User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-6
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 attenuation through the
L-pad is lowered, causing signal stretch,
which is 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, cut-
in, for the corrector stage is set by
controlling where CR8 and CR9 turn on.
This is accomplished by adjusting cut-in
resistor R31, which forms 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, and to diode CR9. 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 that isolates 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
1:4 impedance transformation. Resistors
R42, R59, R60, and R43 form an L-pad
that lowers the level of the signal. The
signal is applied to amplifier U7 to
compensate for the loss in level through
the L-pad.
TP1 is a test point that gives the operator
a place to measure the level of the in-
phase IF signal that is connected to
mixer stage Z2. The amplitude corrector
can be disabled by moving jumper W3 on
J10 to the Disable position; this will move
the breakpoint past sync tip so that it will
have no affect on the signal.
4.1.6.3 Output Circuit
The phase-corrected signal from pin 1 on
combiner Z2 exits the board at IF output
jack J4 after passing through a matching
network consisting of six resistors.
4.1.7 (A8) ALC Board (1265-1305;
Appendix D)
The ALC board provides for automatic
level control (ALC) and amplitude
linearity correction of the IF signal. The
ALC adjusts the level of the IF signal
through the ALC board which, in turn,
controls the output power of the
transmitter.
The visual + aural IF input (0 dBm)
signal from the modulator enters the
board at modulator IF input jack J32 and,
if the (optional) receiver tray is present,
the visual + aural IF input (0 dBm) from
the receiver tray connects to J1, the
receiver IF input jack. The modulator IF
input connects to relay K3 and the
receiver IF input connects to relay K4.
The two relays are controlled by the
Modulator Select command that is
connected to J30 on the board. Modulator