Motorola M12 Oncore User's Guide download pdf (Page 7)

Chapter 3 – Receiver Description

Motorola GPS Products - Oncore User’s Guide

Antenna Feed Circuit

(Continued)

For the over current circuit, when the voltage drop across the current sense resistor

is equal to the over current threshold (set at about 90 mA for room temperature) the

output of the amplifier starts shutting down the pass transistor. At this point, the

voltage to the antenna starts to decrease and a logic level is provided for the digital

circuit to trigger an alarm bit that indicates an overcurrent condition.

Figure: 3.2

An additional feed back path between this output voltage and the over current

operational amplifier causes a further decrease in the output current depending on

the output voltage level. This action results in folding back the current such that the

short circuit amount is about 45 mA, which is less than the 90 mA threshold. This

prevents the over heating of the series pass transistor should the shorted coax

condition occur. A chart of the typical output voltage vs. the load current is shown in

figure 3.2 above.

The output current limit is higher than previous versions of Oncore receivers. This is

to support longer cable runs through the use of higher gain antennas or in-line

amplifiers so that the signal does not drop below the tracking threshold when it

reaches the GPS receiver.

Revision 5.0 08/30/02

3.7

1 2 3 4 5 6 7 8 ... 50

No comments

Motorola M12 Oncore User's Guide Page 7