UBS Axcera LU500AL User Manual

LX Series Power Amplifier Assembly Chapter 4, Circuit Descriptions

Chapter 4

Circuit Descriptions

The RF from the exciter/driver assembly
connects from the RF Output “N” Jack J25,
through a RG-55 cable, to the PA RF Input
SMA Jack J200, located on the rear of the
PA chassis assembly. The RF Input is
cabled through UT-141 to port J111 on
the main chassis. Jack J1 on the PA
module assembly connects to the J111
port when the module assembly is slid
into place.

The RF output from the 2 way UHF
combiner connects to the PA RF Output
Jack J2, located on the rear of the PA
Module. Jack J2 on the PA module
connects to the J115 port, on the main
chassis assembly, when the module
assembly is slid into place.

4.1 (A4) Power Amplifier Module
Assembly (1302868; Appendix B)

The Power Amplifier Module Assembly
contains (A1) a UHF Phase/Gain Board
(1303213), (A2) a 150W Driver Pallet,
Dual Output (1303293), (A3 & A4) UHF
RF Module Pallet Assemblies (1300116),
(A5) a 2 Way UHF Combiner Assembly
(1303208), (A6) an Amplifier Control
Board (1301962) and (A7) a
Temperature Sensor IC.

4.1.1 (A1) UHF Phase/Gain Board
(1303213; Appendix B)

The RF input from J1 on the PA assembly
connects to J1 on the Phase/Gain Board.
The UHF phase/gain board provides the
circuits that adjust the gain and the
phase of the RF signal for the PA
amplifier assembly in which it is
mounted. The input signal connects to
the gain circuit through the capacitor
C13. The gain circuit consists of U1,
R16, CR4, R22, R17, CR5, R23, R27 and
the gain pot, R25. U1 is a 90°, 2-way
splitter. The signal at pin 1 of U1 is split
and applied to pins 3 and 4. The signal
reflects off CR4 and CR5 and is passed to

pin 2. The gain between pins 1 and 2
changes with the voltage applied across
CR4 and CR5. This voltage is controlled by
the gain-adjust pot R25. The more
positive the voltage, the more the diodes
CR4 and CR5 conduct therefore the less
gain through the circuit. The gain
controlled output is coupled through C14
and the pi-type divider circuit consisting of
R8, R5 and R9 that drops the level before it
applied to the phase-shifter circuit.

The level controlled signal connects to the
phase-shifter circuit that consists of U2,
C20, C21, CR2, and CR3. U2 is a 90°, 2­way splitter. The signal at pin 1 of U2 is
split and applied to pins 3 and 4. The signal
reflects off CR2 and CR3 and is passed to
pin 2. The phase shift between pins 1 and
2 changes with the voltage applied across
CR2 and CR3. This voltage is controlled by
the phase-adjust pot R24 through R26,
R18 and R19. +12 VDC from an external
switching power supply is applied to J3 on
the board and is used as the reference that
is applied to the phase-control pot. The
gain and phase controlled output connects
to J2 on the board.

4.1.2 (A2) 150 Watt Driver Pallet
Assembly, Dual Output (1303293;
Appendix B)

The output of the Phase/Gain Board is
connected to the input J1 of (A2) the 150
Watt UHF amplifier assembly. The
assembly contains a 150 Watt CW UHF
Driver Board, Dual Output (1303169).

4.1.3 150 Watt Driver, Dual Output
(1303169; Appendix B)

The board 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 150 watts
(CW) with approximately 14 dB of gain.

LX Series, Rev. 0 4-1

LX Series Power Amplifier Assembly Chapter 4, Circuit Descriptions

The amplification circuit consists of
LDMOS transistors Q1 and Q2 connected
in parallel and operating class AB. The
paralleling network is achieved with the
aid of 3 dB couplers U3 and U4. The
quiescent current settings are achieved
by means of potentiometers R6 and
R10. C39 and C38 are adjusted for best
response. The settings are factory
implemented and should not be altered.

PIN diode VR1 is a variable-damping
circuit that is used to adjust the
amplification of the module. The
adjustment is performed with the Gain
potentiometers R10 and R6. A
readjustment of the amplification may
be required, after repair work, to ensure
that the PAs in multiple PA translators
deliver the same output power.

4.1.4 (A3 & A4) UHF Module
Assembly, RF Module Pallet, Philips
(1300116;Appendix B)

Z 3 Z 5

Matching

Network

RF

Input

3 dB Coupler

Z 1

R 1

R 10

R 9

R 11 R 12

The UHF Module Assembly, 250-watt
module (Figure 4-1) is a broadband
amplifier for the frequency range 470 to
860 MHz. The amplifier is capable of
delivering an output power of 70 W
The amplification is approximately 13 dB.

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.

V 1

Matching

Network

3 dB Coupler

+Uop

Z 2

R 2

RF
Output

Dynamic

N 1

rms

.

Matching

Network

Z 4 Z 6

V 2

Figure 4-1. UHF Amplifier Module, 250 Watts

4.1.5 (A5) 2 Way UHF Combiner
Assembly (1303208; Appendix B)

The 2 Way UHF combiner board assembly
combines the two outputs of the UHF
Module Assemblies and also provides
forward and reflected power samples of
the output to (A6) the amplifier control
board where it connects to the input of
the overdrive-protection circuit.

The RF inputs to the 2 way UHF combiner
assembly, from the UHF amplifier
modules, are soldered to the external
connection points J3 and J4. The RF is

combined by the stripline tracks and R5 a
100 ohm matching resistor across the two
inputs, to the RF Output solder connection
point at J5. A hybrid-coupler circuit picks
off a power sample that is connected to
SMA type connector jack J1 as the forward
power sample. Another power sample is
taken from the coupler circuit that is
connected to SMA type connector jack J2
as the reflected power sample. Two 50Ω
terminations, created from two 100 ohm
resistors in parallel, used as dissipation
loads, connect from the forward and
reflected ports to ground.

LX Series, Rev. 0 4-2

Matching

Network