8525B/8528 Technical Service Manual Technical Description 4-25
Publication No: 15-02036 Issue 6
4.9 PA and Filters (04-02452)
4.9.1 Introduction
8528S Series transceivers may be fitted with a PA and Filter PCB, forming
an internal PA stage, or with a PA Exciter Interface PCB for use with an
external PA unit. Software appropriate to the board fitted is installed in the
controlling EPROM on the Microprocessor Control PCB. Transceivers fitted
for use with an external PA assembly are identified by a letter H suffixed to
their type numbers on the rear panel. The board fitted for the internal PA
version is described in this section. The PCB fitted for the other version is
described in Section 4.10.
4.9.2 PTT Control
The driver and output stages are permanently connected to the supply
voltage when the Power On relay K7 is energised. Power to the rest of the
PA and the bias for the driver and output stages is switched by V14 when
the PTT input (D rail, P1 pin 2) goes high causing pin 10 of IC1 to sink base
current from V14 through the antenna change-over relay K6 coil. In
transmit, the input stage V7 and V9 becomes active when its emitter circuit
is connected to ground on the C rail through D12 on the RF Mixer and Dual
Synthesizer PCB (04-02450).
4.9.3 Gain Control Stage
The RF input to the PA is terminated in R21 and drives the common base
long tailed pair V7 and V9 through R29 and R30 in parallel for signal
currents.
The input voltage is also buffered by V8 and additional input current fed to
V7 and V9 through L17, C66 and R32 to compensate for PA high frequency
gain roll-off.
The gain of V9 is controlled by the ratio of emitter DC currents in V7 and
V9. The RF input current is split between the emitters of V7 and V9 in
inverse proportion to their input impedance. The gain of V9 is therefore
reduced when the ALC increases the current in V7.
4.9.4 Pre-Driver Stages
The output current of V9 drives the collector-base feedback stage V10
which is direct coupled to the transformer feedback stage V12. High
frequency peaking is provided by C72. D11 and D12 provide temperature
compensation for V10 and V12.
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