UHF Analog Driver/Transmitter/
Chapter 4, Circuit Descriptions
Translator
LX Series, Rev. 3
4-14
temperature compensates for the two
diodes in each corrector stage.
4.4 (A5) VHF/UHF Upconverter
Module (1303829; Appendix B)
This module contains the Downconverter
Board Assembly, the First Conversion
Board, LX Series, L-Band PLL Board, LX
Series and the Upconverter Control
Board, LX Series. This module takes an
external IF and converts it to the final
RF output frequency using two internally
generated local oscillator frequencies.
4.4.1 (A1) Downconverter Board
Assembly (1303834; Appendix B)
This board converts a signal at an input
frequency of 1044 MHz to a broadcast
VHF or UHF TV channel.
The IF at 1044MHz is applied to the
board at J7, and is converted down to
VHF or UHF by the mixer IC U6. The LO
frequency is applied to the board at a
level of +20 dBm at J8. The output of
the mixer is applied to a 6 dB attenuator
and then to a 900 MHz Low Pass filter.
The filter is intended to remove any
unwanted conversion products. The
signal is next connected to the amplifier
U2, and then a pin diode attenuator
consisting of DS4, DS5 and their
associated components. The attenuator
sets the output level of the board and is
controlled either by a manual gain pot
R7, or an external AGC circuit. This
automatic or manual mode of operation
is controlled by the switch SW1. When
in manual mode, the LED DS6 is
illuminated.
The output of the pin attenuator is
applied to another amplifier U3 and
another low pass filter, before reaching
the final amplifier U1. The output of the
board is at J5 with a sample of the
output available at J6, which is 20 dB in
level below the signal at J5.
4.4.2 (A2) L-Band PLL Board
(1303846; Appendix B)
This board generates an LO at a frequency
of 1.1-1.9 GHz. The board contains a PLL
IC U6, which controls the output
frequency of a VCO. The PLL IC divides
the output of the VCO down to 100kHz,
and compares it to a 100kHz reference
created by dividing down an external 10
MHz reference that is applied to the board
at J1 pin 4. The IC generates an error
current that is applied to U3 and its
associated components to generate a bias
voltage for the VCO's AFC input.
There are two VCOs on the board, U4,
which operates at 1.1-1.3 GHz for VHF
channels, and U5, which operates at 1.5-
1.9 GHz for UHF channels. The VCO in
use is selected by a signal applied to J1
pin 20. This input enables the power
supply either U1 or U2 for the appropriate
VCO for the desired channel. U7 is a
power supply IC that gen5V for
the PLL IC U6.
The output of each VCO is filtered by a
low pass filter to remove any harmonic
content and applied to a pin diode switch
consisting of CR1, CR2, and their
associated components. The selected
signal is amplified by U9 and U10, then
applied to a high pass filter and finally
amplified to a level of approxi21
dBm by U11. The output is connected to
a low pass filter to remove any unwanted
harmonic content and leaves the board at
J3 at a level of +20 dBm.
4.4.3 (A3) First Conversion Board
(1303838; Appendix B)
This board generates a 1 GHz LO signal
using the VCO U9, the PLL IC U12, and
the loop filter, C30-C33 and R49. The PLL
IC compares a divided down sample of the
VCO to a divided down sample of the
transmitter's system 10 MHz reference.
The output of the VCO is amplified by U10
and U11 and applied to a low pass filter
before being connected to an image
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