Tektronix 1S1 Instruction Manual page 30

tions. Use only the VERT POSITION control for positioning
the display vertically on the crt screen.
Use of Smoothing
Time and amplitude noise may sometimes be objectionable
when operating at minimum deflection factors or maximum
sweep rates. The SMOOTHING control may be used to re-
duce random noise, when necessary, by decreasing the gain
of the sampling fedback loop. Fig. 2-13 shows the advan-
tage of using smoothing when observing a low-amplitude
signal.
Fig. 2-13. Use of the SMOOTHING control for decreasing
display noise when viewing a low-amplitude signal. Displayed
waveform is a 5-mv 2-nsec pulse. (A) SMOOTHING control at
NORM; (B) SMOOTHING control counterclockwise.
Normally the SMOOTHING control will not significantly
affect the risetime of the display if the dot density is
sufficient. If, however, the waveform shape is affected when
the SMOOTHING control is adjusted, a compromise must be
made between smoothing and dot density (see "Selecting
Display Dot Density" above). Fig. 2-14 illustrates the effect
produced by using smoothing when the dot density is too
Iow.
"False" Displays
Due to the nature of the sampling display, it is sometimes
possible to obtain an apparent waveform on the crt screen
that is not a true representation of the input signal, with
Operating Instructions – Type 1S1
Fig. 2-14. Typical waveform illustrating the use of smoothing
in conjunction with a low dot density. The waveform is the
same 5-mv 2-nsec pulse shown in Fig. 2-13. The distortion can
be removed by either increasing the dot density or decreasing
the amount of smoothing.
respect to equivalent time, if the sampling rate is very
close to a sub-multiple of the signal frequency. This type of
display appears as a waveform of a much lower frequency
than the input signal and is caused by sampling at such a
slow rate that the samples are taken on widely-separated
portions of the signal. Each sample represents the correct
amplitude at the instant of sampling, but not enough samples
are taken to trace out the correct waveform.
In the Type 1S1, this type of display is rarely obtained
because of the continuously variable selection of sampling
dot density with the SAMPLES/CM control. A false display
can be detected by merely changing the dot density. The
timing of the waveform should not change. If the display is
found to be false, increase the sweep with the TIME/CM
switch until the individual cycles of the waveform are dis-
played and the apparent timing does not change with dis-
play dot density (see Fig. 2-15).
Single Sweep
A single-sweep presentation of the crt display may be
obtained with the DISPLAY MODE switch set to SINGLE
SWEEP position. This feature may be used for viewing or
photographing a waveform on a slow-moving sweep or a
waveform that changes shape over a relatively short period
of time.
To display a single sweep of the crt:
1. Set up the triggering controls with the DISPLAY MODE
switch set to NORMAL.
2. Set the DISPLAY MODE switch to SINGLE SWEEP. The
sweep will be held off following the completion of the cur-
rent sweep.
3. Press the START button. The sweep circuit is then armed
and the sweep will occur immediately if the triggering
information is still arriving.
4. Release the START button. This allows the single-sweep
circuit to be reset. (The START button must be released to
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