Rockwell RFX144V24-S23, Руководство дизайнера

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RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
1070 14-9
14.2.2 Electromagnetic Interference (EMI) Considerations
The following guidelines are offered to specifically help minimize EMI generation. Some of these guidelines are redundant or
similar to the general guidelines but are mentioned to reinforce their importance.
In order to minimize contribution of MONOFAX modem-based design to EMI, the designer must understand the major
sources of EMI and how to reduce them to acceptable levels.
1. Keep the crystal/oscillator and other related external components as close to the device XTLI and XTLO pins as
physically possible.
2. Keep any traces carrying high frequency signals as short as possible.
3. Have a good ground plane. In some cases a multilayer board might be required with full layers for ground and power
distribution.
4. Decouple power to ground with decoupling capacitors as close to the device as possible.
5. Eliminate ground loops, which are unexpected current return paths to the power source.
6. Decouple the phone line cables at the telephone line jacks using series inductors or ferrite beads.
7. Decouple the power cord at the power cord interface with decoupling capacitors.
8. Locate high frequency circuits in a separate area to minimize capacitive coupling to other circuits.
9. Locate cables and connectors to avoid coupling from high frequency circuits.
10. Layout the highest frequency signal traces next to the ground grid.
11. If a multilayer board design is used, the following design rules are recommended:
a) Make no cuts in the ground or power planes.
b) The ground plane must cover all traces.
14.2.3 Crystal Oscillator EMI Considerations
A source of EMI radiation in a MONOFAX modem design can be the crystal oscillator circuit which produces fundamental
energy at 49.92 MHz/ 53.76 MHz and third harmonic energy at 149.76 MHz/161.28 MHz.
The third harmonic energy can easily be reduced by adding some series resistance between the XTLO pin and the load
capacitor as illustrated in Figure 14-1 and Figure 14-2. This resistor and the XTLO capacitor create a low pass filter which
can attenuate the third harmonic component energy. It is important to note that, although a larger resistor value will further
reduce the third harmonic component, it will also attenuate the fundamental frequency and may cause an oscillator circuit
start-up problem. A careful selection of this resistor is very important.