______________________________________________HARRIS
888-9058-001
WARNING: Disconnect and lockout AC primary power prior to servicing
181
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 FETs 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.
If any phase or amplitude difference exists between the signal in the upper and lower
amplifier halves a voltage will develop across R15.
This RF voltage will be coupled through toroid transformer T9, to CR1, an RF
detector which produces a DC signal proportional to the amount of imbalance. This DC
signal is called the ISO voltage sample, and it is sent to the PCM system through J1-2.
8.2.6 High Band Quarter Module
(Refer to High Band Quarter Module Schematic, Tab 1000 in drawing package)
The RF input to the quarter module passes through TL1 (Phase setting coax) and AT1
(Attenuator which sets the gain of the quarter module to 14.25 dB). The RF input then
passes through a two-way Wilkinson power divider, consisting of two 75 ohm
microstrip sections. R1 provides isolation between the divider outputs.
The upper and lower amplifier halves on the schematic are identical. In the upper
amplifier, C9 couples RF into the amplifier while blocking DC. T1 is a coaxial balun
transformer, which provides both a step-down impedance transformation and an
unbalanced-to-balanced transformation. Its two output signals differ in phase by 180,
which establishes push-pull operation in the RF FET pair Q1 and Q2.
To Next Page
Loading...
