User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 35/76
Section 6 – UHF ATSC Amplifier
Module 4442
General Functional Description
AT7120 transmitter has two Modules 4442 connected in parallel, which work as digital TV end
amplifiers in all UHF band.
Module 4442 offers high linear precision that works satisfactorily with digital TV signals. However, to
achieve optimal gain, this module was divided in 3 operational channel bands, which are:
Channels 14 to 25
Channels 26 to 38
Channels 39 to 53
For the four UHF channel bands described above, the same circuit board is used.
UHF/ATSC amplifier module 4442
CIRCUIT
CIM
CIP
DRIVER
3431
8431
What defines which UHF band of Module 4442 is used are certain components that might or not be
excluded or alter the value.
Module 4442 is made of two Push-Pull amplifiers cells, class AB. Each cell has a double Push-Pull
transistor, with LDMOS technology. This module can offer up to 70W.
To better understand how the amplifier works, refer to the block diagram in the upcoming pages.
The amplifier is made of the following components:
Input Coupler
The input RF coupler equally divides the UHF signal present in the input connector for the two
amplification cells, maintaining 50Ω impedance.
The 3dB 90˚ hybrid coupler SMD built by “sandwiching” together two small boards called stripline.
This hybrid coupler has 4 gates (terminals); in one of this gates is applied an input signal, which will be
divided internally; the two resulting signals are applied in the direct and adjacent gates. There is also an
isolated gate, where a 50Ω balance charge is placed. This charge is delivered by the resistor, R1.
This charge basically guarantees a constant impedance of 50Ω, even if one of the cells comes to fail. By
coupling, the other line receives the input signal and transfers it electronically to the two amplification
cells.