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Theory of Operation
The Oscilloscope Board
Time Base The time base provides the sample clocks and timing necessary for data
acquisition. It consists of the 100 MHz reference oscillator and time base hybrid.
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 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.
DC Cal The DC Cal output, a rear panel signal, is used for self-calibration. It is one
output from the 16-channel DAC.
Digital Interface The Digital Interface provides control and interface between the
system control and digital functions in the acquisition circuitry.
Analog Interface The Analog Interface provides control of analog functions in the
acquisition circuitry. It is primarily a 16-channel DAC with an accurate reference and
filters on the outputs. It controls channel offsets and trigger levels, and provides the DC
Cal output.
8–12
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