Tektronix 2230, Service Manual

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General Information— 2230 Service
Finite resolution affects any measurement using
discrete numbers. All digital storage stores amplitude
values as discrete numbers and associates those ampli­
tude numbers with discretely numbered times. Many mea­
surements must be rounded or truncated. The size of the
truncation or rounding becomes a part of the measure­
ment error. For example, the following line is 1.5 units
long. If it must be drawn as a line connecting points one
unit apart, then it may be drawn as a line one unit long or
two units long, depending on how it occurs relative to the
points.
Case 1: Line approaches three points:
___________ Input line
Measurement resolution
________________ Output line
Case 2: Line approaches two points:
__________ Input line
. . . Measurement resolution
________ Output line
There are several places where measurements are
quantified, and a one-count error in the measurement can­
not be detected. The input channels are digitized to an 8-
bit resolution, where one division is (ignoring expansion
and compression) 25 counts. This means there is an
inherent error of 1/25 of a division in any voltage measure­
ment at acquisition time. Averaging can increase the reso­
lution of a voltage measurement above the sampler's
eight-bit limit. To use the increased resolution, the display
has a 10-bit dynamic range in the vertical axis, as well as
the horizontal axis. An averaged signal has a resolution of
100 points per division (ignoring expansion and compres­
sion). In addition, the averaged number is stored with up
to twelve bits of resolution. Expansion is required to view
the eleventh and twelfth bits of increased resolution.
Time is quantified to determine when each sample
occurred and which display interval gets each sample.
Time is resolved by storing, for example, 4K points. If 4K
points are stored, 4K time intervals are represented. How­
ever, in 4K mode, not ail of the 4K-point resolution may be
displayed on the 10-bit (IK-point) screen. Therefore, if 4K
COMPRESS is selected to present the whole picture on­
screen at once, only IK resolution remains in the display.
When peak-detected information is acquired, events with
high-frequency content such as fast steps, or short pulses,
can only be located within the time interval from which the
peaks came. Even though two display points result from
the interval, the event cannot be tied with certainty to the
first or second point in the interval.
Time is also quantified to determine where to put points
in REPETITIVE; acquisitions, where the points acquired at
50 ns intervals fill only part of the screen. A counting
device producer a number to represent the portion of
50 ns between the samples acquired and the ones that
would have included the trigger. This number ranges from
0 to about 205, which allows accurate placement into the
display record. The display record will have at most 100
slots to choose from on the basis of the 0-205 number
(this is where each slot represents 0.5 ns of acquisition
time, and the counter’s resolution is about 0.244 ns per
count).
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