Keysight N9912A Service Manual page 52

Theory of Operation
Measurement Group
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 FieldFox
User's Guide 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 SA input
signals. Since the NA-R and NA-A signals are sampled simultaneously, phase
relationship is preserved.
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.
5-12 Keysight N9912A Service Guide