High power—Technical description
228 NGT Transceiver System Technical Service Manual
Transmit path
ALC/PTT Switch PCB
The RF from the NGT is connected to the RF In connector. From here it is connected via
the PTT relays to either the power amplifier path for high-power transmission, or to the
output connector for receive or amplifier-disabled operation.
The path from the high-power transmitter filter to the PTT relay includes the
forward/reflected power detector, comprising transformer T1 and diodes D1 and D2.
This produces envelopes of forward and reflected power, which are used by the
Control PCB for ALC control.
This assembly also detects a small amount of the input power for calculation of the
amplifier gain, and as such, detects the low-gain fault state.
Splitter 2-way (500 W) PCB and Splitter 4-way (1 kW) PCB
The RF is presented to the splitter, which has an adjustable attenuator. This sets the
overall amplifier gain and moves the input impedance closer to 50 Ω for the NGT. This
attenuated signal is then split using standard hybrid splitter architecture.
Power Amplifier PCB
The RF from the splitter is then fed to the amplifier sections, each of which is Class AB
push-pull. These produce 250 W PEP from approximately 7 W PEP in. Gain
stabilisation over the bandwidth is provided by feedback networks R7 and R10, and R8
and R9, and impedance correction networks across the amplifier inputs. V1 and V2
transistor collectors are coupled to the 50 Ω combiner input via a 1:2 transmission line
transformer.
The power supply voltage is varied based on maintaining the VCE of the output
transistors above a minimum saturation voltage. This minimum voltage is sensed by
VCE_MIN detectors on the Power Amplifier PCB.
The common bias supply consists of a sense transistor V3 and a series pass transistor V5.
This circuit produces a temperature-controlled bias voltage to set the crossover point of
the Class AB output stage for minimum distortion.
Combiner 2-way (500 W) PCB and Combiner 4-way (1 kW) PCB
The amplifier outputs are fed to the combiner. Each combiner has standard hybrid
combiner architecture, and includes detection windings that detect combiner imbalance,
which may be caused by greatly differing amplifier outputs.
Drawing 04-03422
Drawings 04-03419 and 04-03420
Drawing 04-03416
Drawings 04-03428 and 04-03429
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