2-7
IM 701730-01E
Explanation of Functions
2
Relationship between the Time Axis Setting and Sampling Mode
Depending on the time axis setting, you can switch the mode used to sample the input signal
(sampling mode). The time axis settings that allow the sampling mode to be changed vary
depending on the acquisition mode and other settings. For more details about this
relationship, see appendix 1, “Relationship between the Time Axis, Sample Rate, and Record
Length.”
•
Realtime Sampling Mode
By changing the time axis setting, the sample rate changes, allowing sampling of data at
up to 1 GS/s (or 500 MS/s when interleave mode is OFF; for a description of interleave
mode, see section 7.5). The input signal is sampled sequentially, and the data is stored
in the acquisition memory.
In this mode, the instrument can only display waveforms correctly up to one-half the
frequency of the sample rate (the number of samples per second, in units of S/s) as
defined by the sampling theorem.* Therefore, this mode is best suited for observing
waveforms that undergo slow changes relative to the sample rate.
* If the sample rate is comparatively low with respect to the input signal frequency, the
harmonics contained in the signal are lost. In this case, some of the harmonics will appear
at low frequencies due to the effects described by the Nyquist sampling theorem. This
phenomenon is called aliasing. You can prevent aliasing by acquiring waveforms with the
acquisition mode set to envelope.
Aliased signal
Input signal
Sampling point
•
Repetitive Sampling Mode
In repetitive sampling mode, you can set the time axis to a setting that will cause the
sample rate to exceed 1 GS/s (2 GS/s when interleave mode is ON). In this mode, one
waveform is created from several cycles of a repetitive signal. This is equivalent to
sampling the signal at a higher sample rate than the actual sample rate. The maximum
apparent sample rate is 100 GS/s on the instrument.
In addition, even in realtime sampling mode, if the relationship of the time axis and the
display record length would cause the sample rate to exceed 1 GS/s (or 2 GS/s when
interleave mode is ON), the mode automatically switches to repetitive sampling. There are
two types of repetitive sampling.
One is sequential sampling in which the data is sampled by intentionally offsetting the
sampling points by a certain time with respect to the trigger point. The other is random
sampling in which the data that is offset randomly from the trigger point is sampled and
resorted with respect to the trigger point. The instrument employs random sampling which
enables the waveform before the trigger point (trigger position, see section 2.3) to be
observed.
Time Axis Setting and Roll Mode Display
If T/div is set to a certain range (see appendix 1, “Relationship between the Time Axis,
Sample Rate, and Record Length), the waveforms are displayed in roll mode. In roll mode,
the displayed waveform is not updated using triggers (update mode). Rather, the oldest data
is deleted as new data is acquired, and the waveform is shifted from right to left on the
screen. Roll mode display allows waveforms to be observed in the same way as on a pen
recorder. It is useful in observing slowly repeating signals or signals that change slowly. It is
also useful in detecting glitches (spikes in the waveform) that occur intermittently.
*
Roll mode display is also used when the trigger mode is set to Single. However, the
displayed waveforms stop when a trigger is activated.
2.2 Vertical and Horizontal Axes
