User's Manual
300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-14
leak from one input port to any of the
other ports. This property is enhanced by
having inputs and outputs of the mixer at
50Ω impedance. The reactive filter that is
externally connected to J3 of the board
does not appear as a good 50-Ω load at
all frequencies. The pad, in the output
line of the board, consists of R5, R4, R6,
and R7. The pad buffers the bad effects
of the reactive filter load and makes it
appear as a 50-Ω impedance. The RF
signal is amplified by U1, a modular
amplifier, and includes within it biasing
and impedance matching networks that
makes U1 act as a wideband-RF amplifier
device. This amplifier, in a 50-Ω system,
has approximately 12 dB of gain. U1 is
powered from the +12 VDC line through
RF decoupling components R27, R28,
C30, R8, and L1. Inductor L1 is a
broadband-RF choke and is resonance
free through the UHF band. The amplified
RF connects to SMA RF output jack J3
which is cabled to the external filter.
The RF input signal from the external
filter re-enters the board at J4 (-11 to
-14 dBm) and is capacitively coupled to
the pin-diode attenuator circuit consisting
of CR1, CR2, and CR5. The pin-diode
attenuator acts as a voltage-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