Tait TM8100 mobiles Service Manual page 69

Power Control Loop
>25W
25W
>25W
25W
Directional Coupler
>25W
25W
Temperature Sensor
TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
The high-level RF signal passes via the directional coupler, the transmit-
receive PIN switch, and the LPF, through to the antenna. The LPF is used
to attenuate unwanted harmonic frequencies.
Calibration is used to adjust the power control loop, thus setting the output
of the transmitter to one of four preferred power levels:
10, 15, 25, and 50W (VHF), and
■
10, 15, 20, and 40W (UHF) for 40W/50W radios
2, 5, 10, and 30W (762–806 MHz), and
■
2, 5, 10, and 35W (806–870 MHz)) for 30W/35W radios
1, 5, 12 and 25W (all bands) for 25W radios.
■
The loop maintains these power settings under changing environmental
conditions. The control mechanism for this loop is via the DAC IC204 and
one of the operational amplifiers making up IC301. The power control loop
will be inhibited if for any reason an out-of-lock signal is detected from the
synthesizer. This ensures that no erroneous signals are transmitted at
any time.
With the 40W/50W radio, the power control loop processes the voltages
from the forward and reverse power sensors in the directional coupler.
This signal is fed to the buffer and a band-limited operational amplifier back
to the gate of Q306. In this way, the transmitter is protected against bad
mismatches.
With the 30W/35W radio, the power control loop processes the voltages
from the forward and reverse power sensors in the directional coupler.
This signal is fed back to the exciter device Q303 as well as to the driver
Q306. This is to maintain the power control dynamic range.
With the 25W radio, the power control loop senses the forward power by
means of the diode D304. This signal is fed to the buffer and a band-limited
operational amplifier back to the gate of Q306.
A voltage clamp (one of the operational amplifiers of IC301) for Q306 limits
the maximum control-loop voltage applied to its gate.
With the 40W/50W and 30W/35W radios, the directional coupler
actively senses the forward power and the reverse power, and feeds them
back to the power-control circuit.
With the 25W radio, the directional coupler actively senses the forward
power and feeds it back to the power-control circuit. If the directional
coupler detects too much reverse power, indicating a badly matched
antenna, the transmitter will be reduced to the lowest power setting.
For added protection, a temperature sensor ensures that the transmitter
power is reduced to very low levels should a temperature threshold be
exceeded. If the temperature does not decrease, the transmitter is
switched off.
Circuit Descriptions 69