User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-16
the board. The +12 VDC also connects
to U5, a 5-VDC regulator that provides
the voltage needed to operate U4. The
-12 VDC is applied to J5, pin 1, and is
isolated and filtered by L5 and C35
before it is connected to the rest of the
board.
4.3.9 (A11) UHF Upconverter Board
(1265-1310; Appendix D)
The UHF upconverter board provides
upconversion processing by mixing the
IF and LO signals in mixer Z1 to
produce the desired RF frequency
output. The RF output is connected
through J3 to an external filter and
applied back to the board at J4 where
the gain is set by R10. The RF is
amplified and connected to the RF
output jack of the board at J5.
The IF signal (0 dBm) enters the board
at J1, an SMA connector, and is applied
through a filter circuit consisting of L10
and C25 to C28 to a matching pad. This
pad consists of R1, R2, and R3, which
presents a relatively good source
impedance, and feeds the signal to pins
3 and 4, the I input of mixer Z1. The
local oscillator signal (+13 dBm) from
the x8 multiplier connects to the board
at jack J2, an SMA connector, through
a UHF channel filter and is connected
directly to pin 8, the L input of the
mixer.
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
sidebands 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 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-