AN93
Rev. 1.3 53
4.4.2. Module Design and Application Considerations
Modem modules are more susceptible to radiated fields and ESD discharges than modems routed directly on the
motherboard because the module ground plane is discontinuous and elevated from the motherboard’s ground
plane. This separation also creates the possibility of loops that couple interfering signals to the modem. Moreover,
a poor motherboard layout can degrade the ESD and EMI performance of a well-designed module.
4.4.2.1. Module Design
Particular attention should be paid to power-supply bypassing and reset-line filtering when designing a modem
module. Trace routing is normally very short on modules since they are generally designed to be as small as
possible. Care should be taken to use ground and power planes in the low-voltage circuitry whenever possible and
to minimize the number of vias in the ground and power traces. Ground and power should each be connected to
the motherboard through only one pin so as not to create loops. Bypassing and filtering components should be
placed as close to the modem chip as possible with the shortest possible traces to a solid ground. It is
recommended that a pi filter be placed in series with the module VCC pin with a filter such as the one shown in
Figure 22 on the reset line. This filter also provides a proper power-on reset to the modem. Careful module design
is critical since the module designer often has little control over the motherboard design and the environment in
which the module will be used.
4.4.2.2. Motherboard Design
Motherboard design is critical to proper modem module performance and immunity to EMI and ESD events. First
and foremost, good design and layout practices must be followed. Use ground and power planes whenever
possible. Keep all traces short and direct. Use ground fill on the top and bottom layers. Use adequate power supply
bypassing, and use special precautions with the power and reset lines to the modem module. Bypass VCC right at
the modem module connector. Be sure the modem module is connected to VCC through a single pin. Likewise, be
sure ground is connected to the modem module through one pin connected to the motherboard ground plane. The
modem reset line is sensitive and must be kept very short and routed well away from any circuitry or components
that could be subjected to an ESD event. Finally, mount the modem module as close to the motherboard as
possible. Avoid high-profile sockets that increase the separation between the modem module and the
motherboard.
25 Minimize the area of the loop formed from U2 pin 4 to R9 to U2 pin 15.
26 Cathode marking for Z1.
27 Pin 1 marking for U1 and U2.
28 Space and mounting holes to accommodate for fire enclosure if necessary.
29 IGND does not extend under C3, D1, FB1, FB2, R15, R16, C8, C9, or RV1.
30 Size Q1, Q3, Q4, and Q5 collector pads to safely dissipate 0.5W (see text).
31 Submit layout to Silicon Laboratories for review.
Table 34. Layout Checklist (Continued)P # Layout Items Required