Transmitter Power Amplifier (PA) 40 W 2-5
3.5 Bi-Directional Coupler
The Bi-directional coupler is a microstrip printed circuit, which couples a small amount of the forward
and reverse power of the RF power from Q4441. The coupled signal is rectified to an output power
proportional dc voltage by the diodes D4451 & D4452 and sent to the RFIN of the PCIC. The PCIC
controls the gain of stage U4401 as necessary to hold this voltage constant. This ensures the
forward power out of the radio is held to a constant value.
3.6 Antenna Switch
The antenna switch utilizes the existing dc feed (A+) to the last stage device (Q4441). Basic
operation is to have both PIN diodes D4471 and D4472 turns on during key-up by forward biasing
them. It is achieve by pulling down the voltage at the cathode end of D4472 to around 11.8V (0.7V
drop across each diode). The current through the diodes needs to be set around 80mA to fully open
the transmit path through resistor R4496. Q4472 is a current source controlled by Q4471 and is
eventually connected to pin ANO of PCIC. VR4471 ensures the voltage at the resistor R4511 never
exceeds 5.6V.
3.7 Harmonic Filter
Inductors L4491, L4492, L4493 and capacitors C4448, C4493,C4494, C4496 and C4498 form a
low-pass filter to attenuate harmonic energy from the transmitter. R4491 is used to drain any
electrostatic charges that might otherwise build up on the antenna. The harmonic filter also prevents
high level RF signals above the receiver passband from reaching the receiver circuits, improving
spurious response rejection.
3.8 Power Control
The transmitter uses the Power Control IC (PCIC, U4501) to control the power output of the radio. A
portion of the forward RF power from the transmitter is sampled by the bi-directional coupler and
rectified, to provide a dc voltage to the RFIN port of the PCIC (pin 1) which is proportional to the
sampled RF power.
The PCIC has internal digital to analog converters (DACs) which provide the reference voltage of the
control loop. The reference voltage level is programmable through the SPI line of the PCIC. This
reference voltage is proportional to the desired power setting of the transmitter, and is factory
programmed at several points across the frequency range of the transmitter to offset frequency
response variations of the transmitter’s power detector circuits.
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