Agilent Technologies FieldFox N9923AN Service Manual page 52

Theory of Operation RF Vector Network Analyzer
Measurement Group N9923AN
The System Monitor controls the on/off functions of the analog power supplies. All are on
or off together except for power sequencing.
This circuitry also monitors the Ext DC input and, if below 10.8 volts or above 21 volts, it
disconnects this input from the Unreg DC.
There are two DC power supply lines: (1) Ext DC, and (2) Batt +. Each has a 5 amp fuse
immediately after the connector on the printed circuit board.
FPGA
This FPGA:
• provides the interface between the A6 SOM board and the ADC,
• provides miscellaneous interfaces for circuitry on the A4 RF board, and
• communicates with the CPU via the USB interface.
Time Base
The Time Base is an internal TCXO that produces 30 MHz signals for use throughout the
analyzer.
A 10 MHz signal at the EXT TRIG/EXT REF input may be used to provide and external
time base. The external time base function is controlled via the user interface. Refer to the
"External Frequency Reference" section of the N9923AN FieldFox RF Vector Network
Analyzer User's Guide (Part Number N9923-90006) for additional information.
There is no external trigger (EXT TRIG) function at the time of this printing.
Filters, MUX, and ADC's
The three Filters filter the IF signals in preparation for the ADC. Filter characteristics are
fixed (not programmable).
The ADC's digitize each of the IF signals received from the A4 RF board. The input to one
of the ADC's is multiplexed between the NA-A and NA-B input signals.
The 14-bit ADC's sample at a rate of 30 mega samples per second.
Each ADC may be put into a low power mode when not active.
Temperature Sensors
The SB1 Temp Sensor is located on the A4 RF board side of the A5 system board near the
headphone jack (J3).
The SB2 Temp Sensor is on the A4 RF board side of the A5 system board near the edge of
the board against the battery compartment. This temperature sensor is not under a shield.
Audio Output Jack
When headphones are plugged into the audio output jack, the internal speaker is disabled.
LAN Connector
The LAN connector supports the LAN interface on the A6 SOM board. See "LAN Interface"
on page 5-25 for more details.
5-12 Service Guide N9923-90005