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TDS 620A, 640A, & 644A User Manual
Appendix B: Algorithms
TDS 600A Digitizing Oscilloscopes can take 25 automatic measurements. By
knowing how they make these calculations, you may better understand how
to use your TDS 600A and how to interpret your results.
TDS 600A Digitizing Oscilloscopes use a variety of variables in their calcula-
tions. These include:
High, Low
is the value used as the 100% level in measurements such as fall time
and rise time. For example, if you request the 10% to 90% rise time, then the
oscilloscope will calculate 10% and 90% as percentages with represent-
ing 100%.
is the value used as the 0% level in measurements such as fall time and
rise time.
The exact meaning of and depends on which of two calculation
methods you choose from the High-Low Setup item of the Measure menu.
These are
Min-max
and
Histogram
.
Min-Max Method — defines the 0% and the 100% waveform levels as the
lowest amplitude (most negative) and the highest amplitude (most positive)
samples. The min-max method is useful for measuring frequency, width, and
period for many types of signals. Min-max is sensitive to waveform ringing
and spikes, however, and does not always measure accurately rise time, fall
time, overshoot, and undershoot.
The min-max method calculates the High and Low values as follows:
=
and
=
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).
Measurement
Variables