Oscilloscope Block Diagram and Theory of Operation
A
3000 X-Series Oscilloscopes Advanced Training Guide
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timebase setting of 1 ms/div. Let also assume for simplicity that the
scope’s acquisition memory depth is just 1000 points. Using these
assumptions, the scope should acquire 500 points before the trigger event
followed by acquiring 500 points after the trigger event. At this timebase
setting, the scope will acquire 1000 points across a 10 ms time span
(1 ms/div x 10 divisions). Even though the scope’s maximum specified
sample rate may be 2 GSa/s, at this timebase setting the timebase block
will reduce the scope’s continuous sample rate to 100 k samples/sec
(Sample Rate = Memory/Time- span = 1000 samples/10ms = 100 kSa/s).
When the Run key is pressed, the timebase block enables continuous
storing of digitized data into the scope’s “circular” acquisition memory at
the appropriate sample rate (100 kSa/s). While the timebase block
increments addressing of the circular acquisition memory buffer after each
sample, it also counts the number of samples taken up to 500 (assuming a
memory depth of 1000 and also assuming triggering at center screen).
After the timebase block determines that a minimum of 500 samples have
been stored (meaning that at least half of the acquisition memory is full),
the timebase block then enables triggering and begins to look for the first
rising edge of the output trigger comparator (assuming simple edge
triggering mode). While looking for the trigger event, acquisitions continue
to be stored in the scope’s circular acquisition memory buffer. If the
trigger event is very infrequent, stored samples can actually be overwritten
while waiting on the trigger event. But this is okay. Once the trigger event
is detected, the timebase block then begins counting up to 500 again.
When an additional 500 samples have been stored, the timebase block then
disables (shuts down) sampling. This means that the last 500 stored
samples represent sequential points on the waveform that occurred
after
the trigger event, while the previous 500 points represent sequential points
on the waveform that occurred
before
the trigger event. At this point,
operation is turned over to the Display DSP block.
Although we used an example of triggering at center- screen, using the
horizontal delay/position control you can position the trigger point at any
point. For example, if you adjust the delay such that the trigger point
should occur at the 75% point along the horizontal axis (relative to the left
side of the screen), then the timebase block would set the counter to
initially store 750 points (assuming a memory depth of 1000 points) before
enabling the trigger, and then capture an additional 250 points after
detection of the trigger event.
Display DSP Block
Once an acquisition is completed, the Display DSP block then backs out
the stored data from the acquisition memory block in a last- in- first- out
sequence. Not only can the Display DSP block quickly perform digital
signal processing on the stored data such as running a Sin(x)/x digital