AR 10044831, Operating And Service Manual

Page: 41 / 52

Rev C 23
3. THEORY OF OPERATION
3.1 INTRODUCTION
The Model 175A400 RF amplifier consists of a .01-400 MHz RF amplifier assembly. The RF amplifier
assembly consists of a Pre-Amplifier (Pre-Amp) and two final amplifier modules.
The power supply section consists of an AC input filter, a circuit breaker, +5V, -15V, +24V and two +40V
power supplies, an Operate/Control circuit, and an interface board.
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 Assembly (Schematic No. 10041189,
10041352, 10043496)
The Pre-Amplifier Assembly consists of 3 sub-assemblies: the A1 Pre-Amplifier Assembly, the A2 Switch
Assembly, and the A3 Output Amplifier Assembly.
3.2.1.1 A1 Pre-Amplifier Assembly (Schematic 1001189)
The Pre-Amplifier PWB Assembly consists of a variable attenuator circuit (Q1-Q4), a gain stage (U4), a
resistive splitter (R13, R14, R16), an RF power detector (U6), and associated control circuitry. The overall
gain of the pre-amplifier assembly is approximately 22 dB at minimum attenuation. The power detector (U6)
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.
3.2.1.2 A2 Switch Assembly (Schematic 10041352)
The Switch Assembly is made up of a variable attenuator circuit (Q1-Q4) and associated control circuitry.
The switch can either be in an on or off state depending on the Inhibit input signal (E4). In the off state, when
E4 is pulled low, the switch will reduce the amplifier gain by approximately 40 dB.
3.2.1.3 A3 Output Amplifier Assembly (Schematic 10043496)
The Output Amplifier Assembly has a single gain stage (U1 ) that has a gain of approximately 15 dB.
3.2.2 A2 Two-Way Splitter
The Two-Way Splitter is a broadband splitter. The input signal is split into four equal-amplitude, equal-phase
signals. The amplitude of each signal is 3-3.5 dB below the input signal when all outputs are terminated into
50
Ω
loads.