SimCom SIM808, Проектирование аппаратного обеспечения

Страница: 53 / 67

Smart Machine Smart Decision
5.2 GPS Power on/down Scenarios
5.2.1 Power on GPS engine
User can power on GPS engine by sending AT command “AT+CGPSPWR=1”.
5.2.2 Power down GPS engine
User can power down GPS engine by sending AT command “AT+CGPSPWR=0”.
5.3 GPS Antenna Interface
5.3.1 GPS Antenna Interface
The customer’s GPS antenna also can be located in the customer’s main board and connect to module’s
GPS_ANT pad through microstrip line or other type RF trace which impendence must be 50
Ω .
Figure 51: GPS antenna matching circuit
In Figure 51, the components R101, C101 and C102 is used for antenna matching, the components’ value only
can be got after the antenna tuning. Usually, matching components’ value is provided by antenna vendor, the
default value of R101 is 0
Ω , and users need to reserve the place of C101 and C102 without soldering. The traces
in bold type should be treated as 50 Ω impedance controlled line in PCB layout.
5.3.2 GPS Antenna Choice Consideration
To obtain excellent GPS reception performance, a good antenna will always be required. The antenna is the most
critical item for successful GPS reception in a weak signal environment. Proper choice and placement of the
antenna will ensure that satellites at all elevations can be seen, and therefore, accurate fix measurements are
obtained.
Most customers contract with antenna design houses to properly measure the radiation pattern of the final
mounted configuration in a plastic housing with associated components near the antenna. Linear antennas are
becoming more popular, and the gain is reasonable, since a smaller ground plane can be used.
User can consider following factors as:
z Choose a linear antenna with a reasonably uniform hemispherical gain pattern of >-4dBi.
z Use of an antenna with lower gain then this will give less than desirable results. Please note that a RHCP
antenna with a gain of 3dBi, equates to a linear polarized antenna of 0dBi.
SIM808_Hardware Design_V1.00 2014.03.27 53