SARA-G450 - System integration manual
UBX-18046432 - R08
Design-in
Page 69 of 143
C1-Public
2.2.1.10
Guidelines for VCC supply layout design
Good connection of the module VCC pins with the DC supply source is required for correct RF
performance. Guidelines are summarized in the following list:
All the available VCC pins must be connected to the DC source.
VCC connection must be as wide as possible and as short as possible.
Any series component with Equivalent Series Resistance (ESR) greater than few milliohms must
be avoided.
VCC connection must be routed through a PCB area separated from sensitive analog signals and
sensitive functional units: it is good practice to interpose at least one layer of PCB ground between
VCC track and other signal routing.
Coupling between VCC and audio lines (especially microphone inputs) must be avoided, because
the GSM burst has a periodic nature of approximately 217 Hz, which lies in the audible audio range.
The tank bypass capacitor with low ESR for current spikes smoothing described in
should be placed close to the VCC pins. If the main DC source is a switching DC-DC
converter, place the large capacitor close to the DC-DC output and minimize the VCC track length.
Consider using separate capacitors for DC-DC converter and cellular module tank capacitor.
The bypass capacitors in the pF range described in
close as possible to the VCC pins. This is highly recommended if the application device integrates
an internal antenna.
Since VCC is directly connected to RF Power Amplifiers, voltage ripple at high frequency may
result in unwanted spurious modulation of transmitter RF signal. This is more likely to happen with
switching DC-DC converters, in which case it is better to select the highest operating frequency
for the switcher and add a large L-C filter before connecting to the SARA-G450 modules in the
worst case.
If VCC is protected by transient voltage suppressor to ensure that the voltage maximum ratings
are not exceeded, place the protecting device along the path from the DC source toward the
cellular module, preferably closer to the DC source (otherwise protection functionality may be
compromised).
2.2.1.11
Guidelines for grounding layout design
Good connection of the module GND pins with application board solid ground layer is required for
correct RF performance. It reduces EMC / EMI issues and provides a thermal heat sink for the module.
Connect each GND pin with application board solid ground layer. It is strongly recommended that
each GND pin surrounding VCC pins have one or more dedicated via down to the application board
solid ground layer.
The VCC supply current flows back to main DC source through GND as ground current: provide
adequate return path with suitable uninterrupted ground plane to main DC source.
It is recommended to implement one layer of the application board as ground plane; keep this layer
as wide as possible.
If the application board is a multilayer PCB, then all the board layers should be filled with ground
plane as much as possible and each ground area should be connected together with complete via
stack down to the main ground layer of the board. Use as many vias as possible to connect the
ground planes.
Provide a dense line of vias at the edges of each ground area, in particular along RF and high speed
lines.
If the whole application device is composed by more than one PCB, then it is required to provide a
good and solid ground connection between the ground areas of all the different PCBs.
Good grounding of GND pins also ensures thermal heat sink. This is critical during call connection,
when the real network commands the module to transmit at maximum power: clean grounding
helps prevent module overheating.
