User's Manual
Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-34
network consisting of R7, R9, and
adjustable resistor R6. R6 is adjusted
to set up the operating current at 5
amps. When the opto-isolator LED
turns on, it causes the transistor
portion to also turn on. When the
transistor portion turns on, it biases on
Q2, which acts as a shunt regulator for
the base current of Q1.
Negative feedback for the circuit is
preset so that if Q1 draws more than
the desired amount of collector current,
the voltage drop across A4-A4, the
metering resistor, becomes greater
than normal. This increase causes the
voltage across the opto-isolator to
increase; as a result, the opto-isolator
LED is brighter than normal and
forward biases Q2 even more. This
action tends to make the available
current that was fed through R11 and
Q1 to flow more through Q2, from
collector to the emitter ground, setting
up the negative feedback path. The
negative feedback tends to stabilize the
current flow through Q1. Capacitor C8
provides an active filtering action for
this current source, which provides an
improved response in Q1 during the
vertical interval average current
changes that takes place.
4.8.5 (A4-A2) Coupler Board
Assembly (1227-1316; Appendix D)
The UHF coupler assembly is mounted
in the UHF amplifier tray and provides a
forward power sample of the input
drive level to the dual stage amplifier
assembly, class AB. The drive-level
sample from J3 is cabled to the
amplifier control board where it
connects to the input of the overdrive-
protection circuit.
The RF input to the UHF coupler
assembly from the dual stage amplifier
assembly, class A, connects to SMA
jack J1. The RF is connected by a
stripline track to SMA output jack J2. A
hybrid-coupler circuit picks off a
forward sample that is connected to
SMA-type connector jack J3. R1 is a
dissipation load for the reject port of
the coupler.
4.8.6 (A6) Dual Peak Detector
Enclosure (1227-1317; Appendix D)
The dual peak detector enclosure
provides EMI and RFI protection for the
dual peak detector board, single supply
(1227-1333), which is mounted inside
the enclosure.
The module has two inputs: a forward
power sample at SMA jack J1 and a
reflected power sample at SMA jack J2.
The module has two peak-detected
sample outputs: a forward power
sample at FL3 from J4-4 on the board
and a reflected power sample at FL2
from J4-2 on the board. The module
also has a forward power sample
output at SMA jack J3.
The voltage, +28 VDC, needed to
operate the board connects to FL1 on
the assembly, which is wired to J4-7 on
the board.
4.8.7 (A6-A1) Dual Peak Detector
Board, Single Supply (1227-1333;
Appendix D)
The function of the dual peak detector
board is to detect forward and reflected
samples of visual or aural RF signals
and generate an output voltage
proportional to the power levels of the
sampled signals for metering purposes.
There are two identical signal paths on
the board: one for forward power and
one for reflected power. A sample of
forward output power enters the board
at SMA jack J1. Resistors R1, R2, and
R3 form an input impedance-matching
network of 50Ω. The forward power
signal is detected by CR1, R4, R5, R7,
R10, C1, and C2. The output is buffered
by operational amplifiers U3B and U1C
before it is connected to forward power
output jack J4-4. U3 has a very high
input impedance that makes the IC less