User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-5
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 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.
4.1.6.2 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