Application Notes
50
6. If the signal is Type 0, (No crossings detected) no measurements can be performed and the routine is terminated,
pending the next reload of the data buffers.
7. The process locates the bottom amplitude (baseline) using the IEEE histogram method. A histogram is generated
for all samples in the lowest 12.8 dB range of sample values. The range is subdivided into 64 power levels of 0.2
dB each. The histogram is scanned to locate the power level with the maximum number of crossings. This level is
designated the baseline amplitude. If two or more power value have equal counts, the lowest is selected.
8. The process follows a similar procedure to locate the top amplitude (top line). The power range for the top histogram
is 5 dB and the resolution is 0.02 dB, resulting in 250 levels. The level-crossing histogram is computed for a single
pulse, using the samples which exceed the transition threshold. If only one transition exists in the buffer (Types 2
and 3), the process uses the samples that lie between the edge of the screen and the transition threshold (see figure
). For a level to be designated the top amplitude, the number of crossings of that level must be at least 1/16 the
number of pixels in the pulse width; otherwise, the peak value is designated the top amplitude.
Figure 5.6
Time Interpolation
9. The process establishes the proximal, mesial, and distal levels as a percentage of the difference between top amplitude
and bottom amplitude power. The percentage can be calculated on a power or voltage basis. The proximal, mesial,
and distal threshold values are user settable from 1% to 99%, with the restriction that the proximal < mesial < distal.
Normally, these values will be set to 10%, 50%, and 90%, respectively.
10. The process determines horizontal position, in pixels, at which the signal crosses the mesial value. This is done to
a resolution of 0.1 pixel, or 1/5000 of the screen width. Ordinarily, the sample values do not fall precisely on the
mesial line, and it is necessary to interpolate between the two nearest samples to determine where the mesial crossing
occurred. This process is demonstrated in the example above (figure
):
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