User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-17
variable attenuator in which each pin
diode functions as a voltage-variable
resistor that depends on the DC bias
supplied to the diode for the resistance
value. The pin diodes, because of a
large, intrinsic region, cannot rectify
signals at this RF frequency; therefore,
they act as a linear voltage-variable
resistor.
The pin diodes are configured in shunt
configuration: CR1 is the first shunt
element, CR2 is the second shunt
element, and CR5 is the series
element. The manual gain AGC, W1 on
J10 between pins 1 and 2, is used in
most cases. The control voltage from
manual gain pot R10 sets up a current
path through R11 and the diodes in the
pin attenuator. The level-controlled RF
signal from the pin-diode attenuator
circuit is amplified by wideband-hybrid
amplifier IC U2 which is configured in
the same way as U1. The RF signal is
buffered by Q1 and applied to the
push-pull class A amplifier circuit
consisting of Q2 and Q3. At the input to
the transistors, the RF is converted to a
balanced, dual feed by balun L4, which
is made from a short length of UT-141
coaxial cable.
Capacitors C12 and C13 provide DC
blocking for the input signal to the
amplifier devices. The RF outputs at the
collectors of the transistors are applied
through C19 and C20, which provide
DC blocking for the output signals. The
RF signals connect to L7, which consists
of UT-141 coaxial cable. L7 combines
the RF back to a single-RF output at a
50-Ω impedance to L8, which provides
a sample of the RF. The main path
through L10 is to J5, the RF output jack
of the board (+10 to +20 dBm). The
sample of the RF connects to a splitter
that provides a sample output (0 dBm)
at J6 of the board. The other output of
the splitter connects to a peak-detector
circuit consisting of CR3 and U3, which
provides a DC level at J7 that
represents the RF output of the UHF
exciter to the front panel meter. R29
sets up the calibration of the front
panel meter for 100% in the UHF
exciter position when the output power
of the exciter is at +17 dBm peak
visual or +10 dBm average for digital
applications.
The board is powered by ±12 VDC that
is produced by an external power
supply. +12 VDC enters the board
through J8, pin 3, and is filtered and
isolated by RF choke L9 and shunt
capacitors C24 and C33. This circuit
isolates the RF signals from the board
away from those of other devices
connected to the same +12 VDC line
external to the UHF upconverter board.
The +12 VDC is then applied to the rest
of the board.
The -12 VDC enters the board through
J8, pin 5, and is filtered and isolated by
RF choke L11 and shunt capacitors C34
and C35. This circuit isolates the RF
signals from this board away from
those of other devices connected to the
same
-12 VDC line that is external to the UHF
upconverter board; the -12 VDC is then
applied to the rest of the board.
4.3.10 (A17) Transmitter Control
Board (1265-1311; Appendix D)
The transmitter control board provides
the system control functions and the
operational LED indications that can be
viewed on the front panel of the
transmitter. The main control functions
are for Operate/Standby and
Auto/Manual selection. When the
transmitter is switched to Operate, the
board supplies the enables to any
external amplifier trays. When the
transmitter is in Auto, the board also
performs the automatic switching of the
transmitter to Standby upon the loss of
the video input. The board contains a
VSWR cutback circuit. If the VSWR of
the transmitter increases above 20%,
the VSWR cutback circuit will operate
and cut back the output level of the