User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-16
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
that allows 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.
The threshold for the first corrector stage
is set by controlling where CR1 and CR2
turn on. This is accomplished by
adjusting R3 that forms 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
through the choke L2. The .9 VDC
reference is connected to U9D, a unity-
gain amplifier, whose output is wired to
CR2 through the choke L3. The L2 and
L3 chokes form a high impedance for RF
to isolate the op-amps from any RF. The
adjusted signal is next applied to the
amplifier U2 that 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 a matching, isolation
transformer T2 and then to a third
corrector stage through the matching,
isolation transformer T3. The other two
corrector stages operate in the same
manner as the first. Each stage is
independent and do not interact with
each other. The Quadrature corrected IF
output of the third corrector stage, at
unity gain through the three corrector
stages, is applied to pin 6 of the
combiner Z2.
When jumper W1 on J8 is connected
from pin 2 to pin 1, 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 pin 2 to pin 3, center 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.
The phase correctors can be bypassed by
moving the jumper W2 on J9 to the
Disable position, between pins 2 and 3.
This action will move all of the threshold
points past sync tip so that they will have
no effect. R68 is 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.
4.1.6.2 Amplitude Corrector Circuit, In
Phase Correction
The amplitude corrector circuit, in phase,
uses one stage of correction to correct
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 the
+6.8 VDC reference. The diodes CR11
and CR12 provide the other reference of
.9 VDC, V
ref,
which temperature
compensates for the two diodes, CR8 and
CR9, 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.