Appendix B: Algorithms
TDS 684A, TDS 744A, & TDS 784A User Manual
B–3
The oscilloscope also measures several values itself that it uses to help calculate
measurements.
RecordLength — is the number of data points in the time base. You set it with
the Horizontal menu Record Length item.
Start — is the location of the start of the measurement zone (X-value). It is 0.0
samples unless you are making a gated measurement. When you use gated
measurements, it is the location of the left vertical cursor.
End — is the location of the end of the measurement zone (X-value). It is
(RecordLength – 1.0) samples unless you are making a gated measurement.
When you use gated measurements, it is the location of the right vertical cursor.
Hysteresis — The hysteresis band is 10% of the waveform amplitude. It is used
in MCross1, MCross2, and MCross3 calculations.
For example, once a crossing has been measured in a negative direction, the
waveform data must fall below 10% of the amplitude from the MidRef point
before the measurement system is armed and ready for a positive crossing.
Similarly, after a positive MidRef crossing, waveform data must go above 10%
of the amplitude before a negative crossing can be measured. Hysteresis is useful
when you are measuring noisy signals, because it allows the oscilloscope to
ignore minor fluctuations in the signal.
MCross1, MCross2, and MCross3 — refer to the first, second, and third MidRef
cross times, respectively. (See Figure B–1.)
The polarity of the crossings does not matter for these variables, but the
crossings alternate in polarity; that is, MCross1 could be a positive or negative
crossing, but if MCross1 is a positive crossing, MCross2 will be a negative
crossing.
The oscilloscope calculates these values as follows:
1. Find the first MidRefCrossing in the waveform record or the gated region.
This is MCross1.
2. Continuing from MCross1, find the next MidRefCrossing in the waveform
record (or the gated region) of the opposite polarity of MCross1. This is
MCross2.
3. Continuing from MCross2, find the next MidRefCrossing in the waveform
record (or the gated region) of the same polarity as MCross1. This is
MCross3.
Other Variables
MCross Calculations
