Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-31
The frequency of the LO is the sum of
the IF frequency above the required
visual carrier. For instance, in system
M, the IF visual frequency is at 45.75
MHz and the relative location of the
aural would be 4.5 MHz lower, or 41.25
MHz. For digital applications, the LO is
the center frequency of the digital
channel added to the 44-MHz IF
frequency. By picking the local
oscillator to be 45.75 MHz above the
visual carrier, a conversion in frequency
occurs by selecting the difference
product. The difference product, the
local oscillator minus the IF, will be at
the required visual carrier frequency
output. There will also be other signals
present at the RF output connector J3
at a lower level. These are the sum
conversion product: the LO and the IF
frequencies. Usually, the output
product that is selected by the tuning
of the external filter is the difference
product: the LO minus the 45.75-MHz
IF. The difference product has its side-
bands flipped so that the visual carrier
is lower in frequency than the aural
carrier.
If a bad reactive load is connected to
the mixer, the LO signal that is fed
through it can be increased because
the mixer no longer serves as a double-
balanced mixer. The mixer has the
inherent property of suppressing
signals that may leak from one input
port to any of the other ports. This
property is enhanced by having the
inputs and outputs of the mixer at a
50-Ω 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
contains 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 its resistance
value. The pin diodes, because of a
large, intrinsic region, cannot rectify
signals at this RF frequency; as a
result, 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 that 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