Specifications

AN93
158 Rev. 0.8
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 above the motherboard
ground plane. This separation also creates the
possibility of loops that couple these interfering signals
to the modem. Additionally, system designers can
adversely impact the ESD and EMI immunity and
performance of a properly-designed module with a poor
motherboard layout.
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 one pin
only to avoid the creation of 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 29, 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 frequently has little control over the
motherboard design and the environment the module
will be used in.
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 top and bottom layers. Use adequate
power supply bypassing and use special precautions
with the power and reset lines to the modem module.
Bypass V
CC
right at the modem module connector. Be
sure the modem module is connected to V
CC
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.
Figure 29. Modem Module V
CC
and RESET Filter
RESET
Motherboard
Connector
1.0 µF.01 µF.01 µF1.0 µF
10 kΩ
2.2 µF
To RESET
GND
To Modem Chip V
CC
(Si2493/57/34/15/04
pins 5, 21)
(Si2493/57/34/15/04 pin 12)
GND
VCC
Murata BLM 18A
G601 SN1