User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-8
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 that
temperature compensates the corrector
circuits from the affects 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 and forming 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
that isolates the op-amps from the RF.
The adjusted signal is next applied to
amplifier U2, which compensates 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 two other corrector stages
operate the same 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 will move all of
the threshold points past sync tip so
that they will have no affect. R68 can
be adjusted and set for the required
correction range. 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.
4.3.6.2 Amplitude Corrector Circuit
The amplitude corrector circuit adjusts
for any amplitude nonlinearities of the
IF signal using one stage of correction.
The stage has a variable threshold
control, R31, and a variable magnitude
control, R35. The threshold control
determines the point where 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 diodes CR11 and CR12
provide a .9-VDC reference voltage that
is used to temperature compensate for
the two diodes in the corrector stage.
When the amplitude corrector circuit is
operating, the IF signal from J7 is
applied to transformer T4, which
doubles 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 can be 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, putting it in
parallel with the L-pad. When R35 is in
parallel with the resistors, the