Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-19
4.2.5.12 ±12 VDC Needed to Operate
the Board
The ±12 VDC connects to the board at
J14. The +12 VDC connects to J14-3
and is filtered by L30, L41, and C80
before it is applied to the rest of the
board. The -12 VDC connects to J14-5
and is filtered by L31 and C81 before it
is applied to the rest of the board.
The +12 VDC also connects to U16, a
5-VDC regulator IC, which produces the
+5 VDC needed to operate timing IC
U17.
4.2.6 (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 may
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 they are combined in
Z1, it provides the required, adjustable
phase correction to the IF signal.
The IF input signal enters at J1 and is
AC coupled to U1. U1 amplifies the IF
before it is connected to Z1, a splitter
that creates 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.
4.2.6.1 Phase Corrector Circuit
The phase corrector circuit corrects 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 at which
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 correction,
so that the gain can be increased either
above or below the threshold and either
black or white stretch can be achieved.
In the phase-corrector circuit, the IF
signal from J6 is applied to transformer
T1; T1 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
through the L-pad is lowered, causing
black stretch.
Two reference voltages are utilized in
the corrector stages and both are
derived from the +12 VDC line. Zener
diode VR1, with R46 as a dropping
resistor, provides +6.8 VDC from the
+12 VDC line. Diodes CR11 and CR12
provide a .9 VDC reference to
temperature compensate the corrector
circuits from the effects of the two
diodes in each corrector stage.
The threshold for the first corrector
stage is set by controlling where CR1
and CR2 turn on. This is accomplished
by adjusting R3 to form a voltage
divider from +6.8 VDC to ground. The
voltage at the wiper of R3 is buffered
by U9C, a unity-gain amplifier, and
applied to CR1. The .9 VDC reference is
connected to U9D, a unity-gain
amplifier, whose output is wired to
CR2. These two references are
connected to diodes CR1 and CR2
through chokes L2 and L3. The two
chokes form a high impedance for RF to
isolate the op-amps from the RF. The