Tektronix TDS 500C, TDS 600B, TDS 700C HighRef, MidRef, LowRef, Mid2Ref

Appendix B: Algorithms
B–2 TDS 500C, TDS 600B, & TDS 700C User Manual
Histogram Method — attempts to find the highest density of points above and
below the waveform midpoint. It attempts to ignore ringing and spikes when
determining the 0% and 100% levels. This method works well when measuring
square waves and pulse waveforms.
The oscilloscope calculates the histogram-based  and  values as
follows:
1. It makes a histogram of the record with one bin for each digitizing level (256
total).
2. It splits the histogram into two sections at the halfway point between 
and  (also called ).
3. The level with the most points in the upper histogram is the  value, and
the level with the most points in the lower histogram is the  value.
(Choose the levels where the histograms peak for  and )
If  gives the largest peak value within the upper or lower histogram, then
return the  value for both  and  (this is probably a very low
amplitude waveform).
If more than one histogram level (bin) has the maximum value, choose the
bin farthest from .
This algorithm does not work well for two-level waveforms with greater than
about 100% overshoot.
The user sets the various reference levels, through the Reference Level selection
of the Measure menu. They include:
HighRef — the waveform high reference level. Used in fall time and rise time
calculations. Typically set to 90%. You can set it from 0% to 100% or to a
voltage level.
MidRef — the waveform middle reference level. Typically set to 50%. You can
set it from 0% to 100% or to a voltage level.
HighRef, MidRef, LowRef,
Mid2Ref