Tait TM8100 mobiles Service Manual page 62

2.6.3 RF Transmitter
RF Power Amplifier
and Switching
(>25W Radio)
>25W
RF Power Amplifier
and Switching
(25W Radio)
25W
Output of RF Power
Amplifier
Power Control
>25W
25W
62 Description
The RF power amplifier and exciter of the 40W/50W radio is a five-stage
line-up with approximately 40dB of power gain. The output of the
frequency synthesizer is first buffered to reduce kick during power ramping.
The buffer output goes to a discrete exciter that produces approximately 300
to 400mW output. This is followed by an LDMOS driver producing up to
8W output that is power-controlled. The final stage consists of two parallel
LDMOS devices producing enough power to provide 40 to 50W at
the antenna.
The RF power amplifier and exciter of the 30W/35W K5 band radio is a
four-stage line-up with approximately 38dB of power gain. The output of
the frequency synthesizer is first buffered to reduce kick during power
ramping. The buffer output goes to a discrete exciter that produces
approximately 500mW output. This is followed by an LDMOS driver
producing up to 6W output. The final stage consists of two parallel
LDMOS devices producing enough power to provide 30W at 762 to
806MHz and 35W at 806 to 870MHz at the antenna.
The RF power amplifier of the 25W radio is a four-stage line-up with
approximately 37dB of power gain. The output of the frequency synthesizer
is first buffered to reduce kick during power ramping. The buffer output
goes to a broad-band exciter IC that produces approximately 200mW
output. This is followed by an LDMOS driver producing up to 2W output
that is power-controlled. The final stage consists of two parallel LDMOS
devices producing enough power to provide 25W at the antenna.
The output of the RF power amplifier passes through a dual-directional
coupler, used for power control and monitoring, to the PIN switch.
The PIN switch toggles the antenna path between the receiver and
transmitter in receive and transmit modes respectively. Finally, the output is
low-pass-filtered to bring harmonic levels within specification.
The steady-state power output of the transmitter is regulated using a
hardware control loop.
With the 40/50W radios and 30/35W K5 band radios, the sum of the
forward power output from the RF power amplifier and reverse power
reflected from the load is sensed by the directional coupler and fed back to
the power control loop.
With the 25W radios, the forward power output from the RF power
amplifier is sensed by the directional coupler and fed back to the power
control loop. The PA output power is controlled by varying driver gate bias
voltage that has a calibrated maximum limit to prevent overdrive. The power
control signal is supplied by a 13-bit DAC driven by custom logic.
TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007