User's Manual

720 Hardware User Guide

3.2 GPS RF Design and PCB Layout

3.2.1 GPS Impedance

The 92nd pin is the GPS interface of the module, which also requires a 50 . The PCB layout for

GPS is similar to that for GPRS. For details, refer to the previous section. Figure 3-5 shows the

internal structure of the GPS RF.

Figure 3-5 GPS RF structure

In addition to the basic rules, the GPS routing has higher requirements because the air wireless

GPS signal has lower strength, which results in weaker electrical signal after the antenna

receives. Weaker signals are more susceptible to interference. Therefore, active antenna are

commonly used for GPS. The active GPS antenna amplifies the weak signals received to

stronger signals through the low-noise amplifier (LNA) and then transmits the signals through the

feeder.

If you use a passive antenna, add LNA near the feeder because the module does not embed

one.

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 layout and antenna layout.

3.2.2 Active GPS Antenna Design

Ceramic GPS chip antenna is monly used. In general, it is recommended that you use the active

ceramic antenna. After the antenna receives GPS satellite signals, the LNA amplifies them first

and then they are transmitted to the 92nd pin (GPS_ANT) through the feeder and PCB traces.

50 resistance is required for both the feeder and PCB traces and the traces should be as short