Harris Platinum HT EL 2000LS, Technical Manual

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A.2.3
PA Module
(Refer to the RF PA Module Schematic 843-4999-637)
PA modules consist of four parallel class AB amplifier blocks.
Low Band PA modules produce 18.5 dB gain overall, and the
gain for a high band PA is 13.7 dB.
The module RF input signal feeds the 2-way Divider assembly.
On this divider assembly there is in the signal path a microstrip
directional coupler (which provides a forward drive power sam-
ple for overdrive protection), a microstrip trombone line section
(for phase adjustment), and a Wilkinson 2-way microstrip di-
vider.
The Wilkinson combiner in the Low Band module is a fore-
shortened Wilkinson combiner. Resistors are used in the Wilkin-
son divider and combiner circuits to provide isolation between
ports.
The Wilkinson divider’s two outputs feeds the two 2-way Wilkin-
son microstrip/stripline dividers on the 2X2-Way Divider assem-
bly. The 2X2-Way Divider assembly’s four outputs feeds the four
class AB amplifiers.
The outputs of the four amplifiers feed into the two 2-way
Wilkinson combiners on the 2X2-way Combiner assembly. The
output of the two combiners feeds into the two inputs of the
2-way Wilkinson Combiner assembly. The output of this com-
biner passes through a directional coupler to the RF output jack.
The directional coupler sends a voltage sample of the output port
reflected power to the PCM system.
The Low Band and High Band PA modules are rated at 1050
Watts peak-of-sync visual, and 1050 Watts CW in aural service.
A.2.4
RF Quarter Modules
The RF amplifier subassemblies within a driver or PA module
are called “quarter modules.” The quarter modules use n-channel
Field Effect Transistors, or FETs, as their active devices. FETs
offer several advantages over bipolar junction transistors (BJTs),
including improved ruggedness, better linearity, and less suscep-
tibility to thermal runaway.
N-channel FETs operate similarly to NPN Bipolar Junction
Transistors. In a common-emitter bipolar amplifier, a small
change in base-emitter voltage results in a small change in base
current. The base current modulates the collector current, and the
output is taken at the collector. Similarly, in a common-source
FET amplifier, a small change in gate-source voltage modulates
the drain current, and the amplifier output is taken at the drain.
Each quarter module uses four RF FETs. The input contains a
gain matching pad, a phase matching coax line and a two-way
power divider. Divider outputs each drive a push-pull FET pair.
The FET outputs are recombined in a two way combiner, whose
output is the output of the quarter module.
Temperature compensated bias voltage for each RF FET is
generated from a 15 Volt supply. The supply is part of the module
control card (PCM) and switches on with application of 50 Volts
to the quarter modules. The quarter module supplies voltages
representing temperature and ISO voltage to the module PCM
system.
For any given channel, class A and class AB amplifier blocks use
the same quarter module circuit. The bias voltage adjustment poten-
tiometer controls the quiescent drain current for each FET, which
determines each quarter module’s class of operation.
In cases where quarter modules are biased class AB, as in the 1
kW PA module, each quarter module is capable producing 280
Watts output into a 50 ohm load. The excess power is necessary
to overcome losses in the combining stage.
When the quarter modules are biased class A, as in driver
modules, they exhibit improved linearity and about 1-2 dB higher
gain. The tradeoff, however, is lower power output capability and
reduced efficiency. Thus, class A stages are used as pre-amp and
driver stages, and class AB stages are used as intermediate and
final power amplifier stages.
Because low band and high band quarter modules utilize slightly
different architectures, the circuits are described individually below.
A.2.5
Low Band Quarter Module
(Refer to Low Band Quarter module Schematic 839-7900-001)
The RF input signal first passes through TL1 (Phase setting coax)
and then through AT1 Which sets the gain of the quarter module
to 19.25 dB. The RF input signal then passes to T1, a two-way
coaxial power divider which also performs an impedance trans-
formation. R5 provides isolation between the two divider output
ports.
The upper and lower RF amplifier halves are identical. In the
upper circuit, C1 blocks DC from the input. Components T2/T3
continue the impedance transformation from the divider to the
gates of RF transistors Q1 and Q2. T3 also establishes a 180
phase relationship between the signal voltages sent to the two
transistors, which is the basis for push-pull operation.
R2 and R3 “swamp” the transistor gate input impedances, which
are highly capacitive. C6/C7/C9/C10 block the DC gate bias
from reaching the quarter module input. C8/C5/C11 complete
the input impedance transformation.
An R,L,and C drain-to-gate negative feedback loop exists around
each FET. The feedback will ensure stability at low frequencies.
C25 and C24 block the 50 Volts present at the drains from
reaching the gates through the feedback loops.
L5/L6/C23 form a balanced L-network, which act as both a
low-pass filter and an impedance transformer between the FET’s
and T6. T6 continues the output impedance transformation and
combines the transistor outputs in series. C28, C29/R19, and C4
bypass one port of T6 to ground, and C30 and C31 couple the
RF to T8.
T8 is a two-way combining transformer which combines the
outputs of the upper and lower amplifier halves and completes
the output match. R15 provides isolation between T8’s input
ports.
Platinum™ Series
A-4 888-2457-001
WARNING: Disconnect primary power prior to servcing.