N1_Hardware_User_Guide
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7.3 GPS RF Design and PCB Layout
7.3.1 GPS Impedance
The 116th pin is the GPS interface of the module, which also requires a 50 Ω. The PCB layout for GPS is
similar to that for GPRS. For details, refer to the previous section. Figure 7-7 shows the internal structure
of the GPS RF.
Figure 7-7
GPS RF structure
WCN IC
SAW
ANT_GPS
LAN
SAW
In addition to the basic rules, the GPS routing has higher requirements because the air wireless GPS signal
has lower strength, which results in poor electrical signal after the antenna receives. Poor signals are more
susceptible to interference. Therefore, active antenna is commonly used for GPS. The active GPS antenna
amplifies the poor signals received through the low-noise amplifier (LNA) and then transmits the signals
through the feeder.
If the antenna and layout are not designed reasonably, the GPS will be insensitive, resulting in long time
on positioning or inaccurate position.
Keep the GPRS and GPS far away from each other in antenna layout.
7.3.2 Active GPS Antenna Design
Ceramic GPS chip antenna are commonly used. In general, the active ceramic antenna is recommended.
After the antenna receives GPS satellite signals, the LNA amplifies them first and then they are
transmitted to the 116th pin (GPS_ANT) through the feeder and PCB traces. 50Ω impedance is required
for both the feeder and PCB traces and the traces should be as short as possible. The power supply of the
active antenna is fed by the 100 nH inductance through the signal traces.
Common active antenna requires 3.3V to 5V power supply. Though the active antenna has a low power
consumption, it requires stable and clean power supply. Use high-performance LDO to supply power for
the antenna through a 100 nH inductance, as shown in 7.4 FM RF Design and PCB Layout.
