Development of electronic communication technology has led to increasingly wider applications of wireless RF and control circuit technology. The performance index of RF circuit directly influences the quality of the whole product. All these products have one advantage in common that is miniature, which means that components are distributed with large density, leading to protruding mutual interference between components (including SMD, SMC and bare chip). This article aims to analyze methods on how to maximize the implementation of performance index of circuitry in order to meet requirements of EMC (Electromagnetic Compatibility) based on Protel se software.
General Principles of PCB Design
General principles of PCB design contain selection of PCB, dimension of PCB, component layout, circuit routing, pad, filling, crossing lines and ground among which component layout and circuitry routing are the most essential to the performance of PCBs.
• Component layout
First, the size of PCB has to be considered. If the size of PCB is too large with long printing wires, the impedance will improve, anti noise capacity will decrease and cost will rise as well. If the size of PCB is too small, PCB will suffer from bad heat dissipation and adjacent wires will be interfered. After the determination of PCB size, positions of components are determined. Generally, analog signals, high-speed circuitry and noise source (such as relay, switch with large curent) should be divided reasonably to minimize the signal coupling between them. Furthermore, layout should be implemented on all the components in circuitry according to the function unit of circuit.
• Circuit routing
a. Principle of circuit routing
1). Leads at the terminal of input and output should be adjacent and parallel as much as possible and ground lines should be added to stop from feedback coupling.
2). The minimum width of leads is mainly determined by adhesion strength between leads and insulated substrate and the current flowing through them. When the thickness of copper foil is 0.05mm and width 1 to 15mm with a current of 2A passing through, the temperature won't be higher than 3°C. Therefore, leads whose width is 1.5mm are capable of meeting the requirement. For integrated circuits, especially digital circuits, leads whose width is in the range from 0.02mm to 0.3mm are usually picked up. Of course, wide leads should be picked up whenever possible if it is permitted, especially power lines and ground lines. The minimum spacing between leads is primarily determined by insulated resistor between lines and breakdown voltage in the worst circumstance.
b. Requirements of routing for PCB in RF circuitry
In the process of PCB in RF circuitry, the correct routing of power lines and ground lines seems extremely important and reasonable design is the most important method to fight against EMI (Electromagnetic interference). Quite a lot of interference source on PCBs is generated through power and ground lines and noise interference caused by ground lines is the largest. The main reason for the generation of EMI by ground lines lies in the fact that the impedance takes place on ground lines. When current flows through ground lines, voltage will be generated on ground lines so that ground line loop current is produced, leading to loop interference of ground lines. When multiple circuits share the same ground line, public interference coupling will be formed so that so-called ground line noise will be generated. Therefore, some considerations have to be taken into in terms of PCB routing in RF circuit. Take five-channel tank as an example. If complete grounding processing is implemented on high-frequency part, the stability of receiver will be influenced and it will suffer from electromagnetic interference. The solution to this problem lies in maintaining gap and incomplete sealing, which is shown in Figure 1.
Anti-Interference Measures for Circuit Boards and Circuitry
Anti-interference design of PCBs is closely related with specific circuits and anti-interference measures for PCBs should be considered from the following three aspects.
• Design of power lines
According to the amount of current in PCB, the width of power lines should be enlarged as much as possible and loop resistance should be decreased. Moreover, the direction of power lines and ground lines should be compatible with data transmission, which is helpful to the enhancement of anti-noise capacity.
• Design of ground lines
a. For lines on PCB, power lines and ground lines are the most important. The key method to defeat EMI lies in grounding. For double-layer PCBs, single-point grounding is implemented in that power and ground are connected to PCB from two ends of power, that is, power with one end and ground with the other end. Multiple returning ground lines should be arranged on PCBs and concentrated to the end that is connected to power, which is so-called single-point grounding. For communication with signals that are external from PCBs, shielded cable is generally applied. For high-frequency and digital signals, both ends of shielded cable should be connected with ground. The shielded cable applied by low-frequency analog signals should be connected with ground with one end. Majority of interference problems can be settled through the suitable combination between grounding and shielding. In electronic equipment, ground lines can be classified into system ground, enclosure ground (shielded ground), digital ground (logic ground) and analog ground.
b. The principle of ground line design is to separate digital ground and analog ground. For PCBs with both logic circuitry and linear circuitry, they should be separated from each other as much as possible and connected with ground lines at power terminal. Furthermore, grounding area of linear circuitry should be enlarged as much as possible.
c. Single-point parallel grounding should be implemented on the ground in low-frequency circuitry and part of lines can be connected in series first and then in parallel when practical routing suffers from setback. Multi-point series grounding should be applied on high-frequency circuitry and ground lines should be short and thick. Grid shaped ground coil with massive area should be distributed adjacent to high-frequency components.
d. When working frequency fall in the range from 1 to 10MHz, if a little grounding is applied, the length of ground lines shouldn't be more than 1/20 of wave length. Otherwise, multi-point grounding has to be applied. Ground lines should be designed to be closed loop. In the process of designing ground line system of PCB that is composed only by digital circuitry, closed loops can be formed by ground lines in order to obviously increase anti-noise capacity. The reason is that many integrated circuitry components on PCB, especially those components with large electricity consumption, generate relatively large potential difference under the limitations of thickness of ground lines, which leads to the decreasing of anti-noise capacity. If, however, loop is formed by grounding, potential difference will be decreased with anti-noise capacity improved.
• Reasonable setting of decoupling capacitor
High-frequency decoupling capacitor with excellent performance is capable of eliminating high-frequency element that is as much as 1GHz. Ceramic capacitor or multi-layer ceramic have excellent high-frequency performance. Decoupling capacitor has two functions. On one hand, it is responsible for eliminating high-frequency noise of components. In digital circuits, typical decoupling capacitor with capacitance of 0.1μF features distributed inductance of 5nH. Its parallel frequency is approximately 7MHz, which means that it plays a role in decoupling for noise under 10MHz while it does little influence on noise of more than 40MHz. Capacitors of 1μF and 10μF perform better in high-frequency noise elimination as a result of their resonant frequency that is more than 20MHz. On the other hand, energy storage capacitor in this integrated circuit provides and absorbs the transient charging and emission energy. Charging and emission capacitor should be added for 10 integrated circuits, or storage and emission capacitor. The capacitance of capacitor should be 10μF. Electrolytic capacitors shouldn't be applied since they have a rolling structure that performs as inductance in high frequencies. It's best to apply bile capacitor or poly carbonate capacitor.
The configuration principles of decoupling capacitor include:
a. An electrolytic capacitor of 10 to 100μF should be connected with power terminal. If possible, it's best to connect a capacitor of more than 100μF.
b. Generally, a ceramic capacitor of 0.01μF should be arranged on each IC chip. If PCBs have limited space, a tantalum capacitor should be arranged every 4 to 8 chips.
c. For components that features low anti-noise capacity and large power change such as RAM and ROM storage components, a decoupling capacitor should be accessed between power lines and ground lines of chip.
d. Capacitor leads mustn't be too long and high-frequency bypass capacitor shouldn't have leads.