With the emergence of high-speed DSPs (digital signal processors) and peripherals, new product designers are facing an increasingly serious threat of electromagnetic interference (EMI). In the early days, transmission and interference issues were referred to as EMI or RFI (radio frequency interference). Now use the more definite word "interference compatibility" instead. Electromagnetic compatibility (EMC) encompasses both the emission and sensitivity aspects of the system. If the interference cannot be completely eliminated, it must also be minimized. If a DSP system meets the following three conditions, then the system is electromagnetic compatible.

1. It does not cause interference to other systems.

2. Not sensitive to the emission of other systems.

3. It does not cause interference to the system itself.

Definition of interference

When the energy of interference puts the receiver in an unwanted state, it causes interference. The generation of interference is either direct (through conductor, common impedance coupling, etc.) or indirect (through crosstalk or radiation coupling). Electromagnetic interference is generated through conductors and radiation. Many electromagnetic emission sources, such as lighting, relays Both DC motors and fluorescent lamps can cause interference. AC power lines, interconnect cables, metal cables, and internal circuits of subsystems may also generate radiation or receive unwanted signals. In high-speed digital circuits, clock circuits are usually the largest source of broadband noise. In fast DSP, these circuits can generate harmonic distortion up to 300MHz, which should be removed in the system. In digital circuits, the reset line, interrupt line, and control line are the most easily affected.

Conductive EMI

The most obvious and often overlooked path that can cause noise in a circuit is through a conductor. A wire passing through a noisy environment can detect noise and send it to another circuit to cause interference. Designers must avoid wire picking noise and use decoupling methods to remove noise before it causes interference. The most common example is noise entering the circuit through the power cord. If the power supply itself or other circuits connected to the power supply are sources of interference, it must be decoupled before the power line enters the circuit.

Common impedance coupling

When current from two different circuits flows through a common impedance, common impedance coupling occurs. The voltage drop on impedance is determined by two circuits. The ground current from two circuits flows through a common ground impedance. The ground potential of circuit 1 is modulated by ground current 2. The noise signal or DC compensation is coupled from circuit 2 to circuit 1 through a common ground impedance.

Radiative coupling

The coupling through radiation is commonly known as crosstalk, which occurs when a current flows through a conductor, generating an electromagnetic field that induces transient currents in adjacent conductors.

Radiation emission

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

Factors affecting EMC

Voltage - The higher the power supply voltage, the greater the voltage amplitude and more emissions, while a lower power supply voltage affects sensitivity.

Frequency - High frequencies generate more emissions, while periodic signals generate more emissions. In high-frequency digital systems, current spikes are generated when the device is turned on or off; In a simulation system, a current spike signal is generated when the load current changes.

Grounding - There is nothing more important for circuit design than a reliable and perfect power system. Among all EMC issues, the main issue is caused by improper grounding. There are three signal grounding methods: single point, multi-point, and mixed. When the frequency is below 1MHz, a single point grounding method can be used, but it is not suitable for high frequencies. In high-frequency applications, it is best to use multi-point grounding. Hybrid grounding is a method of using single point grounding for low frequencies and multi point grounding for high frequencies. The layout of the ground wire is crucial. The ground circuit of high-frequency digital circuits and low-level analog circuits must not be mixed.

PCB design - Proper wiring of printed circuit boards (PCBs) is crucial for preventing EMI.

Power Decoupling - When the device is turned on or off, transient currents are generated on the power line, and these transient currents must be attenuated and filtered out from high di/dt sources, causing ground and line trace "emission" voltage. High di/dt generates a wide range of high-frequency currents, which excite components and cable radiation. The change in current and inductance flowing through the wire can cause a voltage drop, and reducing the inductance or the change in current over time can minimize this voltage drop.

Technologies for reducing noise

There are three methods to prevent interference:

1. Suppress source emission.

2. Make the coupling path as ineffective as possible.

3. Minimize the sensitivity of the receiver to transmission.

Below is an introduction to board level noise reduction technology. Board level noise reduction technology includes board structure, circuit arrangement, and filtering.

The plate structure noise reduction technology includes:

*Using ground and power supply tablets

*The flat surface area should be large to provide low impedance for power decoupling

*Minimize surface conductors

*Using narrow lines (4 to 8 mils) to increase high-frequency damping and reduce capacitive coupling

*Separate digital, analog, receiver, transmitter ground/power lines

*Separate circuits on PCBs based on frequency and type

*Do not cut the PCB, as the traces near the cut may cause unwanted loops

*Using multi-layer boards to seal the wiring between the power supply and the floor layer

*Avoid large open loop plate layer structures

*The PCB connector is connected to the chassis ground, which provides shielding to prevent radiation at the circuit boundaries

*Using multi-point grounding to reduce high-frequency ground impedance

*Keep the ground pin shorter than 1/20 of the wavelength to prevent radiation and ensure low impedance line arrangement. Noise reduction technology includes using 45. Instead of 90. Track turning, 90. Turning will increase capacitance and cause changes in the characteristic impedance of the transmission line

*Maintain a distance between adjacent excitation traces greater than the width of the trace to minimize crosstalk

*The clock signal loop area should be as small as possible

*The high-speed line and clock signal line should be short and directly connected

*Sensitive traces should not be parallel to traces that transmit high current fast switching signals

*Do not have floating digital inputs to prevent unnecessary switching and noise generation

*Avoid power supply traces under crystal oscillators and other inherent noise circuits

*The corresponding power, ground, signal, and loop traces should be parallel to eliminate noise

*Keep the clock line, bus, and chip enabled separate from input/output lines and connectors

*Route clock signal orthogonal I/O signal

*To minimize crosstalk, the traces are crossed at right angles and ground wires are scattered

*Protect critical traces (use 4 to 8 mil traces to minimize inductance, route close to the floor layer, sandwich structure between board layers, with ground on each side of the protective interlayer)

Filtering techniques include:

*Filter the power cord and all signals entering the PCB

*At each point of the IC, use high-frequency low inductance ceramic capacitors (0.1UF for 14MHz, 0.01UF for over 15MHz) to decouple the original pins

*All power supply and reference voltage pins of the bypass analog circuit

*Bypass fast switching device

*Decoupling power/ground at device leads

*Using multi-level filtering to attenuate multi band power supply noise

Other noise reduction design techniques include:

*Insert the crystal oscillator into the board and ground it

*Add shielding in appropriate places

*Using series terminals to minimize resonance and transmission reflection, impedance mismatch between load and line can cause partial signal reflection, which includes instantaneous disturbance and overshoot, resulting in significant EMI

*Arrange adjacent ground wires to be close to signal lines in order to more effectively prevent the occurrence of electric fields

*Place the decoupling line driver and receiver appropriately close to the actual I/O interface, which can reduce coupling to other PCB circuits and reduce radiation and sensitivity

*Shield and twist interfering leads together to eliminate mutual coupling on the PCB

*Using clamping diodes on inductive loads

EMC is an important issue to consider in DSP system design, and appropriate noise reduction techniques should be adopted to ensure that the DSP system meets EMC requirements