The fast development of electronic technology contributes to the high density of electronic components, which prompts the ability of anti-interference for PCB designers. In the process of PCB design, designers must conform to the generic principles of PCB design and requirement of anti-interference. The ability of anti-interference in PCB design has a direct relation to the validity and stability of electronic products, even regarded as the key point of design. When the requirement of anti-interference is taken into full consideration in the procedure of design, time will be also saved since anti-interference remedial measures don't need to be made afterwards.
Source of Interference Generation in PCB
The source of interference generation in PCB comes from the following elements:
a. Interference source refers to the components, devices or signals that generate interference such as relays, silicon controlled rectifiers, electric machines and high frequency clocks.
b. Sensitive components refers to the objects that are easily susceptible such as A/D (D/A) converters, single chip microcomputer (SCM), digital IC etc.
c. Transmission path refers to the path or media through which interference travels from its source to the sensitive components. According to interference transmission path, interference can be classified into two categories: conduction interference and radiation interference. The former refers to the interference transmitted through lead to sensitive components. Different from frequency band of useful signals, the transmission of high-frequency interference noise can be cut down by adding filters on leads and sometimes adding isolated opto-coupler also can work. Radiation interference refers to the interference transmitted to sensitive components through space. The general solution is to enlarge the distance between interference source and sensitive components or to isolate them through ground wires.
Principles of Anti-Interference in PCB Design
The generic principles of anti-interference should include inhibit interference source, cut down interference transmission path and increase the anti-interference capacity of sensitive components. The specific measures of each principle will be displayed in the following content:
• To inhibit interference source
a. For relays, two measures can be taken to inhibit interference source. Interference source refers to the components, devices or signals that generate interference such as relays, silicon controlled rectifiers, electric machines and high frequency clocks.
1). Flyback diode can be added to relay coil in order to eliminate the generated interference of back electromotive force with coil off.
2). Spark suppression circuit can be connected to the pins of relays parallel in order to reduce the interference of spark.
b. For electric machines, filter circuit can be added to them. Note that leads of capacitor and inductor should be as short as possible.
c. For silicon controlled rectifiers, RC interference circuit can be connected to pins of silicon controlled rectifier to reduce noise generated by silicon controlled rectifier.
d. A high frequency capacitor in the range from 0.01ΜF to 0.1ΜF should be connected to each IC on board in order to decrease the interference generated by IC to power. Note that in terms of routing of high frequency capacitor, wires should be close to power and be short and thick. Otherwise, equivalent series resistance would be increased with filter effect influenced.
• To cut down interference transmission path
Specifically, ordinary measures of cutting down interference transmission path include:
a. The influence of power to SCM should be taken into full consideration. Many SCMs are quite sensitive to the noise of power and filter circuit or voltage regulator should be added to SCM power in order to decrease the interference from power noise to SCM.
b. If I/O ports in SCM are used to control noise components, isolation (Π-shape filter wave) should be added between I/O ports and noise source.
c. Crystal oscillator routing should be noticed. Crystal oscillator should be close to SCM pins with ground wire isolating the clock zone. The shell of crystal oscillator is connected to the ground and stabilized.
d. The board should have a reasonable partition based on strong or weak signals, digital or analog signals. Interference source such as electrical machine or relay should be isolated from sensitive components such as SCM.
e. Ground wires should be used to isolate digital zone from analog zone, digital ground from analog ground that will be connected to power ground at one end. This principle is also suitable for A/D and D/A chip routing.
f. Ground wires of SCM and high-power components should be connected to the ground independently in order to decrease the mutual interference. Besides, high-power components should be placed at the edge of the board.
g. Anti-interference components such as Ferrite bead, Ferrite tube, power filter and shielding case are used on some key places on board such as SCM I/O ports, power wire and PCB connecting lines so as to drastically increase the anti-interference capacity of circuit.
• To increase the anti-interference capacity of sensitive components
This refers to the measures that the pickup of interference noise should be reduced from sensitive components and fast recovery from abnormal conditions. Ordinary measures to increase the anti-interference capacity of sensitive components include:
a. The area of circuit loop should be enlarged when routing in order to reduce induced noise.
b. When routing, both power line and ground wire should be as thick as possible, which is capable of reducing pressure drop and decoupling noise.
c. The idle I/O ports on SCM should be connected to ground or power, and so should other IC idle ports without changing the system logic.
d. Power monitor and circuit of watchdog should be used on SCM so that anti-interference capacity of the whole circuit can be drastically increased.
e. IC components should be directly welded on the board instead of IC sockets.
f. As present speed can meet requirements, crystal oscillator of SCM should be decreased and low-speed digital circuit should be picked up.
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