Xircom An Intel Company GEM3501 Manual De Usuario
Core Engine GSM/GPRS Modem Developer Guide Preliminary Draft: 7/6/2001
25
Part Number: 07100026, Revision: 002
Confidential
© 2001 Xircom, Inc., an Intel company All rights reserved.
All trademarks and copyrights are the property of Xircom, Inc., an Intel company
All trademarks and copyrights are the property of Xircom, Inc., an Intel company
NOTE: Refer to the Core Engine Programmer Reference documentation for the syntax
of how to use these software commands.
of how to use these software commands.
5.3 Transmit Power
The duration of a single transmit burst is 577 microseconds (uS). In Multislot Class 10
operation, two transmit slots may be concatenated, for a total of (577 uS x 2 = ) 1.54 mS.
The current required during the transmit burst is somewhat less than 2 Amps. This is
when running the full transmit power (30 dBm for PCS). The current required is
substantially less at the lower power levels.
operation, two transmit slots may be concatenated, for a total of (577 uS x 2 = ) 1.54 mS.
The current required during the transmit burst is somewhat less than 2 Amps. This is
when running the full transmit power (30 dBm for PCS). The current required is
substantially less at the lower power levels.
A good way to estimate the current required at each power control level is to calculate the
current required to provide the transmit power to the antenna with a typical power
amplifier efficiency of 50%. For instance, to achieve 33 dBm transmit power to the
antenna, add about 2 dB to account for losses in the transmit filters and switches, so 35
dBm is required from the power amplifier output. Add another 3 dB to account for the
(typically) 50% power amplifier efficiency, so the power that must be delivered to the
power amplifier is 38 dBm. This is equal to 6.3 Watts. This requires about 1.7 amps at
3.7 Volts. This is the current required for the power amplifier stage only. The remainder
of the transmitter requires an additional 200 mA during the transmit burst (regardless of
transmit power level), for a total requirement of 1.9 Amps.
current required to provide the transmit power to the antenna with a typical power
amplifier efficiency of 50%. For instance, to achieve 33 dBm transmit power to the
antenna, add about 2 dB to account for losses in the transmit filters and switches, so 35
dBm is required from the power amplifier output. Add another 3 dB to account for the
(typically) 50% power amplifier efficiency, so the power that must be delivered to the
power amplifier is 38 dBm. This is equal to 6.3 Watts. This requires about 1.7 amps at
3.7 Volts. This is the current required for the power amplifier stage only. The remainder
of the transmitter requires an additional 200 mA during the transmit burst (regardless of
transmit power level), for a total requirement of 1.9 Amps.
The capacitance required to sustain the transmit burst current can be estimated by
subtracting the current available from the power supply from the total burst current
required, and determining a suitable voltage droop during the burst. For instance, if 500
mA is available from the power supply, the capacitor will have to supply (1.9 - 0.5) = 1.4
Amps during the transmit burst time. If 300 mV is an acceptable voltage droop during the
transmit burst, the capacitance required would be C = (i*t)/V which would be
(1.4*0.00154)/0.3 = 7.2 milliFarad (7,200 uF).
subtracting the current available from the power supply from the total burst current
required, and determining a suitable voltage droop during the burst. For instance, if 500
mA is available from the power supply, the capacitor will have to supply (1.9 - 0.5) = 1.4
Amps during the transmit burst time. If 300 mV is an acceptable voltage droop during the
transmit burst, the capacitance required would be C = (i*t)/V which would be
(1.4*0.00154)/0.3 = 7.2 milliFarad (7,200 uF).
Capacitor ESR must also be considered. Since the ESR multiplied by the current
produces a voltage step that increases the droop during the transmit burst, the lower the
ESR the better; 50 milliohms or less is preferred. Some of the "supercap" solutions on the
market may have unacceptably high ESR values.
produces a voltage step that increases the droop during the transmit burst, the lower the
ESR the better; 50 milliohms or less is preferred. Some of the "supercap" solutions on the
market may have unacceptably high ESR values.
These values are conservative estimates, and depending on the application, less
capacitance may give satisfactory performance. Dropping to a single transmit slot
operation (for example, Multislot Class 8 which uses 1 transmit and 4 receive slots) cuts
the capacitance required by half.
capacitance may give satisfactory performance. Dropping to a single transmit slot
operation (for example, Multislot Class 8 which uses 1 transmit and 4 receive slots) cuts
the capacitance required by half.