How to eliminate the electromagnetic interference of the single chip system? (I)

As single-chip systems are more and more widely used in consumer electronics, medical, industrial automation, intelligent instrumentation, aerospace and other fields, single-chip systems are facing increasing threats of electromagnetic interference (EMI).

Electromagnetic compatibility (EMC) covers both the emission and sensitivity of the system.

If a microcontroller system meets the following three conditions, the system is electromagnetically compatible:

1 does not cause interference to other systems;

2 is not sensitive to the transmission of other systems;

3 No interference to the system itself.

If the interference cannot be completely eliminated, but also to minimize interference. The generation of interference is either direct (via conductors, common impedance coupling, etc.) or indirect (by crosstalk or radiative coupling). Electromagnetic interference is generated by conductors and by radiation. Many electromagnetic sources, such as light, relays, DC motors and fluorescent lamps, can cause interference. The internal circuits of AC power lines, interconnecting cables, metal cables and subsystems can also generate radiation. Or receive an unwanted signal. In high-speed microcontroller systems, clock circuits are often the largest source of broadband noise, which can produce harmonic distortions of up to 300 MHz, which should be removed in the system. In addition, in the microcontroller system, the most susceptible are the reset line, the interrupt line and the control line.

1, the coupling method of interference

(1) Conductive EMI

One of the most obvious and often overlooked paths that cause noise in the circuit is through the conductor. A wire that passes through a noisy environment picks up noise and sends it to other circuits to cause interference. Designers must avoid wire pick-up noise and decoupling to remove noise before noise causes interference. The most common example is the noise entering the circuit through the power line. If the power supply itself or other circuitry connected to the power supply is the source of interference, the power supply must be decoupled before it enters the circuit.

(2) Common impedance coupling

Co-impedance coupling occurs when current from two different circuits flows through a common impedance. The voltage drop across the impedance is determined by two circuits, and the ground current from the two circuits flows through the common ground impedance. The ground potential of circuit 1 is modulated by ground current 2, and a noise signal or DC compensation is coupled from circuit 2 to circuit 1 via a common ground impedance.

(3) Radiation coupling

The coupled radiation is known as crosstalk. Crosstalk occurs when an electric current is generated by a current flowing through a conductor, and an electromagnetic field induces a transient current in an adjacent conductor.

(4) Radiation emission

There are two basic types of radiated emissions: differential mode (DM) and common mode (CM). Common mode radiation or monopole antenna radiation is caused by an unintentional voltage drop that raises all ground connections in the circuit above the system ground potential. In terms of electric field size, CM radiation is a more serious problem than DM radiation. To minimize CM radiation, the common mode current must be reduced to zero with a realistic design.