Measurement concepts
Measurement variables
By knowing how the instrument makes calculations, you may better understand how to use your instrument and how to interpret
your results. The instrument uses a variety of variables in its calculations. These include:
Definition of Base and Top
Base is the value used as the 0% level in measurements such as fall time and rise time.
Top is the value used as the 100% level in measurements such as fall time and rise time. For example, if you set the 10% to
90% rise time, then the instrument calculates 10% and 90% as percentages of Top and Base, with Top representing 100%.
The exact value of Base and Top depends on which Base Top Method you select in the Reference Levels panel of a
Measurement configuration menu. It also depends on if you set the reference level to be Global (applies to all measurements set
as Global in the Reference Levels panel), or Local (just applies to the measurement that is set to Local).
Base, Top calculation methods
The Base Top calculation method is set in the Reference Levels panel of a Measurement configuration menu.
Auto is the default method, and automatically determines the best Base Top method to use. Most commonly sets the Base Top
method to Histogram Mode.
MinMax defines the 0% and the 100% waveform levels as the lowest value and the highest value samples of the waveform
record. This setting is best for examining waveforms that have no large, flat portions at a common value, such as sine waves and
triangle waves - almost any waveform except for pulses.
The MinMax method calculates the Top and Base values as follows:
Top = Max
and
Base = Min
Histogram Mean uses histogram analysis to calculate the mean or average value using all values above and below the
waveform midpoint. Top is set to the mean high value, and Base is set to the mean low value. This setting is best for examining
eye patterns and optical signals.
Histogram Mode uses histogram analysis to select the most common values either above or below the midpoint. Since this
statistical approach ignores short-term aberrations (overshoot, ringing, and so on), Mode is the best setting for examining pulses.
The oscilloscope calculates the histogram-based Top and Base values as follows:
MSO54, MSO56, MSO58, MSO58LP, MSO64 Help 429
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