Manual Chapter 3

500-Watt UHF Transmitter Chapter 3, Circuit Descriptions
425A, Rev. 0 3-22
3.1.9 (A19) Visual/Aural Metering
Board (1265-1309; Appendix D)
The visual/aural metering board provides
detected outputs of the visual, aural, 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 attained from
samples of the forward power and
reflected power outputs of the tray.
A forward power sample, visual + aural,
is applied to SMA jack J1 on 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
visual + aural signal is amplified by U6B
and its output is split. One amplified
output of U6B connects to the aural level
circuit and the other output connects to
the visual level circuit.
3.1.9.1 Aural Level Circuit
One of the detected visual + aural level
outputs of U6B connects through C6 to
the intercarrier filter circuit that consists
of R13, R14, L1, C7, and C8; C8 and L1,
the intercarrier filter, can be adjusted for
a maximum aural reading. The filter
notches out the video + aural and only
leaves the 4.5-MHz difference frequency
between the visual and aural, which is a
good representation of the aural level.
The 4.5-MHz signal is fed to buffer
amplifier U6A. The output of U6A is
detected by diode detector CR3 and U1A
and then fed through aural calibration
control R20 to amplifier U2D. The
amplified output of U2D is split, with the
main output connected through R21 to
J6, pin 1, which supplies the aural level
output to the front panel meter for
monitoring. The other output of U2D is
connected to aural null adjust R51 and
offset null adjust R48, which are adjusted
to set up the visual power calibration.
3.1.9.2 Visual Level Circuit
The other detected visual + aural level
output from U6B is connected to U1C
and, if there is no scrambling, connects
directly to intercarrier notch L3, which is
adjusted to filter out the aural and the
4.5-MHz intercarrier frequencies, leaving
only a visual-with-sync output. The
visual-with-sync output is fed to a peak-
detector circuit consisting of CR5 and
U2A. The signal is then fed through visual
calibration control R28, which is adjusted
for a 100% visual reading with no aural,
to amplifier U2B. The amplified visual
peak of sync output is connected to
comparator U2C. The other input to U2C
is the level set by aural null adjust R51,
which is adjusted for 100% visual power
after the aural is added and the peak
power is adjusted back to the reference
level. Inputs to U2C also come from
offset null adjust R48, which is adjusted
for 0% visual 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
supply the peak of sync visual level
output to the front panel meter for
monitoring.
If this board is operated with scrambling,
using suppressed sync, the visual level
circuit operates differently than described
above because there is no peak of sync
present on the forward sample input. For
the board to operate properly, a timing
pulse from the scrambling encoder must
connect to the board at J4. This timing
pulse is converted to sync pulses by U4A
and U4B, which control the operation of
Q2. Intercarrier notch L2 is tuned to
remove any visual + aural signal that
may remain.
The sync amplitude is controlled by gate
amplitude adjust R25 and then applied to
the minus input of U1C. At this point, it is
inserted into the visual + aural signal
that is connected to the plus input of
U1C, producing a peak of sync in the
signal. The output of U1C is connected to