User`s guide

Chapter 3 - Receiver Descriptions
Antenna Drive and Protection Circuitry
The M12+ is capable of detecting the presence of an antenna. The receiver utilizes an antenna
sense circuit that can detect under current (open condition), over current (shorted or exceeding
maximum receiver limits), or a valid antenna connection. The M12+ is designed to provide up to
80 mA of current via the antenna power supply circuit. The circuit contains short protection and a
means for detecting over current and open circuit conditions of the connection between it and the
antenna. This allows the user a degree of confidence that the antenna is connected properly and
is drawing current. This feature can eliminate hours of troubleshooting, especially in a new
installation.
The antenna power supply circuit consists of a current sense resistor, two rail-to-rail output
operational amplifiers, a pass transistor and a voltage divider to set the upper and lower limits of
the under current and over current thresholds. The operational amplifiers compare the voltage
developed across the current sense resistor with these thresholds. If the antenna is drawing 15
mA or more, the first operational amplifier will produce a logic level to the digital circuits,
indicating that an antenna is attached. If the signal is absent, indicating an under current
condition, an alarm bit is set to alert the user. Having this alarm bit high does not prevent the
receiver from operating, and may in fact be high all the time when utilizing an antenna with low
current draw, or when supplying the antenna with power through an external source using a
bias-T.
The over current detection circuit operates in a similar manner. When the voltage drop across the
current sense resistor is equal to the over current threshold (set at about 90 mA at room
temperature) the output of the sense amplifier starts shutting down the pass transistor. The
receiver will automatically fold-back the antenna feed current to approximately 45mA until the
fault is cleared. As with the undercurrent sensor, a logic level is provided to the digital circuits to
trigger an alarm bit that indicates the over-current condition.
The antenna sense circuit was designed to operate with the Motorola Hawk and Timing 2000
GPS antennas, therefore non-Motorola antennas may exceed the threshold limits as listed below:
Under current detect @ 25°C:
Good indication: greater than 15 mA
Undercurrent indication: less than 15 mA
Over current detect @ 25°C: 80 mA maximum for normal operation
NOTE: An external power source such as a
bias-T must be used if the antenna circuit power
requirement exceeds the upper limit.
Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05
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