User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-15
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.
4.3.7.11 ±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.
4.3.8 (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.
4.3.8.1 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 digital calibration control that
is adjusted for a 100% digital reading,
to amplifier U2B. The amplified digital
output is connected to comparator U2C.
The other input to U2C is the level set
by aural null adjust R51, which is not
used with digital. Pots R51 and R20
should be set full CCW. The offset null
adjust R48 is adjusted for 0% digital
power with the transmitter in Standby.
The adjusted output is amplified by
U3D and connected to the other input
of U2C. The output of U2C connects to
J6, pins 2 and 3, which supplies the
forward digital level output to the front
panel meter for monitoring.
The scrambling circuits are not used
with digital operation.
4.3.8.2 Reflected Level Circuit
A reflected power sample is applied to
J3 of the board and is detected by
diode detector CR7 and U3B. The
detected output is fed through R39, the
reflected calibration pot, which can be
adjusted to control the gain of U3C.
The output of U3C connects to J6, pin
7, which supplies a reflected power
level output to the front panel meter.
4.1.8.3 Voltages for Circuit Operation
The ±12 VDC is applied to the board at
J5. +12 VDC is connected to J5, pin 3,
and is isolated and filtered by L4 and
C34 before it is connected to the rest of