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–

–

–

8

9

Antenna Considerations

The F4 Series module is designed to utilize a wide variety of external

antennas, but care must be taken in antenna selection to ensure optimum

performance. For example, a handheld device may be used in many

varying orientations so an antenna element with a wide and uniform

pattern may yield better overall performance than an antenna element with

high gain and a correspondingly narrower beam. Conversely, an antenna

mounted in a fixed and predictable manner may benefit from pattern and

gain characteristics suited to that application. Evaluating multiple antenna

solutions in real-world situations is a good way to rapidly assess which will

best meet the needs of your application.

For GPS, the antenna should have good right hand circular polarization

characteristics (RHCP) to match the polarization of the GPS signals.

Ceramic patches are the most commonly used style of antenna, but

there are many different shapes, sizes and styles of antennas available.

Regardless of the construction, they will generally be either passive or

active types. Passive antennas are simply an antenna tuned to the correct

frequency. Active antennas add a Low Noise Amplifier (LNA) after the

antenna and before the module to amplify the weak GPS satellite signals.

For active antennas, a 300 ohm ferrite bead can be used to connect the

the RFIN line to an external supply for the antenna. This bead prevents the

RF from getting into the power supply, but allows the DC voltage onto the

RF trace to feed into the antenna. A series capacitor inside the module

prevents this DC voltage from affecting the bias on the module’s internal

LNA.

Maintaining a 50 ohm path between the module and antenna is critical.

Errors in layout can significantly impact the module’s performance. Please

review the layout guidelines elsewhere in this guide carefully to become

more familiar with these considerations.

Power Control

The F4 Series GPS Receiver module offers four power control modes:

Full Power, Adaptive Trickle Power, Push-to-Fix and Hibernate. In Full

Power mode the module is fully active and and continuously tracking.

Measurements are of the highest quality and are continuously output by the

module. This is the highest current consumption state.

In Adaptive Trickle Power mode, the receiver powers on at full power to

acquire and track satellites and obtain satellite data. It then powers off the

RF stage and only uses its processor (CPU) to determine a position fix.

Once the fix is obtained, the receiver goes into Hibernate mode. After a

user-defined period of time, the receiver wakes up to track the satellites for

a user-defined period of time, updates its position using the CPU only, and

then resumes standby. The initial acquisition time is variable, depending

on whether it is a cold start or assisted, but a maximum acquisition time is

definable. This cycling of power is ideal for battery-powered applications

since it significantly reduces the amount of power consumed by the

receiver while still providing similar performance to the full power mode.

Push-to-Fix mode is for applications that require infrequent position

reporting. The module stays in Hibernate mode until either the ON_OFF

line is triggered or a user-defined time period has expired. The Push-to-Fix

Period is set by a serial command and can be between 10 seconds and

two hours. An edge on the ON-OFF line triggers an immediate position fix.

When the module wakes up it acquires a new position fix and outputs the

NMEA messages before going back into Hibernate mode.

Hibernate mode is the lowest power setting. The tracking and processor

blocks are powered down, but the RTC is still running and the memory

blocks are still powered, enabling a hot start.

The module switches between these states by toggling the ON_OFF line.

The ON_OFF line must go high for at least 100ms to trigger the change of

state and must remain low for at least 100ms to reset the edge detector.

If the module is in Full Power mode, a pulse on the ON_OFF line initiates an

orderly shutdown into Hibernate mode. If the module is in Hibernate mode,

a pulse transistions the module into Full Power Mode. If the module is in

Push-to-fix mode a pulse initiates a single push-to-fix cycle. If the module is

in Adaptive Trickle Power mode, a pulse initiates one Trickle Power cycle.

ON_OFF

Full PowerHibernate

Full Power

Module

Power

100ms

100ms

Figure 7: F4 Series GPS Receiver Power Control