With the emergence of high-speed DSP (digital signal processors) and peripherals, new product designers faced electromagnetic interference (EMI) increasingly serious threats. Early, transmitted launch and interference issues EMI or RFI (radio frequency interference). Now use more identified words "interference compatibility". Electromagnetic compatibility (EMC) contains two problems of the transmission and sensitivity of the system. If the interference cannot be completely eliminated, it is also necessary to minimize interference. If a DSP system meets the following three conditions, the system is electromagnetic compatible.
1. No interference is made to other systems.
2. Insensitive to other systems.
3. Do not disturb the system itself.
Interference definition
When the energy of the interference causes interference when the receiver is in an undesired state. The generation of interference is not direct (by conductor, common impedance coupling, etc.) is indirect (by crosstalk or radiation coupling). The generation of electromagnetic interference is through conductor and through radiation. Many electromagnetic densities, such as illumination, relays, DC motors, and fluorescent lamps can cause interference. The internal circuits of the AC power cord, the interconnect cable, the metal cable and subsystems can also generate radiation or receiving undesirable signals. In the high speed digital circuit, the clock circuit is usually the maximum source of broadband noise. In the fast DSP, these circuits produce harmonic distortion of up to 300 MHz, which should be removed in the system. In the digital circuit, it is most likely to affect the reset line, the interrupt line, and the control line.
Transducer EMI
A path that is most obviously negligible can cause noise in the circuit is conducted. A wire passing through the noise environment can check the noise and send noise to another circuit to cause interference. Designers must avoid wire picking noise and remove noise with decoupling as the noise causes interference. The most common example is that the noise enters the circuit through the power cord. If the power supply itself or other circuit connected to the power supply is an interference source, it must be decoupled before the power cord enters the circuit.
Common impedance coupling
Co-impedance coupling is generated when current from two different circuits flows through a common impedance. The pressure drop on the impedance is determined by two circuits. The ground current from the two circuits flows through a common impedance. The ground potential of the circuit 1 is modulated by ground current 2. The noise signal or DC compensation is coupled to the circuit 2 from the circuit 2.
Radiation coupling
The radiated coupling generally referred to that crosstalk occurs when current flows through the conductor, while the electromagnetic field is inductive in the adjacent conductor.
Radiation launch
Two basic types of radiation emissions: Differential Mode (DM) and Currency (CM). Common-mode radiation or unipolar antenna radiation is caused by unintentional pressure drop that enables all ground connections to the system to the systemically potential. In terms of electric field size, CM radiation is a more serious problem than DM radiation. In order to minimize the CM radiation, the common mode current must be reduced to zero with the actual design.
Factor affecting EMC
The higher the voltage - the supply voltage is, which means that the larger the voltage amplitude is more, and the launch is more, while the low supply voltage affects the sensitivity.
Frequency - high frequency produces more emission, and periodic signals produce more emission. In the high frequency digital system, a current spike signal is generated when the device switch; in the simulation system, the current spike signal is generated when the load current changes.
Ground - There is no more important thing than the circuit design than a reliable and perfect power system. In all EMC issues, the main problem is inappropriate grounding. There are three signal grounding methods: single, multi-point and mixing. A single point grounding method can be used at a frequency below 1 MHz, but is not suitable for high frequency. In high frequency applications, it is best to use multi-point grounding. Mixed grounding is a low-frequency single-point grounding method for multi-point grounding. Ground layout is critical. The high-frequency digital circuit and the low-level analog circuit are absolutely not mixed.
PCB Design - Appropriate Printed Circuit Board (PCB) wiring is critical to prevent EMI.
Power supply decoupling - When the device switch, the transient current is generated on the power cord, which must be attenuated and filtered away. These transient currents from the transient current from the high Di / DT source resultively and the track "emission" voltage. High Di / DT produces a wide range of high-frequency current, excitation parts, cable radiation. Current changes and inductance flowing through the wire can cause pressure drop, reducing the inductance or current over time, can minimize the pressure drop.
Technology to reduce noise
Three methods for preventing interference:
1. Inhibit the source emission.
2. Keep the coupling path as much as possible.
3. Make the receiver to the sensitivity of the transmitter as small as possible.
The board-level noise reduction technology is described below. Plate-level noise reduction techniques include board structures, line arrangements and filters.
Board structure noise reduction techniques include:
* Using ground and power plate
* The flat area is large to provide low impedance for power supply decoupling
* Minimize surface conductors
* Use narrow lines (4 to 8 mils) to increase high frequency damping and reduce capacitive coupling
* Separate digital, simulation, receiver, transmitter / power cord
* Separate the circuit on the PCB based on frequency and type
* Do not cut the PCB, the strand near the cutting can lead to undesirable loops
* The line of sealing power and floor layers is used in multilayer board sealing
* Avoid large open loop layers
* PCB coupler pick-up housings, which provides shielding to prevent radiation at the boundary
* Using multi-point grounding to reduce high frequency impedance
* Keep the ground pin less than 1/20 of the wavelength to prevent radiation and ensure that the low impedance line arrangement noise reduction technology includes 45. Instead of 90. The strand turns, 90. Steering increase capacitance and causes change in transmission line characteristics
* Keep the spacing between adjacent excitation stations greater than the width of the track to minimize crosstalk
* Clock signal loop area should be small
* High-speed lines and clock signal lines are short and directly connected
* Sensitive tracks do not focus on the transfer signal of the transmission high current quick switch
* Do not have floating digital inputs to prevent unnecessary switching conversions and noise generation
* Avoid supplying electricity lines under crystal and other inherent noise circuits
* The corresponding power, ground, signals, and back path should be parallel to eliminate noise
* Keep the clock line, bus and film can be separated from the input / output line and connector
* Route clock signal orthogonal I / O signal
* In order to make crosstalk, lineage cross and bulkline
* Protect critical strands (with 4 mils to 8 mils to make the inductance, the route is close to the floor layer, the laminated structure between the plates, the protective interlains)
Filtering technology includes:
* Filtering the power cord and all signals entering the PCB
* Perform decoupling with high-frequency low inductive ceramic capacitance (14 MHz with 0.1uf, more than 15 MHz with 0.01uF)
* All power supply and reference voltage pins of the bypass analog circuit
* Bypass fast switching device
* Wheavs the power / ground in the device leads
* Decay multi-band power noise with multi-stage filtering
Other noise reduction design techniques are:
* Embed the crystal excitation to the board and ground
* Plus mask in proper places
* Use the series terminal to minimize resonance and transmission reflection, the impedance mismatch between the load and the line can cause the signal part to reflect, reflecting including instantaneous disturbance and overshoot, which will produce a large EMI
* Arrange the neighboring ground line to close the signal line to more effectively prevent the appearance of electric fields
* Apply the decoupling line driver and the receiver at the immediate I / O interface, which can reduce the coupling of other PCBs and reduce the radiation and sensitivity.
* Shielding and twisting with interference leads to eliminate mutual coupling on the PCB
* Clamp diodes in an inductive load
EMC is an important issue to consider by the DSP system design. Appropriate noise reduction techniques should make DSP systems comply with EMC requirements.
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