User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-12
ALC circuit as well as the AGC reference
to the transmitter control board (1265-
1311).
Voltage TP4 should be the same in either
the normal or the encoded video mode.
Monitor J9, pins 3 and 4, with a spectrum
analyzer, check that the board is in the
AGC mode, and tune C103 to notch out
the aural IF carrier.
4.1.7.11 Fault Command
The ALC board also has circuitry for an
external mute fault input at J19, pin 6.
This is a Mute command and, in most
systems, it is part of the interface for the
protection circuits of high-gain output
amplifier devices. The Mute command is
intended to protect the amplifier devices
against VSWR faults. In this case, it is
required that the actions occur faster
than just pulling the ALC reference down.
Two different mechanisms are employed:
one is a fast-acting circuit to increase the
attenuation of pin-diode attenuator CR3,
CR1, and CR2, and the second, as just
described, is the reference voltage being
pulled away from the ALC amplifier
device. An external mute is a pull-down
applied to J19, pin 6, that provides a
current path from the +12 VDC line
through R78 and R139, the Mute
Indicator LED DS4, and the LED section
of opto-isolator U11.
These actions turn on the transistor
section of U11 that applies -12 VDC
through CR21 to U10A, pin 3, which pulls
down the reference voltage. This is a
fairly slow action that is slowed down by
the low-pass filter function of R81 and
C61. When the transistor section of U11
is on, -12 VDC is also connected through
CR22 to the pin-diode attenuator circuit.
This establishes a very fast muting
action, by reverse biasing CR3, in the
event of an external VSWR fault.
4.1.7.12 ±12 VDC to Operate the Board
The ±12 VDC connects to the board at
J14. +12 VDC connects to J14-3 and is
filtered by L30, L41, and C80 before it is
applied to the rest of the board. The -12
VDC connects to J14-5 and is filtered by
L31 and C81 before it is applied to the
rest of the board.
The +12 VDC also connects to U16, a 5-
VDC regulator IC, which produces the +5
VDC needed to operate timing IC U17.
4.1.8 (A19) Visual/Aural Metering
Board (1265-1309; Appendix D)
Note: This board was originally
designed for analog television signal
operation. For digital applications,
forward digital can be substituted
for forward visual and the aural
circuits along with the scrambling
circuits are not used.
The visual/aural metering board provides
detected outputs of the forward digital
and reflected output samples that are
used for monitoring on the front panel
meter. The board also provides
adjustments for the calibration of the
readings on the meter. These readings
are obtained from the samples of the
forward power and reflected power
outputs of the tray.
A forward power sample is applied to
SMA jack J1 of the board. The input
signal is split, with one path connected to
forward power sample SMA jack J2 for
monitoring purposes. The other path is
connected through C1 to CR2, R4, R5,
R6, C4, and CR1, which make up a
detector circuit. The detected signal is
amplified by U6B and its output is split.
One amplified output of U6B connects to
the digital level circuit, the other output
is not used in digital operation.
4.1.8.1 Digital Level Circuit
The detected digital level output from
U6B is connected to U1C. The intercarrier
notch L3 is not used with digital. The
digital output of U1C is fed to a peak-
detector circuit consisting of CR5 and
U2A and then fed through R28, the visual