VT-MOD-CELL-B48 Hardware User Manual
15
5.2.2
Design Guide
There is an EMI filtering and ESD protection circuit between SIM card interface and
module interface on the user’s board.
The SIM support clock frequencies of 3.25MHz. The SIM interface signals consist of
four signals that are USIM_PWR, USIM_RST, USIM_CLK, and USIM_DATA
(UIM_Vpp isn’t connected also not used in many applications). Due to the relatively
low clock frequencies involved, the concern is not the degradation of the SIM signals
themselves. The main concern is routing of the SIM interface signals through areas
considered to be of high risk for RF noise coupling (crosstalk and RF contamination)
which can desensitize the radio circuitry.
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The general guidelines that should be followed are listed as follows:
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It is recommended that these signals should be routed over a contiguous
ground plane.
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SIM interface signals should not be routed near high transient signals
(power supply chokes and DC/DC switching FETs).
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Avoid routing of these signals near output connectors.
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Keep SIM interface signals isolated from other signals. 2x width spacing
(1.5xmin) between SIM interface signals and all other signal routing is
recommended.
Since the SIM is a CMOS device, ESD protection devices should be placed near to
the SIM connector to provide protection before connecting to the module. In addition,
all the SIM interface signals should be bypassed with a 10 pF capacitor.
5.3 Power Interface
Externally standard voltage from 3.15 V to 4.2 V (with the typical value of 3.3 V) is
used as VT-MOD-CELL-B48 module power supply. When the network signal is very
weak, the antenna emission with maximum power will happen, with 1A of transient
maximum current. So, it is recommended to use LDO with more than 1A or switching
power supply.
Besides considering the voltage drop when high-power launching, a large
capacitance should be added to the module power supply port.
Note: Because EMC interference for switching power supply is rather big, the circuit
line should not be close to the antenna part.
