Echelon 42150, User Manual, Page: 73 / 99

LonWorks Router User’s Guide 63
•
Most of the RF noise originates in the CPU portion of the RTR-10
router—which effectively means the entire board. Most of the RF noise
originates with the Router 5000 chip.
• Most of the EMI will be radiated by the network cable and the power
cable.
•
Filtering is generally necessary to keep RF noise from getting out on the
power cable.
•
EMI radiators should be kept away from the RTR-10 router or Router
5000 chip to prevent internal RF noise from coupling onto the radiators.
• The RTR-10 router must be well grounded to ensure that its built-in EMI
filtering works properly. Likewise, a Router 5000 must be well grounded.
•
Early EMI testing of prototypes at a certified outdoor range is an
extremely important step in the design of level “B” products. This testing
ensures that grounding and enclosure design questions are addressed
early enough to avoid most last-minute changes.
ESD Design Issues
Electrostatic Discharge (ESD) is encountered frequently in industrial and
commercial use of electronic systems. Reliable system designs must consider the
effects of ESD and take steps to protect sensitive components. Static discharges
occur frequently in low-humidity environments when operators touch electronic
equipment. The static voltages generated by humans can easily exceed 10 kV.
Keyboards, connectors, and enclosures provide paths for static discharges to
reach ESD-sensitive components, such as the Neuron Chip in the RTR-10 or the
Router 5000.
Designing Systems for ESD Immunity
ESD hardening includes the following techniques:
•
Provide adequate creepage and clearance distances to prevent ESD hits
from reaching sensitive circuitry
• Provide low-impedance paths for ESD hits to ground
•
Use diode clamps or transient voltage suppression devices for accessible,
sensitive circuits
The best protection from ESD damage is circuit inaccessibility. If all circuit
components are positioned away from package seams, the static discharges can
be prevented from reaching ESD-sensitive components. There are two measures
of “distance” to consider for inaccessibility: creepage and clearance.
•
Creepage is the shortest distance between two points along the contours
of a surface.
• Clearance is the shortest distance between two points through the air.
An ESD hit generally arcs farther along a surface than it will when passing
straight through the air. For example, a 20 kV discharge will arc about 10 mm
(0.4 inches) through dry air, but the same discharge can travel over 20 mm (0.8
inches) along a clean surface. Dirty surfaces can allow arcing over even longer
creepage distances.