MDEV-GPS-RM datasheet(7/27 Pages) LINX

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Antenna Considerations

The RM 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.

The 1PPS Output

The 1PPS line outputs 1 pulse per second on the rising edge of the GPS

second when the receiver has an over-solved navigation solution from five

or more satellites. The pulse has a duration of 100ms by default with the

rising edge on the GPS second. This line is low until the receiver acquires a

3D fix. The pulse width can be adjusted with a serial command.

The GPS second is based on the atomic clocks in the satellites, which are

monitored and set to Universal Time master clocks. This output and the

time calculated from the satellite transmissions can be used as a clock

feature in an end product. It has a ±11ns accuracy relative to other RM

Series GPS receiver modules.

Power Control

The RM 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 135µA of current. An external processor

can be used to power the module on and off to conserve battery power.

Standby mode is configured by command 161.

Note: The receiver duty cycle mode was removed from modules with

date code 1612 and later.

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