User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-7
On, the drain goes low and removes
the high to pin 1 of J1, which is
connected to the Lock LED, on the LED
display board, which is extinguished.
Voltage Requirements
The ±12 VDC needed for the operation
of the board enters through jack J11.
The +12 VDC is connected to J11-3,
which is filtered and isolated by L6 and
C38 before it is connected to the rest of
the board. The -12 VDC is connected to
J11-5, which is filtered and isolated by
L7 and C39 before it is connected to
the rest of the board.
The +12 VDC is connected to U7, a 5-
volt regulator IC, that provides the +5
VDC operating voltage to the U2, U4,
U5, and U9 ICs.
4.3.5 (A1) Power Entry Module
Assembly (1227-1206; Appendix
D)
The power entry module assembly
provides overvoltage and surge
protection for the input AC lines that
connect to the exciter tray. The AC
input plug connects to J14; J14 is part
of the power entry module on the rear
panel of the exciter tray.
The module assembly contains two
130-VAC varistors and one 250-VAC
varistor that connect across the AC
lines and to ground. The module also
contains two 4-amp fuses, one in each
input line for overcurrent protection.
4.3.6 (A9) IF Phase Corrector
Board (1227-1250; Appendix D)
The IF phase corrector board has
adjustments that pre-correct for any IF
phase modulation distortion that might
occur in output amplifier devices such
as Klystron power tubes and solid-state
amplifiers. Two separate, adjustable IF
paths are on the board: a quadrature
IF path and an in-phase IF path. The
quadrature IF is 90° out of phase and
much larger in amplitude than the in-
phase IF. When these paths are
combined in Z1, they provide the
required adjustable phase correction to
the IF signal.
The IF input signal enters at J1 and is
capacitively coupled to U1. U1 amplifies
the IF before it is connected to Z1, a
splitter, to create two, equal IF outputs.
IF output #1 is connected to J2 and IF
output #2 is connected to J3. IF output
#1 at J2 is jumpered through coaxial
cable W4 to jack J6, the quadrature
input, on the board. IF output #2 at J3
is jumpered through coaxial cable W5
to jack J7, the in-phase input, on the
board.
4.3.6.1 Phase Corrector Circuit
The phase corrector circuit adjusts for
any amplitude nonlinearities of the IF
signal. It is designed to work at IF and
has three stages of correction. Each
stage has a variable threshold and
magnitude control. The threshold
control determines the point where the
gain is changed and the magnitude
control determines the gain change
once the breakpoint is reached. The
second stage has a jumper that
determines the direction of the
correction so that the gain can be made
to increase either above or below the
threshold and either black or white
stretch can be accomplished in stage
two.
When the phase corrector circuit is
operating, the IF signal from J6 is
applied to transformer T1, which
doubles the voltage swing using a 1:4
impedance transformation. Resistors
R8, R61, R9, and R48 form an L-pad
that attenuates the signal. This
attenuation is adjusted by adding R7, a
variable resistor, in parallel with the L-
pad. R7 is only in parallel when the
signal reaches a level large enough to
bias on CR1 and CR2 and allow current
to flow through R7. When R7 is put in
parallel with the L-pad, the attenuation