Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-20
adjusted signal is next applied to
amplifier U2 to compensate for the loss
through the L-pad. U2 is powered
through L4 and R10 from the +12 VDC
line. After the signal is amplified by U2,
it is applied to the second corrector
stage through T2 and then to a third
corrector stage through T3. The other
two corrector stages operate in the
same manner as the first; they are
independent and do not interact with
each other.
When jumper W1 on J8 is connected
from center to ground, R15 is put in
series with ground. In this
configuration, black stretch (white
compression) is applied to the IF signal
by controlling the attenuation through
the path. When W1 is connected from
the center pin to the end that connects
to T2, R15 is put in parallel with the L-
pad. In this configuration, black
compression (white stretch) is applied
to the IF signal by controlling the
attenuation through the path.
The phase correctors can be bypassed
by moving jumper W2 on J9 to the
Disable position. This action will move
all of the threshold points past sync tip
so that they will have no effect. R68
can be adjusted and set for the
correction range that is needed. TP2 is
a test point that gives the operator a
place to measure the level of the
quadrature IF signal that is connected
to pin 6 on combiner Z2.
4.2.6.2 Amplitude Corrector Circuit
The amplitude corrector circuit uses
one stage of correction to adjust for
any amplitude nonlinearities of the IF
signal. The stage has a variable
threshold control, R31, and a variable
magnitude control, R35. The threshold
control determines the point at which
the gain is changed and the magnitude
control determines the amount of gain
change once the breakpoint is reached.
Two reference voltages are needed for
the operation of the corrector circuit.
Zener diode VR1 with R46 provides
+6.8 VDC and the diodes CR11 and
CR12 provide a .9 VDC reference
voltage to temperature compensate for
the two diodes in the corrector stage.
In the amplitude corrector circuit, the
IF signal from J7 is applied to
transformer T4 to double the voltage
swing by means of a 1:4 impedance
transformation. Resistors R36, R55,
R56, and R37 form an L-pad that
lowers the level of the signal. The
amount that the level is lowered is
adjusted by adding more, or less,
resistance, using R35 in parallel with
the L-pad resistors. R35 is only in
parallel when the signal reaches a level
large enough to turn on diodes CR8 and
CR9. When the diodes turn on, current
flows through R35 and puts it in
parallel with the L-pad. When R35 is in
parallel with the resistors, the
attenuation through the L-pad is
lowered, causing signal stretch (the
amount of stretch determined by the
adjustment of R35).
The signal is next applied to amplifier
U5 to compensate for the loss in level
through the L-pad. The breakpoint, or
cut-in point, for the corrector stage is
set by controlling where CR8 and CR9
turn on. This is achieved by adjusting
cut-in resistor R31 to form a voltage
divider from +6.8 VDC to ground. The
voltage at the wiper arm of R31 is
buffered by the unity-gain amplifier
U8B. This voltage is then applied to
R34 through L11 to the CR9 diode. The
.9-VDC reference created by CR11 and
CR12 is applied to unity-gain amplifier
U8A. C36 keeps the reference from
sagging during the vertical interval. The
reference voltage is then connected to
diode CR8 through choke L12. Chokes
L11 and L12 form a high impedance for
RF to isolate the op-amp ICs from the
IF.