Linx Technologies RXM-GPS-FM - Manual (Page 7)

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Antenna Considerations
The FM Series module is designed to utilize a wide variety of external
antennas. The module has a regulated power output which simplifies
the use of GPS antenna styles which require external power. This allows
the designer great flexibility, 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
VOUT line to the RFIN line. 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 section carefully to become more familiar with
these considerations.
Power Control
The FM Series GPS Receiver module offers several ways to control the
module’s power. A serial command puts the module into a low-power
standby mode that consumes only 150µA of current. An external processor
can be used to power the module on and off to conserve battery power.
In addition, the module includes a duty cycle mode where the module will
power on for a configurable amount of time to obtain a position fix then
power off for a configurable amount of time. In this way the module can
handle all of the timing without any intervention from the external processor.
There are four times that are configured with duty cycle mode. The on
time and standby times are the amount of times that the module is on and
in standby in normal operation. There are also cold start on and standby
times. These are used to keep the module on longer in the event of a cold
start so that it can gather the required satellite data for a position fix. After
this, the module uses the normal operation times.
In the event that the module’s stored ephemeris data becomes invalid the
module supports and extended receive time to gather the required data
from the satellites. Figure 7 shows the power control times.
The module supports MediaTek’s proprietary AlwaysLocate TM mode. In
this mode, the module automatically adapts the on and standby times to
the current environmental conditions to balance position accuracy and
power consumption. In this mode, any byte sent to the module triggers it to
output the current position data.
Standby mode is configured by command 161. Extended receive time is
configured by command 223. Command 225 configures which duty cycle
mode is used.
ON
Standby
Cold Start On Time
Cold Start
Standby Time Standby Time On Time On Time Extended RX Time
Figure 7: FM Series GPS Receiver Power Control