Tektronix TDS 500C Manual De Usuario

Appendix B: Algorithms

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.