With the emergence of high-speed DSP (Digital Signal Processors) and peripherals, new product designers face an increasingly severe threat from electromagnetic interference (EMI).
Initially, the issues of transmission and interference were referred to as EMI or RFI (Radio Frequency Interference). Now, a more definitive term, “interference compatibility,” is used instead. Electromagnetic compatibility (EMC) encompasses both the emission and sensitivity aspects of a system.
If interference cannot be completely eliminated, it must be minimized. A DSP system is considered electromagnetically compatible if it meets the following three conditions:
1. It does not generate interference to other systems;
2. It is not sensitive to emissions from other systems;
3. It does not generate interference to itself.
1. Definition of Interference
Interference occurs when the energy of interference causes the receiver to be in an undesirable state. The generation of interference can be either direct (through conductors, common impedance coupling, etc.) or indirect (through crosstalk or radiated coupling). Electromagnetic interference is generated through conductors and radiation. Many sources of electromagnetic emissions, such as lighting, relays, DC motors, and fluorescent lights, can cause interference. AC power lines, interconnect cables, metal cables, and internal circuits of subsystems can also radiate or receive unwanted signals.
In high-speed digital circuits, clock circuits are often the largest sources of broadband noise. In fast DSPs, these circuits can produce harmonic distortion up to 300MHz, which should be eliminated from the system. In digital circuits, the reset line, interrupt line, and control lines are the most susceptible to interference.
(1) Conductive EMI
One of the most obvious yet often overlooked paths that can introduce noise into circuits is through conductors. A wire passing through a noisy environment can pick up noise and transfer it to another circuit, causing interference. Designers must avoid wires picking up noise and remove noise through decoupling methods before it causes interference. The most common example is noise entering the circuit through the power line. If the power source itself or other circuits connected to the power source are sources of interference, decoupling must be performed before the power line enters the circuit.
(2) Common Impedance Coupling
Common impedance coupling occurs when currents from two different circuits flow through a common impedance. The voltage drop across the impedance is determined by both circuits. Ground currents from both circuits flow through the common ground impedance. The ground potential of Circuit 1 is modulated by the ground current of Circuit 2. Noise signals or DC compensation couple from Circuit 2 to Circuit 1 through the common ground impedance.
(3) Radiated Coupling
Radiated coupling, commonly known as crosstalk, occurs when a current flowing through a conductor generates an electromagnetic field, which induces transient currents in nearby conductors.
(4) Radiated Emissions
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 unintended voltage drops, which raise all ground connections in the circuit above the system ground potential. In terms of electric field magnitude, CM radiation is a more serious problem than DM radiation. To minimize CM radiation, common mode currents must be reduced to zero through practical design.
2. Factors Affecting EMC
Voltage — Higher supply voltage means larger voltage amplitudes and more emissions, while lower supply voltage affects sensitivity.
Frequency — High frequencies generate more emissions, and periodic signals produce more emissions. In high-frequency digital systems, current spikes occur when devices switch; in analog systems, current spikes occur when load currents change.
Grounding — There is nothing more important for circuit design than a reliable and perfect power system. The main issue in all EMC problems arises from improper grounding. There are three methods of signal grounding: single-point, multi-point, and mixed. The single-point grounding method can be used at frequencies below 1MHz but is not suitable for high frequencies. In high-frequency applications, multi-point grounding is preferred. Mixed grounding combines single-point grounding for low frequencies and multi-point grounding for high frequencies. Ground layout is key. The ground return loops of high-frequency digital circuits and low-level analog circuits must not be mixed.
PCB Design — Proper printed circuit board (PCB) layout is crucial for preventing EMI.
Power Decoupling — When devices switch, transient currents are generated on the power lines, which must be attenuated and filtered out. Transient currents from high di/dt sources lead to “emission” voltages in the ground and traces. High di/dt generates a wide range of high-frequency currents, exciting components and cables to radiate. Changes in current flowing through conductors and inductance can cause voltage drops; reducing inductance or the rate of change of current over time can minimize this voltage drop.
3. Noise Reduction Techniques
There are three methods to prevent interference:
1. Suppress source emissions.
2. Render coupling paths as ineffective as possible.
3. Minimize the sensitivity of receivers to emissions.
Below are board-level noise reduction techniques. Board-level noise reduction techniques include board structure, trace arrangement, and filtering.
Board structure noise reduction techniques include:
* Use ground and power planes
* The plane area should be large to provide low impedance for power decoupling
* Minimize surface conductors
* Use narrow traces (4 to 8 mils) to increase high-frequency damping and reduce capacitive coupling
* Separate digital, analog, receiver, and transmitter ground/power lines
* Segregate circuits on the PCB based on frequency and type
* Do not score the PCB, as traces near the scoring may cause unwanted loops
* Use multilayer boards to seal traces between power and ground layers
* Avoid large open-loop board structures
* Connect PCB connectors to chassis ground to provide shielding against emissions at circuit boundaries
* Use multi-point grounding to keep high-frequency ground impedance low
* Keep ground pins shorter than 1/20 of the wavelength to prevent radiation and ensure low-impedance trace arrangements
Trace arrangement noise reduction techniques include using 45-degree instead of 90-degree turns, as 90-degree turns increase capacitance and cause transmission line characteristic impedance changes.
* Maintain a spacing greater than the width of the trace between adjacent excited traces to minimize crosstalk
* Minimize the loop area of clock signal traces
* Keep high-speed traces and clock signal traces short and directly connected
* Do not run sensitive traces parallel to traces carrying high current fast-switching signals
* Avoid floating digital inputs to prevent unnecessary switching and noise generation
* Avoid power traces under oscillators and other inherently noisy circuits
* Keep corresponding power, ground, signal, and return traces parallel to eliminate noise
* Separate clock traces, buses, and chip enables from input/output traces and connectors
* Route clock signals orthogonal to I/O signals
* To minimize crosstalk, traces should cross at right angles and ground lines should be scattered
* Protect critical traces (use 4 mil to 8 mil traces to minimize inductance, routing close to the ground plane, and sandwich structures between board layers, with ground on each side of the sandwich)
Filtering techniques include:
* Filter power lines and all signals entering the PCB
* Use high-frequency low-inductance ceramic capacitors (0.1uF for 14MHz, 0.01uF for over 15MHz) for decoupling at each power pin of the IC
* Bypass all power supply and reference voltage pins of analog circuits
* Bypass fast-switching devices
* Decouple power/ground at device leads
* Use multi-stage filtering to attenuate multi-band power noise
Other noise reduction design techniques include:
* Embed the crystal oscillator onto the board and ground it
* Add shielding where appropriate
* Use series termination to minimize resonance and transmission reflections; impedance mismatches between loads and lines can cause partial signal reflections, including instantaneous disturbances and overshoots, which can generate significant EMI
* Arrange nearby ground lines close to signal lines to more effectively block electric fields
* Properly place decoupling drivers and receivers close to the actual I/O interfaces to reduce coupling to other circuits on the PCB and minimize radiation and sensitivity
* Shield and twist together leads that have interference to eliminate mutual coupling on the PCB
* Use clamp diodes on inductive loads (u=L*di/dt, clamp diodes can clamp u to prevent high-frequency high-voltage pulses)
EMC is an important issue to consider in DSP system design, and appropriate noise reduction techniques should be adopted to ensure DSP systems meet EMC requirements.
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