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Iridium Communications, Inc.
Iridium 9523 Product Developers' Guide
Revision 2.6
3.7.1
Transmit power Control
The radiated power output from the Iridium transmitter can exceed SAR safety limits if the antenna is too
close to the operator's body. An Iridium handset has a telescopic antenna to provide the necessary safe
distance. The GPIO signals ANT_DET_PWR and FULL_POWER_EN are used to control the transmit
power in conjunction with an antenna position-detector switch.
The Iridium 9523 polls the position-detector switch by providing a 3.3V output on ANT_DET_PWR in
order to provide power to the antenna position sensing circuitry. When ANT_DET_PWR is high, the
position sensing circuit must take FULL_POWER_EN high to enable full transmit power.
3.7.2
External GPS Receiver Switch
If the Iridium 9523 is used near a GPS receiver, it is possible that the input circuitry of the GPS receiver
could be damaged by the output power of the Iridium transmitter, especially if the two devices share a
single antenna. The Iridium 9523 has a GPIO signal, TX_ACTIVE, which is high whenever the Iridium
transmitter is active and this could be used to control a switch on the input of the GPS receiver.
The TX_ACTIVE signal goes high 610us before the start of a transmit burst (ie. full power), and goes low
220us after the end of the burst. There is also 100us of power ramp-up before the burst and 100us of
ramp-down after the burst.
3.7.3
Modem/Handset Mode Select Signal
A further GPIO signal, LBT_HSTB, is used to indicate whether the Iridium 9523 should operate as though
it is mounted in a handset or a modem device. It should be pulled to 3.3V for normal modem operation, or
0V for handset mode. The major differences are:
Data/Fax port is operational in modem mode and disabled in handset mode
Background radio scanning for active radio channels is less intensive in handset mode, because
power consumption is a more important issue for a battery-powered handset.
Iridium Communications, Inc.
Proprietary & Confidential Information
Only
Information Contained in this Guide
is Subject to Change Without Notice
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