User`s guide

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Logic Analyzer Concepts
The Analyzer Hardware
The 100 MHz reference oscillator provides the base sample frequency.
The time base hybrid has programmable dividers to provide the rest of
the sample frequencies appropriate for the time range selected. The
time base uses the time-stretched output of the fine interpolator to
time-reference the sampling to the trigger point. The time base has
counters to control how much data is taken before (pre-trigger data)
and after (post-trigger data) the trigger event. After the desired
number of pre-trigger samples has occurred, the time base hybrid
sends a signal to the logic trigger (trigger arm) indicating it is ready for
the trigger event. When the trigger condition is satisfied, the logic
trigger sends a signal back to the time base hybrid. The time base
hybrid then starts the post-trigger delay counter.
When the countdown reaches zero, the sample clocks are stopped and
the CPU is signaled that the acquisition is complete. The fine
interpolator is a dual-slope integrator that acts as a time-interval
stretcher. When the logic trigger receives a signal that meets the
programmed triggering requirements, it signals the time base. The time
base then sends a pulse to the fine interpolator. The pulse is equal in
width to the time between the trigger and the next sample clock. The
fine interpolator stretches this time by a factor of approximately 500.
Meanwhile, the time base hybrid runs a counter with a clock derived
from the sample rate oscillator. When the interpolator indicates the
stretch is complete, the counter is stopped. The count represents, with
much higher accuracy, the time between the trigger and the first
sample clock. The count is stored and used to place the recently
acquired data in relationship with previous data.
AC/DC Cal. The AC Cal is a multiplexer circuit that can provide
several signals to the Probe Compensation/AC Calibrator output on the
rear panel. The signal provided depends on the mode of the
instrument. It can be either a probe compensation signal, a pulse
representing the trigger event, signals used for self-calibration, or the
100 MHz reference oscillator when sample period is 1 ns. The DC Cal
output, a rear panel signal, is used for self-calibration. It is one output
from the 16-channel DAC.