Agilent Technologies Infiniium 80000 Series Service Manual page 155

Chapter 8: Theory of Operation
Acquisition Theory
Time Base
The time base provides the sample clocks and timing necessary for data acquisition. It primarily
consists of a reference oscillator, state machine, and trigger-time interpolator.
• The time base reference can be supplied by the on-board 10 MHz oscillator, or by an external
10 MHz reference signal.
• The state machine sequences through the stages of the acquisition. First it times the pre-
trigger delay, which guarantees that the required amount of data to the left of the trigger has
been captured. When this times out, it sends a signal (ARM) to the trigger multiplexer. The
next time the trigger condition is satisfied, the trigger multiplexer sends a signal (SYSTRIG)
back to the state machine. After receiving SYSTRIG, the state machine counts down the post-
trigger delay time, then stops the acquisition. The stored data now covers the requested time
window relative to the trigger. Finally, the state machine signals the CPU that the acquisition
is complete.
• The trigger-time interpolator measures the time between the trigger event (SYSTRIG) and the
next sample clock. The interpolator circuitry converts this time difference into a voltage and
digitizes it with an analog-to-digital converter (ADC). Using this value, the CPU precisely
positions the acquired samples relative to the trigger for plotting and making measurements.
Calibration
The Calibration circuit provides several signals to the Probe Compensation and Aux Out outputs.
Which signal is driven to the front panel depends on the current selection from the drop-down
menu in the Calibration dialog box. Available signals for Aux Out include a 715 Hz probe
compensation signal, a pulse representing the trigger event, the timebase clock, a DC voltage in
the range –2.5 to +2.5 V, or a high-speed edge used to calibrate the A/D converters. The DC
voltage is used for self-calibration, and is an output from a 16-channel DAC. The calibration
signals are sent to an analog multiplexer, which selects the signal that will be sent to the front
panel.
Microprocessor Interface
The Microprocessor Interface provides control and interface between the system control and
digital functions in the acquisition circuitry.
Analog Interface
The Analog Interface provides analog control of functions in the acquisition circuitry. It is
primarily DACs with accurate references and filtered outputs. The analog interface controls:
• Channel offsets
• Trigger levels
• Mixed Signal Trigger functions
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