boards:
Case 1: PCB thickness 1.6mm
takes into account the impact of PCB thickness It is difficult to fully comply with the 3W principle. It is necessary to ensure the 50Ω wiring and ensure that it does not affect the surrounding components. Therefore, it is recommended that customers do not place other components around the antenna, and the wiring on the PCB should be as far away as possible from the RF section.
After a lot of actual verification, the following design can ensure that the RF impedance is controlled at about 50Ω with minimal impact: The
RF line width is 43mil, the distance from the ground (blue) next to it is 8mil, and the RF line reference is directly below the RF trace Layer, usually intact ground (red). Complete ground protection is required around the RF line, and as many ground holes as possible along the routing direction. It is not recommended to have any other traces or devices around the RF trace and its reference ground.
Theoretical calculation of 50Ω impedance:
Schematic diagram of impedance calculation tool (unit: mil)
Er1, T1, C1, C2, CEr parameters are generally confirmed by the PCB manufacturer, and each PCB manufacturer's craft and material are slightly different, and need to be confirmed with the PCB manufacturer.
H1 is the thickness of the PCB, W1 is the line width, and D1 is the distance from the line to the GND next to it. Errors should be considered in actual production. Case 2 When the PCB thickness is 1.0mm, the RF line width on the PCB is 35mil, and the distance between the line and the GND next to it is 8mil.
Four-layer board impedance design
Four-layer board with a thickness of 1.0mm. The RF line is routed on the surface layer Lay1, refer to Lay2 (GND layer). The stackup of different PCB board factories will be different, the following figure shows the stackup of 4-layer board as an example:
schematic diagram of stacking thickness
The thickness of Lay 1 to Lay 2 is 65um, the RF trace is 4mil, and the distance from RF to GND on both sides is greater than 3 times the width of the RF trace. Lay 1 is blue, red is Lay 2, and the highlighted part is the RF line. Theoretical calculation of the 50Ω impedance of the
RF wiring diagram
: After
the value of D1 is greater than 3 times of W1, the impact on the impedance is very weak.
Schematic diagram of the four-layer board resistance TOP layer trace resistance calculation
The above is some of our accumulated experience in the hardware power supply design process of the module development project. I hope you will avoid detours in the project development process and successfully complete the development task. Finally, we introduce our Miracle IoT. Our products cover 2G, 4G, NB modules based on eSIM technology. We also provide customers with module-related hardware design materials and review services to help customers quickly complete project development and quickly seize the Internet of Things. market.
