Service Manual

PAGE 42 TECHNICAL DESCRIPTION
4.3.3.1 Drivers and PA Stages
The RF output level from the VCO buffer Q604 is typically +5dBm (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 typically +20dBm. 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 typically +25dBm.
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.