30 Functional Description TB7100 Installation and Operation Manual
© Tait Electronics Limited December 2005
3.2.3 RF Power Amplifier
RF Power Amplifier
and Switching
(50W/40WVersion)
The RF power amplifier and exciter of the 50W/40W 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 RF
connector.
RF Power Amplifier
and Switching
(25W Version)
The RF power amplifier of the 25W version 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 RF connector.
Output of RF
Power Amplifier
The output of the RF PA passes through a dual-directional coupler, used for
power control and monitoring. Finally, the output is low-pass-filtered to
bring harmonic levels within specification.
Power Control The steady-state power output of the transmitter is regulated using a
hardware control loop. The forward power output from the RF PA is sensed
by the directional coupler and fed back to the power control loop. The PA
output power is controlled by varying the 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.
Ramping Power ramp-up consists of two stages:
■ bias
■ power ramping
The timing between these two stages is critical to achieving the correct
overall wave shape in order to meet the specification for transient ACP
(adjacent channel power). A typical ramping waveform is shown in
Figure 3.4.
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