AR 10047590, Operating And Service Manual

Page: 41 / 54

Rev A 25
3. THEORY OF OPERATION
3.1 INTRODUCTION
The Model 125S1G2z5 RF amplifier consists of a 1.0–2.5 GHz RF amplifier assembly. The RF amplifier
assembly consists of a Pre-Amplifier (Pre-Amp), a Driver Quad Amplifier (Driver Amp), a two-way splitter,
(2) S1G2z5 quad amplifiers, and a 2-way combiner.
The power supply section consists of an AC input filter with a circuit breaker, two switching power supplies,
and a regulator circuit.
The control system consists of a Control/Fault Board, an Interface Board and remote interfaces for IEEE-488,
RS-232, USB, and Ethernet.
3.2 RF AMPLIFIER OPERATION
3.2.1 A1 Variable Gain Amplifier (Schematics 10042929, 10044992)
The A1 variable gain amplifier consists of 2 subassemblies: the A1 Pre-Amplifier PWB Assembly and the A2
Three-Watt Amplifier Assembly.
3.2.1.1 A1 Pre-Amplifier PWB Assembly (Schematic 10042929)
The Pre-Amplifier PWB Assembly consists of a stage of gain (U1), a variable attenuator (U2), another stage
of gain (U3) a high isolation RF switch (U7) and control circuitry. The overall gain of the pre-amplifier
assembly is approximately 20 dB at minimum attenuation. The high isolation RF switch (U7) is used to
inhibit the RF input signal.
3.2.1.2 A2 Three-Watt Amplifier (10044992)
The Three-Watt Amplifier has a gain of approximately 16 to 18 dB. The Three-Watt Amplifier increases the
overall variable gain amplifier to approximately 36 to 38 dB of gain. The output RF power is approximately
33 dBm at the 1 dB compression point
3.2.2 A2 Driver, A6 and A7 Final S1G2z5 Quadrature-Coupled
Amplifier (Schematics 10032938, 10033124)
The 1-2.5 GHz quad amplifier consists mainly of two separate PWB circuit assemblies, the motherboard
10033123, and a bias board 10032937. The motherboard contains all the RF traces (with the exception of the
through lines connected to the external SMA connectors) as well as the bias sequencing circuitry. Each RF
FET (Q1 and Q2) has an associated bias circuit and fault circuitry contained on a single bias board.
On the motherboard, IC U3 is a voltage comparator used to monitor the negative VGG supply. Q3 turns off
the +28 VDC supply to the bias boards (VDDA) when VGG becomes less negative than approximately
-7.2V.