RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
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7.2 ELECTROMAGNETIC INTERFERENCE (EMI) CONSIDERATIONS
The following guidelines are offered to specifically help minimize EMI generation. Some of these guidelines are redundant
with, or similar to, the general guidelines but are mentioned again to reinforce their importance.
In order to minimize the contribution of the modem-based design to EMI, the designer must understand the major sources of
EMI and how to reduce them to acceptable levels.
These guidelines assume a microcontroller unit (MCU) is connected to the MDP.
Crystal Circuit
1. Place the crystal and related components as close as possible to the MCU and MDP devices, and in particular, the XTLI
and XTLO pins.
2. For MCU and MDP devices that do not have an internal series resistor in the crystal circuit, place a 100-ohm (typical)
resistor between the XTLO pin and the crystal/capacitor node.
3. Connect crystal capacitor ground connections directly to GND pin on the MCU and MDP devices. Do not use common
ground plane or ground trace to route the capacitor GND pin to the corresponding MCU or MDP GND pin.
Digital Components
1. Place digital components close together in order to minimize signal trace length.
2. Use decoupling capacitors (0.1 µF) on each digital component.
3. Place one large 10 µF tantalum capacitor between power and ground near digital components.
Circuit Traces
1. Avoid right angle (90 degree) turns on high frequency traces. Use smoothed radiuses or 45 degree corners.
2. Minimize the number of through-hole connections (feedthroughs) on traces carrying high frequency signals.
3. Keep all signal traces away from crystal circuits.
4. Keep digital signals, EIA/TIA-232 signals, and DAA signals separated from each other.
5. Provide a good ground plane or grid. In some cases, a multilayer board may be required with full layers for ground and
power distribution.
6. Eliminate ground loops, which are unexpected current return paths to the power source.
7. Locate high frequency circuits in a separate area to minimize capacitive coupling to other circuits.
8. Locate cables and connectors so as to avoid coupling from high frequency circuits.
9. Lay out the highest frequency signal traces next to the ground grid.
10. If a multilayer board design is used, make no cuts in the ground or power planes and be sure the ground plane covers
all traces.
11. On 2-layer boards with no ground grid, provide a shadow ground trace on the opposite side of the board to traces
carrying high frequency signals. This will be effective as a high frequency ground return if it is three times the width of
the signal traces.
12. Distribute high frequency signals continuously on a single trace rather than several traces radiating from one point.
Signal Conditioning
1. Keep traces carrying high frequency signals as short as possible.
2. Condition high frequency, long trace, and digital signals by inserting a series resistor (300 ohm typical) in the signal
source.
3. Conditioning the ~READ signal alone may provide adequate EMI reduction.
4. Conditioning the address or data lines might also be necessary.
EIA/TIA-232 Interface Components
1. Place components close to each other and close to the EIA/TIA-232 interface cable connector.
2. Connect power and ground for all RS-232 components to the power and ground source points via separate power and
ground traces that are not connected to the digital power and ground “except” at these source points. Power and ground
source points are the board input pins or a regulator output if used.
3. Connect RS-232 cable signal ground wire to the ground source point.
4. Connect the EIA/TIA-232 cable shield ground to the ground source point.
Содержание RC336DPFL
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