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RH56D-PCI

Host-Controlled V.90/K56flex™ Modem device Family for Desktop Applications

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RH56D-PCI Modem Designer’s Guide

2. On some designs, place a ferrite between bracket ground and digital ground. This helps keep digital noise away from the

connector but has the harmful side effect of making the digital area of the board noisier.

4.2.6 Decoupling

1. Another way to deal with EMI is to short it to ground through proper decoupling capacitors (caps). Because traces and

component leads become inductors at high frequencies, use surface mount caps if possible, and place them close to the

device being decoupled. Decouple power pins of a device directly to associated ground pins of the device between

traces as close to the device as possible, i.e., not to a remote ground plane. If you have power and ground planes,

connect capacitors to the planes with more than one via. This will decrease the lead inductance and increase the

effectiveness of the capacitors.

2. Capacitors are useful in reducing EMI is that they provide a high frequency short to ground. This is good if the ground

plane is properly grounded (i.e., short path to chassis ground.), However if the ground plane isn’t properly grounded, then

the decoupling caps need to be used to source as much noise to the ground plane as it can handle. Also note that the

desired value of the decoupling cap depends on the frequency that you want to eliminate. With higher system clock rates

it is necessary to have a mixture of values for decoupling caps (e.g., 0.001 uF, 0.01 uF and 0.1 uF).

3. Sourcing too much energy to ground can be just as harmful EMI-wise as not sourcing enough should the path to ground

be poor. The capacitor value chosen for the offending frequency can be mathematically correct but also cause excessive

EMI ripple on the ground. Unless the impedance of ground traces can be reduced, the solution is to reduce the capacitor

value. As an example, if the board is excessively radiating at 100 MHz with a 0.01 uF cap, try a 0.047. The idea is to only

source as much energy to ground as the ground can handle, no more or it will radiate.

Capacitor Value

0.1 uF

0.01 uF

0.001 uF

Frequency of Reduced EMI

10 MHz

30 MHz

100 MHz

4. There is some overlap, as these values will change as inductance of the board comes into effect, and the fact you rarely

have just one frequency to contend with. Another rule of thumb, derived from experiences, is 0.1 uF for <80 MHz, 0.01

uF for 60-500 MHz, and 0.001 uF for >400 MHz. The designer should provide several different values for de-coupling

capacitors. These should be spread evenly around the power pins of devices on the board. A few capacitors can be

placed in open areas of the board to stabilize the power and ground.

5. Separate analog power/ground from digital power/ground with inductors or ferrite beads. The side effect of this

separation is that the analog circuit cannot dump their high frequency noise to the chassis ground. The EMI characteristic

of boards can be improved by using adequate decoupling and by limiting the areas of analog power/ground.

6. The de-coupler on the PCICLK guard-band should be 0.001 uF to handle the clock harmonics. Decouple this guard-band

directly to a VCC pin near the PCICLK pin at the PCI connector.

7. Provide several different values for de-coupling capacitors. These should be spread evenly around the power pins of

devices on the board. A few caps can be placed in open areas of the board to stabilize the power and ground.

4.2.7 Crystal Circuit

1. Provide a separate trace(s) for the ground connections of the crystal's bypass capacitors going back to the digital ground

pins of the modem device(s). These traces should be as short as possible, relatively thick, and have minimal sharp

bends.

2. Avoid tying the ground connections of these capacitors to surrounding ground copper island(s). In these cases, the return

high-frequency currents do not necessarily choose the shortest path and instead, they ripple the surrounding ground

distribution with EMI.

3. Lay the crystal flat against a ground plane that has a low impedance path to ground.

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