Oscilloscope Block Diagram and Theory of Operation
A
4000 X-Series Oscilloscopes Advanced Training Guide
155
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 reconstruction filter, but it can also
“pipeline” the stored and/or processed data into the scope’s pixel display memory.
After the data has been “backed out” of acquisition memory, the DSP block then
signals the timebase block that it can begin another acquisition.
Note that early generations of DSOs did not include an explicit Display DSP block.
This function was traditionally handled by the scope’s CPU system, but with much
lower efficiency that produced much slower waveform update rates. With custom
display DSP processing, some of today’s DSOs can update waveforms as fast as
1,000,000 waveforms/second.
To learn more about the fundamentals of oscilloscopes, download Keysight’s application note
titled
Evaluating Oscilloscope Fundamentals
. This publication is listed in
on page 167 with instructions on how to download.
