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Five: High-speed PCB Design Guide: Noise Reduction Technology of DSP System

With the advent of high-speed DSP (digital signal processor) and peripherals, designers of new products are facing an increasingly serious threat of electromagnetic interference (EMI). In the early days, emission and interference problems were called EMI or RFI (Radio Frequency Interference). Now use the more definite word "interference compatibility" instead. Electromagnetic compatibility (EMC) includes two aspects of the system's emission and sensitivity. If the interference cannot be completely eliminated, the interference must be minimized. If a DSP system meets the following three conditions, the system is electromagnetically compatible.

1. No interference to the system itself.

2. No interference to other systems.

3. It is not sensitive to the emission of other systems.

Definition of interference

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

Conducted EMI

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

Radiation coupling

The radiated coupling is known as crosstalk. Crosstalk occurs when an electric current flows through a conductor to generate an electromagnetic field, and the electromagnetic field induces transient currents in adjacent conductors.

Common impedance coupling

Common impedance coupling occurs when currents from two different circuits flow through a common impedance. The voltage drop on the impedance is determined by two circuits. The ground currents from the two circuits flow through the 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 via a common ground impedance.

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, which 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. In order 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 the more emission, and the low power supply voltage affects the sensitivity.

Frequency-High frequencies produce more emissions, and periodic signals produce more emissions. In a high-frequency digital system, a current spike signal is generated when the device is switched; in an analog system, a current spike signal is generated when the load current changes.

Grounding-Nothing is more important to circuit design than a reliable and perfect power system. In all EMC problems, the main problem is caused by improper grounding. There are three signal grounding methods: single-point, multi-point, and mixed. Single-point grounding method can be used when the frequency is lower than 1MHz, but it is not suitable for high frequency. In high-frequency applications, it is best to use multi-point grounding. Hybrid grounding is a single-point grounding method for low frequency and multi-point grounding for high frequency. The ground wire layout is critical. The ground loops of high-frequency digital circuits and low-level analog circuits must not be mixed.

Power supply decoupling-When the device is switched on and off, transient currents will be generated on the power line. These transient currents must be attenuated and filtered out. The transient currents from high di/dt sources cause ground and trace "emission" voltages. The high di/dt generates a wide range of high-frequency currents that excite components and cables to radiate. The current change and inductance flowing through the wire will cause a voltage drop, which can be minimized by reducing the inductance or current change over time.

PCB design-Proper printed circuit board (PCB) wiring is essential to prevent EMI.


Technology to reduce noise

There are three ways to prevent interference:

1. Suppress source emission.

2. Make the coupling path as ineffective as possible.

3. Make the receiver's sensitivity to transmission as small as possible.

The following describes the board-level noise reduction technology. Board-level noise reduction technology includes board structure, line arrangement and filtering.

The noise reduction technology of the board structure includes:

* Adopt ground and power plate

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

* Minimize surface conductors

* Separate ground/power cables for digital, analog, receiver and transmitter

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

* Separate the circuit on the PCB according to frequency and type

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

* Use multi-layer boards to seal the traces between the power supply and the floor layer

* Avoid large open-loop board structure

* Multi-point grounding is used to make high-frequency ground impedance low

* Keep the ground pin shorter than 1/20 of the wavelength to prevent radiation and ensure low-impedance line arrangements. Noise reduction techniques include 45. Instead of 90. Stitch turns, 90. Turning will increase the capacitance and cause the characteristic impedance of the transmission line to change

* Keep the distance between adjacent excitation traces greater than the width of the traces to minimize crosstalk

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

* High-speed lines and clock signal lines should be short and directly connected

* Sensitive traces should not be in parallel with 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 the crystal oscillator and other inherent noise circuits

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

* Keep the clock line, bus and chip enable separated from the input/output line and connector

* Route clock signal quadrature I/O signal

* In order to minimize crosstalk, the traces should be crossed at right angles and ground wires should be scattered

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

* Protect key traces (use 4 mils to 8 mils traces to minimize inductance, the route is close to the floor layer, the sandwich structure between the layers, and each side of the protective layer has ground)

Filtering techniques include:

* Filter the power cord and all signals entering the PCB

* Use high-frequency low-inductance ceramic capacitors (0.1UF for 14MHz, 0.01UF for more than 15MHz) at each point of the IC for decoupling

* Decouple the power/ground at the device leads

* Use multi-stage filtering to attenuate multi-band power supply noise

* Bypass all power supply and reference voltage pins of the analog circuit

* Bypass fast switching devices

Other noise reduction design techniques include:

* Embed the crystal oscillator installation on the board and ground it

* Use series termination to minimize resonance and transmission reflection. Impedance mismatch between the load and the line will cause partial reflection of the signal. The reflection includes instantaneous disturbance and overshoot, which will generate a lot of EMI

* Arrange the adjacent ground wire close to the signal wire to prevent the electric field from appearing more effectively

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

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

* Use clamp diodes on inductive loads

* Add shielding where appropriate

EMC is an important issue to be considered in the design of DSP systems. Appropriate noise reduction technology should be adopted to make the DSP system meet EMC requirements.