Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-14
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.
3.1.6.2 Amplitude Corrector Circuit
The amplitude corrector circuit 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 +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
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. The two chokes L11 and L12
form a high impedance for RF to isolate
the op-amp ICs from the IF.
After the signal is amplified by U5, it is
applied to a second stage through T5.
The transformer doubles the voltage
swing by means of a 1:4 impedance
transformation. Resistors R39, R57, R58,
and R40 form an L-pad that lowers the
level of the signal. The signal is applied
to amplifier U6 to compensate for the
loss in level through the L-pad. After the
signal is amplified by U6, it is applied to a
third stage through T6. The transformer
doubles the voltage swing by means of a