Version 0A 29
panel RELAY control line is held low (below 1 volt), exciter power is routed through input
relay RLY1 to the PD8m power attenuator board.
ATTENUATOR BOARD
The attenuator board has two 3dB attenuators. A relay switches one attenuator out on
six meters. This makes amplifier drive power requirements more consistent across the
operating frequency range.
PA Amplifier
Power amplification comes from a single 600-watt power amplifier module. The PA
module (PAM-606) uses four MFR-150 field effect transistors. Bias each MRF-150 at an
equal quiescent current within the range of 100 mA to 300 mA. It is important to bias
each FET the same. IMD performance changes very little within this bias range.
Transistor conduction angle is slightly over 180-degrees, providing linear class-AB
operation. Normal dc drain operating voltage is approximately 50 volts. The linear
supply is unregulated, and can run as high as 60 volts without harm, although it is much
better to keep voltage below 56 volts. See the power supply manual.
Unlike the standard Motorola based modules, the PAM-606 module uses two
diametrically opposed push-pull pairs. This shortens ground path distance while
simultaneously reducing circuit board ground plane current levels. This greatly improves
VHF performance. The dual diametrically opposed push-pull pairs drive balanced low
impedance striplines. The balanced striplines parallel at a unique 1:9 broadband
matching transformer.
The linear RF power FET’s mount on a forced-air-cooled aluminum heatsink. Two dc
fans cool the PAM-606 module and heatsink. Thermistor (PAM-606, R2) senses power
amplifier transistor flange temperature. Transistor temperature thermistor R2 regulates
bias voltage, reducing bias voltage as transistor temperature increases. This bias
feedback system keeps transistor quiescent current stable independent of transistor
junction temperatures. PAM-606 thermistor R2 also feeds a comparator. The
comparator removes drive when transistor temperatures approach unsafe levels.
A second thermistor (PAM-606 R1) monitors heatsink temperature. Voltage at
thermistor R1 regulates fan speed, increasing fan speed and airflow as the heat sink
warms.
PAM-606 module bias comes from the CB-2 control board assembly. Each transistor
has an individual bias adjustment, with minimum bias counter-clockwise from the top
view.
The PAM-606 module employs significant negative feedback to reduce gain, improve
gain flatness, improve linearity, and ensure stability. The FET’s have direct resistive
voltage feedback across each individual transistor from drain-to-gate, as well as push-
pull transformer (T2) coupled feedback common to the push-pull circuit. Push-pull
operation, negative feedback, and linear biasing of FET’s provide significant pre-filter
harmonic suppression.
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