Tait TM8000 Series, Service Manual

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TM8100 Mobile Radio Service Manual General Description 61
May 2004 © Tait Electronics Limited
RF transmitter
RF Power Amplifier
and Switching
The RF PA (power amplifier) is a four-stage line-up with approximately
42 dB 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 200 mW output.
This is followed by an LDMOS driver producing up to 2 W output that
is power-controlled. The final stage consists of two parallel LDMOS
devices producing enough power to provide 25 W 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, 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.
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 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.
Figure 2.15 Typical ramping waveform
Bias
ramp
Power
ramp
Bias
ramp
Power
ramp
Hi power
Lo power
Power
Time