Operating instructions

9-2 Transmitter
1.1.2 Antenna Switch and Harmonic Filter
The antenna switch comprises of 2 GaAs switches: U900 and U901.
The switches are located between the transmit/receive path and the standard/remote ports
During the transmit mode operation, the ANT_EN line turns on the Q900 switch to route the transmit
signal from pin J3 to J1 of U901 GaAs switch to the harmonic filter which consists of C902,C904,C905
and L901.
After the harmonic filter, another GaAs switch determines the remote and standard port. The VA_SW
from OMAP which controls this switch will remain high to route the transmit/receive signal to/from the
standard port.
Once the VA_SW is high, all the RF signals will be routed to the remote port.
After the second GaAs switch, U900, along the standard port path,
L900,C901,C912,C913,C914,L905 and L906 serves as a diplexer for the RF and the GPS path. The
overall insertion loss of the switch and the harmonic filter is around 1.6dB.
1.1.3 ESD Protection Circuit
C911, L907 and C910, L908 form a high pass filter on the standard port and remote port. The high
pass filter and the ESD protection suppressor (VR901, VR900) are used to suppress the ESD pulse
from the antenna jack and bull eye in order to protect the RF GaAs switch U900.
1.1.4 Power Control
The transmitter power control uses discrete integrator approach, to sense the current of both PA
stages namely, Driver Power Amplifier (Q703) and Final Power Amplifier (Q704).
The basic block diagram of the power control is shown in Figure 6-2. A single sensing resistor (R700)
is used to sense both PA stages current. The power control consists of 2 comparators, U701-1 and
U701-2. U701 is a dual-package op-amp in SOT-8 package. The first stage is an I-V converter
converts current drained by Power Amplifier Driver (Q703) and Power Amplifier Final (Q704) across
R700 into a voltage drop and the second stage is an integrator used to shape the output Vcontrol to
bias both PAs. The slope of the Vcontrol is controlled by the RC time constant of the integrator and
also the ramping slope of reference signal, TX_DAC generated by MAKO IC. The final TX_DAC value
determines the transmit power level and is a factory tuned parameter which is stored inside the
codeplug.