Simoco Xfin Blade, Service Manual

Page: 39 / 76

XFIN-BLADE-SM
TECHNICAL DESCRIPTION
PAGE 39
4.3.3.1 Drivers and PA Stages
The RF output level from the VCO buffer Q604 is typ5dBm (UHF) and +8dBm
(VHF). TX buffer Q606 increases this level by approximately 3dB (UHF) and 11dB (VHF)
and also provides additional VCO isolation.
The following section of the TX buffer Q612 is controlled by the transmitter power control
loop and Q609. Q609 is normally saturated in transmit mode so there is no minimum gain
control applied to this stage. The gain of Q612 is typically 10dB (UHF) and 15dB (VHF) but
the output level is reduced by input and output resistive attenuators to limit the PA driver
input level to typ20dBm. The gain of PA driver Q12 is controlled by the power
control loop to ensure that transmitter output power remains within defined limits. The PA
driver output level is typ25dBm.
PA module U2 utilises three stages (UHF) and two stages (VHF) to achieve the required
final RF output power level of +44dBm (25 watts). Power output settings are derived from
alignment data stored in flash memory during the initial factory alignment. The DSP
processes this data to optimise the power output level relative to the programmed channel
frequencies which may be changed at any time without retuning the radio.
An active filter comprising Q14, 17,18 and 19 provides isolation to minimise power supply
noise at the PA. This is achieved by maintaining a voltage differential of approximately 1V
across Q14 and indirectly filtering its gate voltage. Q14 is switched on only during transmit
via R523 to minimise receiver power requirements.