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In the design of electronic system, in order to reduce detours and save time, the requirements of anti-interference should be fully considered and met to avoid taking anti-interference remedial measures after the design is completed. There are three basic elements of interference:

(1) Interference source refers to the element, equipment or signal that generates interference. It is described in mathematical language as follows: du/dt, where di/dt is large, is the interference source. For example, lightning, relay, thyristor, motor, high-frequency clock, etc. may become interference sources.

(2) Propagation path refers to the path or medium through which the interference propagates from the interference source to the sensitive device. The typical interference propagation path is the conduction of wires and the radiation of space.

(3) Sensitive devices refer to objects that are easily disturbed. Such as: A/D, D/A converter, single-chip microcomputer, digital IC, weak signal amplifier, etc. The basic principle of anti-interference design is to suppress the interference source, cut off the interference propagation path, and improve the anti-interference performance of sensitive devices.

1 Suppress interference sources

Suppressing interference sources is to reduce the du/dt and di/dt of interference sources as much as possible. This is a priority and important principle in anti-jamming design, and often can achieve twice the result with half the effort. The du/dt reduction of the interference source is mainly achieved by paralleling capacitors at both ends of the interference source. Reducing di/dt of interference source is realized by serializing inductance or resistance in the interference source circuit and adding freewheeling diode.

Common measures to suppress interference sources are as follows:

(1) The relay coil adds a freewheeling diode to eliminate the back electromotive force interference generated when the coil is disconnected. Only adding the freewheeling diode will delay the disconnection time of the relay, and the relay can act more times in unit time after adding the voltage regulator diode.

(2) Spark suppression circuit is connected at both ends of relay contact in parallel (generally RC series circuit, resistance is generally selected from several K to dozens K, capacitance is selected from 0.01uF) to reduce the impact of electric spark.

(3) Add a filter circuit to the motor, and pay attention to the capacitor and inductor leads should be as short as possible.

(4) Each IC on the circuit board should be connected with a 0.01 μ F～0.1 μ F high-frequency capacitance to reduce the impact of IC on power supply. Pay attention to the wiring of high-frequency capacitor. The wiring should be close to the power supply end and as thick as possible. Otherwise, it will increase the equivalent series resistance of the capacitor and affect the filtering effect.

(5) Avoid 90-degree broken lines during wiring to reduce high-frequency noise emission.

(6) RC suppression circuit is connected at both ends of the thyristor in parallel to reduce the noise generated by the thyristor (the thyristor may be broken down when the noise is serious).

According to the propagation path of interference, it can be divided into conducted interference and radiated interference.

The so-called conducted interference refers to the interference transmitted to sensitive devices through wires. The frequency band of high frequency interference noise is different from that of useful signal. The propagation of high frequency interference noise can be cut off by adding a filter on the wire, and sometimes it can also be solved by adding an isolation optocoupler. Power supply noise is harmful, and special attention should be paid to treatment. The so-called radiation interference refers to the interference transmitted to sensitive devices through space radiation. The general solution is to increase the distance between the interference source and the sensitive device, isolate them with a ground wire and put a mask on the sensitive device.

2 Common measures to cut off the interference propagation path are as follows:

(1) Fully consider the influence of power supply on single-chip microcomputer. If the power supply is well done, the anti-interference of the whole circuit is solved by more than half. Many single-chip microcomputers are very sensitive to power noise, so a filter circuit or voltage stabilizer should be added to the single-chip power supply to reduce the interference of power noise on the single chip. For example, magnetic beads and capacitors can be used to form a π filter circuit. Of course, 100 Ω resistance can also be used to replace the magnetic beads when the conditions are not high.

(2) If the I/O port of the single chip computer is used to control noise devices such as motors, isolation should be added between the I/O port and the noise source (adding a π filter circuit). For controlling noise devices such as motors, isolation shall be added between I/O port and noise source (adding π filter circuit).

(3) Pay attention to crystal oscillator wiring. The crystal oscillator is as close as possible to the pins of the single chip computer. The clock area is isolated by the ground wire, and the crystal oscillator shell is grounded and fixed. This measure can solve many difficult problems.

(4) The circuit board shall be reasonably zoned, such as strong and weak signals, digital and analog signals. Try to keep interference sources (such as motors and relays) away from sensitive components (such as single-chip microcomputer).

(5) Use the ground wire to isolate the digital area from the analog area. The digital area and the analog area should be separated, and then connected to the power ground at a point. The wiring of A/D and D/A chips is also based on this principle. The manufacturer has taken this requirement into account when allocating the pin arrangement of A/D and D/A chips.

(6) The ground wire of single chip microcomputer and high-power device shall be grounded separately to reduce mutual interference. High-power devices should be placed on the edge of the circuit board as much as possible.

(7) Anti-interference components such as magnetic beads, magnetic rings, power filters, and shielding shields can be used in key places such as the I/O port of the single chip computer, power lines, and circuit board connecting lines to significantly improve the anti-interference performance of the circuit.

3. Improve the anti-interference performance of sensitive devices

Improving the anti-interference performance of the sensitive device refers to the method of reducing the pickup of interference noise as far as possible from the sensitive device and recovering from abnormal state as soon as possible.

Common measures to improve the anti-interference performance of sensitive devices are as follows:

(1) When wiring, minimize the area of the loop to reduce the induced noise.

(2) When wiring, the power line and ground wire should be as thick as possible. In addition to reducing voltage drop, it is more important to reduce coupling noise.

(3) For the idle I/O port of the single chip computer, do not hang in the air, but connect to the ground or power supply. The idle end of other ICs is grounded or powered on without changing the system logic.

(4) The use of power monitoring and watchdog circuits for single chip computers, such as IMP809, IMP706, IMP813, X25043, X25045, can greatly improve the anti-interference performance of the entire circuit.

(5) On the premise that the speed can meet the requirements, try to reduce the crystal vibration of the single chip computer and select low-speed digital circuits.

(6) IC devices should be soldered directly on the circuit board as much as possible, and IC sockets should be used less.

In order to achieve good anti-interference, we often see the ground split wiring mode on the PCB. But not all digital circuits and analog circuits must be divided into ground planes. Because this division is to reduce