Development of communication technology has witnessed gradually wide applications of wireless radio frequency (RF) circuit such as in the fields of mobile phones, Bluetooth products and RF circuit has been the core technology of radio propagation. In recent years, however, the gradual prevalence of 4G and obvious increase in terms of data's transfer order of magnitude lead to challenges to PCB design of RF circuit. After all, the number of signals transferred by RF circuit escalates by hundreds of times every day. Furthermore, since RF circuit is mainly applied in portable devices that have attributes of small scale and portability, the basic requirement of the whole circuit lies in small volume, even and reasonable routing and non-interference between micro components. Nevertheless, it seems unavoidable that electromagnetic interference takes place between components inside mobile phones. Don't worry. Some operations can be applied to effectively reduce the influence caused by electromagnetic interference. This article will bring about a reasonable PCB design for RF circuit and the characteristics of this design include small volume and obvious capability of anti-interference.
Selection of Substrate Material
Since some ICs (integrated circuits) are implemented on substrate, suitable substrate must be picked up for RF circuit first as a template carrying electronic components. In terms of selection of substrate material, the first elements for consideration include dielectric constant, dielectric loss and coefficient of thermal expansion among which dielectric constant is the most significant since it greatly affects the impedance and transmission speed of circuit, especially those circuits with extremely high frequency that have strict requirements to dielectric constant. Therefore, it is generally a rule to pick up substrate material with relatively small dielectric constant.
PCB Design Procedure
• Schematic Diagram Design
The first step of PCB design is to design schematic diagram, which has to be completed with the assistance of computers. The schematic diagram design is implemented through PCB design software that contains all electronic analog components. First of all, circuit diagram is designed by simulating the real circuit in computer. Then, circuit diagram has to be connected with corresponding components. Next, the operation simulation is implemented based on schematic diagram to determine feasibility of basic operation.
• PCB Design
After schematic diagram design, the pattern and size of PCB can be scientifically determined based on the schematic diagram. The pattern and size of PCB can be optimized in accordance with the position, dimension, pattern and other parameters in order to make the whole system reach the optimal performance. In this process, it's necessary to determine the positions of location holes, view eyes and reference holes.
Locate all required omponents. Ordinary components are easy to be found in warehouse. If components aren't available in warehouse, it's necessary to procure or manufacture components. PCBCart has a professional and stable component sourcing system which clients can rely on. Then, components are required to be distributed and routing to be implemented around them. The final step is to detect the operation of circuit in order to ensure that the performance of circuit is capable of meeting the requirement and the operation of circuit can be basically stable.
Different from ordinary components layout, all the components in RF circuit are so small as a result of the small scale of the circuit that SMT (surface mount technology) is applied for component layout and infrared reflow oven for the soldering of microelectronic components. Soldering is an important link in RF circuit design, whose quality directly affects the overall quality of the whole circuit. For the PCB of RF circuit, excellent electromagnetic compatibility needs to be formed between electronic components, which is the most element worth considering. Electromagnetic radiation between different electronic components influences the independent operation of each electronic component so it's necessary to first pick up the components with anti-interference capability.
In addition, in the process of the overall operation of circuit, the current in the circuit tends to lead to the generation of magnetic field. Therefore, from the perspective of RF circuit, besides the consideration of the interference between components, the electromagnetic interference of the circuit to other circuits has to be taken into account. Macroscopic circuit layout is quite critical and the following basic circuit layout principles can be regarded as the reference.
First, the arrangement of components should be arranged in one row. The determination of direction of PCB entry tin coating system is applied to reduce the problems caused by loose soldering. Generally, the spacing between components should be 0.5mm or more so that tin soldering can be implemented between components. Otherwise, soldering can't be implemented owing to the small distance between components.
Second, all the interfaces have to be compatible with each other in PCB system. Both positions, dimensions and shape of component interfaces have to be taken into considerations to ensure smooth connection between them. The complexity of circuit unavoidably leads to difference of electric potential between circuits. As a result of the small space between these differences, short circuits always take place. Therefore, components with high electric potential shouldn't be placed too near to each other to avoid the occurrence of short circuits. More attention has to be paid in the environment of high voltage.
Finally, circuit structure has to be carefully considered as a whole and circuit has to be cut into separate modules each of which has lots of electronic components. Components should be distributed according to different modules. For example, high-frequency amplifying circuit or mixer circuit should be placed together in the process of layout so that the wire loop area can be effectively reduced and so can circuit consumption and electromagnetic radiation. Moreover, it is capable of stopping the mutual interference between different modules.
Routing is implemented after basic layout, classified into detailed routing and overall routing. The former refers to the routing inside different modules in the circuit. Although detailed routing may take place in IC design, preliminary detailed routing is completed prior to the procurement of components. Sometimes just a little modification is required.
Overall routing refers to mutual routing between different modules or the network routing between power and each module. Some aspects have to be taken into consideration in the process of overall routing. Lots of limitations will be caused as a result of position particularity and different distance between modules. If each module is regarded as a point and connection between points is determined, the best plan with the shortest routing length will be generated so as to save the cost of material and make circuit look simple and neat.