User Manual
Iridium Communications, Inc. Information Contained in this Guide
Iridium 9523 Product Developers’ Guide is Subject to Change Without Notice
Revision 2.6
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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.