User's Manual

MeiG hardware design guide
31
5.2.1. Antenna
Antenna part design, SLM756P module has a total of 5 antenna interfaces, they are: RF_MAIN,
RF_DRX, RF_GPS, RF_WIFI, RF_FM. Pay attention to component placement and RF routing:
The RF test head is used to test the conducted RF performance and should be placed as close as
possible to the antenna pins of the module;
 The antenna matching circuit needs to be placed close to the antenna end;
 The connection between the antenna pin of the module and the antenna matching circuit must be
controlled by 50 ohm impedance;
 The devices and connections between the antenna pins of the module and the antenna connector
must be away from high-speed signal lines and strong interference sources to avoid crossing or parallel
with any signal lines in adjacent layers.
 The length of the RF cable between the antenna pin of the module and the antenna connector should
be as short as possible. The situation across the entire PCB should be absolutely avoided.
 If the antenna is connected by a coaxial RF line, care should be taken to avoid coaxial RF lines
across the SIM card, power circuit, and high-speed digital circuits to minimize the effects of each other.
5.2.2 Power supply
Power traces must consider not only VBAT, but also the return GND of the power supply. The trace of
the positive electrode of VBAT must be short. To be thick, the trace must first pass through the large
capacitor, Zener diode and then the power PIN of the module. There are multiple PAD exposed copper
at the bottom of the module. It is necessary to ensure that the GND path of these exposed copper areas
to the power supply is the shortest and most smooth. This ensures that the current path of the entire
power supply is the shortest and the interference is minimal.
5.2.3. SIM card
The size of the SIM card is large, and there is no anti-EMI interference device itself, which is relatively
susceptible to interference. Therefore, in the layout, first ensure that the SIM card is away from the
antenna and the antenna extension cable inside the product, as close as possible to the module. When
the PCB is routed, pay attention to The SIM_CLK signal is protected, and the SIM_DATA, SIM_RST,
and SIM_VDD signals of the SIM card are away from the power source and away from the high-speed
signal line. If the processing is not easy, it may cause problems such as not knowing the card or
dropping the card. Therefore, please follow the following principles when designing:
 Keep the SIM card holder away from the LTE antenna during the PCB layout phase;
 Keep the SIM card away from the RF line, VBAT, and high-speed signal lines, and do not leave the SIM card too
long.
 The GND of the SIM card holder should be in good communication with the GND of the module to make the
GND equipotential between the two.
 To prevent SIM_CLK from interfering with other signals, it is recommended to protect SIM_CLK.
 It is recommended to place a 100nF capacitor on the SIM_VDD signal line near the SIM card holder.
 Place TVS near the SIM card holder. The parasitic capacitance of the TVS should not exceed 50pF. The 51Ω
resistor in series with the module can enhance ESD protection.
 The SIM card signal line increases the capacitance of 22pf to ground to prevent radio frequency interference.
 The VBAT's return path has a large current, so the SIM card trace should avoid the VBAT return path.