Rev A
43
3. THEORY OF OPERATION
3.1 INTRODUCTION
The Model 2500A225C RF amplifier consists of a .01-225 MHz RF amplifier assembly. The RF amplifier
assembly consists of a pre-amplifier/driver module, and a two-way splitter that feeds (2) four-way splitters to
drive (8) final amplifier modules. These 8 final amplifier modules are combined through (1) eight-way
combiner to yield a combined power of 2500 watts or more.
The power supply section consists of circuit breaker CB1, line filter FL1, Relay K2, (+5, +15, -15 volt) triple
output power supply PS1, +34V PS2 for the RF preamp/driver module, +24V PS3 for the cooling fans, and
three +47V power supplies (PS4 through PS6) for supplying the final RF power modules.
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 Pre-Amplifier/Driver Assembly
The Pre-Amplifier/Driver Assembly consists of a variable attenuator circuit, three gain stages, a resistive
splitter, an RF power detector, and associated control circuitry. The overall gain of the pre-amplifier assembly
is approximately 49 dB at minimum attenuation. The power detector is used to sense the input RF power and
it increases the attenuation of the variable attenuator circuit if an input overdrive condition is detected.
3.2.2
A2 Two-Way Splitter
The Two-Way Splitter is a broadband splitter. The input signal is split into two equal-amplitude, equal-phase
signals. The amplitude of each signal is 3-3.5dB below the input signal when all outputs are terminated into
50
Ω
loads.
3.2.3
A3-A4 Four-Way Splitter
The Four-Way Splitter is a 180
°
broadband splitter. The input signal is split into four equal-amplitude signals.
The amplitude of each signal is 6-6.5dB below the input signal when all outputs are terminated into 50
Ω
loads.
3.2.4
A5-A12 Final Module Assembly
The Final Module Assembly consists of RF matching circuits, an RF transistor (Q1), a bias control circuit, and
a fault detection circuit.
The RF input is fed to a 4:1 balun transformer composed of T1 and T2. The output signal of the 4:1 transformer
is connected to the gates of Q1. The drains of Q1 are connected to a 4:1 balun transformer composed of T3 and
T4. Q1 has approxi47 VDC applied to both drains at a total current of 17A. The RF stage has
approximately 19 dB of gain and an output compression point of 250W or greater.
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