Agilent Technologies FieldFox N9923AN Service Manual page 49

RF Vector Network Analyzer Theory of Operation
N9923AN Measurement Group
The settings of the FINE ATTEN and COARSE ATTEN attenuators between the source
and the reflectometer are determined by the Hi/Lo power setting on the user interface.
Each reflectometer performs three functions:
• It passes the source output signal to either PORT 1 or PORT 2.
• It samples the source output to either PORT 1 or PORT 2.
• It samples the incoming signal at either PORT 1 or PORT 2.
The sampled signals are mixed with the LO signal to produce two 390 kHz IF signals,
NA-A and NA-B.
The reflectometer dissipates up to 1.5 watts when active.
PORT 1 and PORT 2 each has a DC isolation capacitor (0.056uF).
Power Management Circuitry
The circuitry on the A4 RF board has five special sections that may be powered down for
the purpose of power management. These sections may be selectively powered down (via
"load switching") when not required for the current operating mode of the analyzer.
The sections that may be selectively powered down are:
• Source—most of the source synthesizer circuitry
• LO—most of the LO synthesizer circuitry
• Reflectometer—put into a low power state
The load switching for the Source, and LO sections is accomplished on the A5 system board
via the Analog Power Supply Load Switching circuitry.
The load switching for the Reflectometer and High-Band Preamp is accomplished on the
A4 RF board.
Other Circuitry
The FPGA (Field Programmable Gate Array) communicates with the FPGA on the
A5 system board via Serial Bus [A]. It provides the digital control for the functions on the
A4 RF board. It also reads the VCO unlock signals.
Serial Bus [B] allows the System Monitor on the A5 system board to read a temperature
sensor and the ID EPROM for the A4 RF board.
RF1 temperature sensor on the side of the A4 RF board that is away from the A5 system
board. It is located near the edge of the A4 RF board that is farthest from the Port 1 and
Port 2 connectors.
Service Guide N9923-90005 5-9