Motorola M12 Oncore User's Guide download pdf (Page 6)

Chapter 3 – Receiver Description

Motorola GPS Products - Oncore User’s Guide

ANTENNA SENSE CIRCUIT

The Oncore receiver 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 antenna sense circuit was designed around the Motorola

GPS antenna; therefore non Motorola antennas may exceed the threshold limits as

listed below

Under current circuit @ 25 degC:

Good indication: greater than 5 mA

Bad indication: less than 5 mA

Over current circuit @ 25 degC:

80 mA maximum for normal operation

45 mA maximum for short circuit

The above information is output in the following I/O messages, @@Ea (8-Channel

Position/Status/Data Output), @@Fa (8-Channel SelfTest), @@Ha

(Position/Status/Data 12-Channel), @@Ia (Self-Test Message 12-Channel). Upon

detecting an over current situation, the receiver will automatically shut down the antenna

feed section until the fault is cleared. Upon detecting an under current situation, the

receiver will continue to operate as normal, but will flag the fault mode in the two 1/O

messages. An external power source must be used if the antenna circuit power

requirements exceed the limits.

ANTENNA FEED CURRENT

The Oncore receiver now provide up to 80 mA of current via the antenna power

supply circuit. The circuit still has a 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 and

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

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 5 mA or more, the first operational

amplifier will produce a logic level to the digital circuits where it is monitored by the

firmware. if the signal is absent, indicating an under current condition, an alarm bit is

set to alert the user.

Revision 5.0 08/30/02

3.6

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Motorola M12 Oncore User's Guide Page 6