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UHF Analog Driver/Transmitter/ Chapter 4, Circuit Descriptions
Translator
LX Series, Rev. 3 4-26
4.7.3 (A2) 40 Watt UHF Amplifier
Assembly (1304490; Appendix B)
(NOTE: Used in 1kW Systems)
The output of the UHF filter is connected
to the input J1 of (A2) the 40 Watt UHF
amplifier assembly (Figure 4-3). The
assembly is made up of a (51-5378-308-
00) module, which operates class AB and
is a highly linear broadband amplifier for
the frequency range of 470 to 860 MHz.
It can deliver an output power of 40
watts (CW) with approximately 13 dB of
gain. With a typical input of +23dBm an
output of +36.2dBm is expected. The
output is set as needed to provide the
drive level to the external PA
Assemblies.
The amplification circuit consists of
LDMOS transistors V804 and V805
connected in parallel and operating in
class AB. The paralleling network is
achieved with the aid of 3 dB couplers
Z802 and Z803. A further 3 dB coupler
Z801, in conjunction with capacitors
C800 and C819, serves as a phase
shifter. Phase alignment, for the
complete amplifier, as well as quiescent
current settings are achieved by means
of potentiometers R807 and R808. The
settings are factory implemented and
should not be altered.
PIN diodes V810 & V811 form a
variable-damping circuit that is used to
adjust the amplification of the 40-watt
module. The adjustment is performed
with the Gain potentiometer R838. A
readjustme nt of the amplification may
be required, after repair work, to ensure
that the PAs in multiple PA transmitters
deliver the same output power.
OR 4.7.3.1 (A3) 250 Watt UHF
Amplifier Assembly (1300116;
Appendix B)
(NOTE: Used in 2kW Systems)
The UHF Module Assembly, 250-watt
module (Figure 4-4) is a broadband
amplifier for the frequency range 470 to
860 MHz. The amplifier is capable of
delivering an output power of 70 W
rms
.
The amplification is approximately 12 dB.
With a typical input of +27dBm an output
of +39dBm is expected.
The amplification circuit consists of the
parallel connected push-pull amplifier
blocks V1 and V2 operating in class AB. In
order to match the transistor impedance
to the characteristic impedance of the
input and output sides, matching networks
are placed ahead and behind the amplifier
blocks. Transformers Z3 to Z6 serve to
balance the input and output signals. The
paralleling circuit is achieved with the aid
of 3-dB couplers Z1 and Z2.
The working point setting is factory
implemented by means of potentiometers
R9, R11, and R12 and should not be
altered.
4.7.4 (A4) Coupler Board Assembly
(1227-1316; Appendix B)
The UHF coupler board assembly provides
forward and reflected power samples of the
output to (A5) the amplifier control board
where it connects to the input of the
overdrive-protection circuit.
The RF input to the UHF coupler assembly,
from the 40 Watt UHF amplifier module,
connects to SMA jack J1. The RF is
connected by a stripline track to the SMA
type connector RF Output jack J2. A
hybrid-coupler circuit picks off a power
sample that is connected to SMA type
connector jack J3 as the forward power
sample. Another power sample is taken
from the coupler circuit that is connected
to SMA type connector jack J6 as the
reflected power sample. Two 50O
terminations, used as dissipation loads,
connect to the reject and reflected ports,
J5 and J4, of the coupler.
4.7.5 (A5) Amplifier Control Board
(1303682 or 1301962; Appendix B)
The amplifier control board provides LED
fault and enable indications on the front
panel of the module and also performs the