Keysight Technologies 3458A Guide D'utilisation page 209

Sub-Sampling
In sub-sampling (also known as sequential-sampling), the multimeter takes one
or more samples on each period of the input signal. With each successive period,
the beginning sample point is delayed further-and more samples are taken. After a
number of periods have occurred and the specified number of samples have been
taken, the samples can be reconstructed to form a composite waveform with a
period equal to that of the input signal.
The advantage of sub-sampling is that samples can be effectively spaced at a
minimum interval of 10 ns versus 10 µs for DCV digitizing and 20 µs for
direct-sampling. This means that sub-sampling can be used to digitize signals
with frequency components up to 12 MHz (the upper bandwidth of the signal
path for sub-sampling). Sub-sampled measurements use the track-and-hold
circuit, which has a 2 nanosecond aperture. Sub-sampling (and direct-sampling)
have less trigger jitter than DCV digitizing (see the
page 409). The disadvantages of sub-sampling are that the input signal must be
periodic (repetitive) and sub-sampling is not a real-time measurement.
You specify sub-sampling using the SSAC or SSDC command. The SSAC
command selects AC-coupled sub-sampling which digitizes only the AC
component of the input signal. The SSDC command selects DC-coupled
sub-sampling, which digitizes the combined AC and DC components of the signal.
Sub-sampling fundamentals
In sub-sampling, the samples in the composite waveform can be spaced very
closely together. This means that the interval between samples in the composite
waveform (effective_interval) can be much smaller (and the effective sampling
rate much greater) than in the DCV or direct-sampling methods. For example,
assume you need to digitize a repetitive 10 kHz input signal with a 5 µs
effective_interval between samples. This is a sampling rate of 1/5E-6 or 200,000
samples per second, (This application would be impossible using DCV or
direct-sampling since their maximum sampling rates are 100,000 and 50,0000
samples per second, respectively.)
using sub-sampling. The effective_interval is specified as 5 µs and specified
number of samples is 20. The effective_interval and the total number of samples
are specified by the SWEEP command. After specifying the effective_interval and
the number of samples, the multimeter calculates how many bursts (a burst is a
group of samples) it needs to make and how many samples will be in each burst.
Keysight 3458A User's Guide
"Appendix A: Specifications"
Figure 5-10 illustrates how this can be done
Digitizing 5
on
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