Tektronix 2212 User Manual page 47

Digital Storage Displays
If the sine wave is only one graticule division in width, it will be
sampled one-tenth as many times (400 samples). This number is
still adequate for accurate reproduction of the stored waveform.
If the SEC/DIV switch is set so that the entire sine-wave period fills
one-tenth of a graticule division, it will be sampled only 40 times
during its acquisition. This means that only 40 samples of the wave-
form will be available to reproduce the waveform for display. In
theory, if a sine wave is sampled at least two times during its pe-
riod, it may be accurately reproduced. In practice, the sine wave
can be reconstructed, using special filters, from slightly more than
two samples.
At 20 µs per division, and a horizontal magnification of
XlO,
a sig-
nal of 2 MHz will be sampled 10 times during the sinewave period.
Consequently the waveform will be accurately reproduced within
95
%
of its true amplitude. This is the accuracy required for useful
storage bandwidth.
If the input frequency is increased beyond 8 MHz, the samples will
soon become less than two times per period. This occurs at 10 MHz
for a 20 MHz sample rate. Past this point, information sampled from
two different sine-wave periods will be used to reconstruct the
displayed waveform. This waveform will not be a correct reproduc-
tion of the input signal. At certain input frequencies, the data sam-
pled will reproduce what appears to be a correct display, when in
fact it is only related to the input signal by some multiple or part of
a multiple of the input signal. This type of display is one type of
"alias" (see Figure 2-2).
The sampling rate is controlled by the SEC/DIV switch, and it de-
creases when the SEC/DIV switch is set to slower settings. When-
ever the SEC/DIV switch is set so that the input signal is sampled
less than 10 times per period of the fastest frequency component,
observable aliases will occur.
2212 User Manual
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