( Electromagnetic interference ) can occur in the frequency range commonly identified as anything greater than DC. RF energy may be transmitted as a byproduct of an electronic device's operation. This energy can be transmitted through free space or conducting wires. To prevent from these electronic component should have relative radiated and conducted immunity. Suppression of RF energy can be achieved using shielding or filtering methods.
Causes of noise sources ( EMI ) within PCB : Noise sources are frequency generation circuits, component radiation within a plastic package, incorrect trace routing, ground bounce from digital logic, and common-mode currents developed within an assembly. This noise is propagated through free space or interconnects. Receptor of RF energy such as amplifiers , controllers , power supplies etc. transfers this harmful energy to easily affected circuits and devices.
Capacitance is always present between two leads. Dielectric between this capacitor is air that is used for RF propagation.
Trace inductance is only one reason RF energy is developed within a PCB. Magnetic fields exist because a current loop network is present. These fields are radiated through free space as unwanted electromagnetic energy.
EMI is often the result of passive component behavior at high frequency. A resistor at high frequency acts as a series combination of inductance within the leads of the resistor, in parallel with a capacitor across the two terminals. A capacitor at high frequency acts as an inductor with a resistor in a series combination on each side of the capacitor plates. An inductor at high frequency performs as an inductor with a capacitor across the two terminals, along with some resistance in the leads ( Fig 1) .One should never assume that a resistor is a resistor, a capacitor is a capacitor, or an inductor is an inductor when used with digital components.
Ground loops are a major contributor to the development and propagation of RF energy. RF current will attempt to return to its source through any available path or medium i.e. components, wire harnesses, power or ground planes, adjacent traces via crosstalk etc.
RF current always exists between source and load. This presence is due to the need to have a closed loop for the signal and return path. This loop develops a voltage potential difference between two devices, regardless of whether inductance exists between these points. Inductance in a transmission line causes magnetic coupling of RF current to occur between a source and victim circuit, increasing RF losses in the return path .
Crosstalk is one important aspect of a layout that must be considered during the design cycle. It refers specifically to unintended electromagnetic coupling between traces, wires, trace-to-wire, cable assemblies, components, and other electrical components subject to electromagnetic field disturbance. These paths include PCB traces., crosstalk represents RF energy coupled from a source trace to a victim trace.
More sensitive parts of a PCB in terms of EMI, ESD, and other forms of radiated and conducted susceptibility are I/O and related interconnects. I/O logic must be located as physically close as possible to the connector in order to minimize trace lengths and the risk that these signals will receive coupling from other circuits. Filtering is often required and is always placed between the driver and connector.
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| Fig 1 |
Ways of suppressing RF energy : The use of planes, voltage, or ground planes internal to the assembly is one important design technique of suppressing common-mode RF energy developed within the PCB. Along with this , proper implementation of decoupling and bypass capacitors for a specific application is required. When we combine a clockwise field of signal current with a counterclockwise field of return path current, a cancellation effect is observed. If unwanted magnetic flux between a source and return path are canceled or minimized, radiated or conducted RF current cannot exist .
Placing RF generating components near or adjacent to ground stitch locations will minimize RF current loops, which will develop into the form of eddy currents within the chassis structure. It is imperative that all unwanted RF energy be diverted into the ( zero-voltage )0V-reference structure.
Decoupling removes RF energy injected into the power distribution network from high-frequency components consuming power at the speed at which the device is switching. Decoupling capacitors also provide a localized source of DC power for devices and components, and is particularly useful in reducing peak current surges propagated across the board.
Bypassing diverts unwanted common-mode RF noise from components or cables coupling from one area to another. This is essential in creating an AC shunt to remove undesired energy from entering susceptible areas, in addition to providing other functions of filtering. Bulk capacitors (tantalum dielectric) are required, in addition to higher self resonant frequency decoupling, to provide DC power for components,minimizing RF modulation in the power distribution network.
Oscillators and crystals must be installed directly on the PCB. Do not use sockets. Sockets add additional lead inductance to the transmission line.The best technique for preventing or minimizing crosstalk between parallel traces is to maximize separation between the traces or bring the traces closer to a reference plane. The best technique for preventing or minimizing crosstalk between parallel traces is to maximize separation between the traces or bring the traces closer to a reference plane.
It is a fact that ferrite devices (bead-on-lead, toroids, cores, split cores, wound beads, etc.) attenuate RF energy.When a ferrite material is introduced, high-frequency impedance results, suppressing high-frequency RF currents.
Moating : Allowing RF currents to propagate to different parts of the board by radiated or conductive means can cause problems not only in terms of EMI, but also functionality. A more general-purpose method of preventing digital currents from circulating into the analog ground plane would be the following (typical of layouts used with A/D converters in excess of 20 MHz with more than 8 bits). This design technique is called moating.
1. Partition the PCB. Divide into analog, digital, and I/O regions.
2. Isolate all plane layers along this partition line with absence of copper between regions. This absence of copper area is identified as a moat.
3. For power and ground planes, use a 0.010 in. (0.25 mm) minimum wide moat.
4. Tie analog ground and digital ground at one and only one point. This section of the ground plane will be the "bridge" that goes across the moat.
5. Locate the analog portion of analog components exactly in the middle across the bridge.
6. Permit no signals whatsoever to cross the moat in any location under any condition.
7. Ensure that any signals that must pass between the analog and digital sections travel only through the bridge, and do so on a layer adjacent to the bridge (maintain RF return current path).
8. Provide filters for analog power and phase lock loop circuits. This filter provides a digital noise-free analog power source.