SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26 Design-in
Page 111 of 217
2.2.1.10 Guidelines for VCC supply layout design
Good connection of the module VCC pins with 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 typical
GSM burst has a periodic nature of approx. 217 Hz, which lies in the audible audio range.
The tank bypass capacitor with low ESR for current spikes smoothing described in Figure 45 and Table 28
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. Otherwise consider using
separate capacitors for DC-DC converter and cellular module tank capacitor.
The bypass capacitors in the pF range and ferrite bead described in Figure 45, Figure 46 and Table 28 should
be placed as 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-G3 and SARA-U2 series 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 significantly reduces EMC issues and provides a thermal heat sink for the module.
Connect each GND pin with application board solid GND 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 GND plane as
much as possible and each GND 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 GND 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: proper grounding helps prevent
module overheating.
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