User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-9
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.
4.3.6.3 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.
4.3.7 (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 select enable/disable jumper
W11 on J29 controls whether the
Modulator Select command at J30
controls the operation of the relays.
With jumper W11 on J29, pins 1 and 2,
the Modulator Select command at J30
controls the operation of the relays but,
with jumper W11 on J29, pins 2 and 3,
the modulator is selected all of the
time.
4.3.7.1 Modulator Selected
With the modulator selected, J11-10
and J11-28 on the rear of the UHF
exciter tray are connected together;
this makes J30 low and causes relays
K3 and K4 to de-energize. When K4 is