DPO6000
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MPO6000 Series Digital Phosphor Oscilloscope Instruction V1.1
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3.To Set the Sample System
To understand the waveform acquisition system of oscilloscope, it is necessary to
understand the sampling principle first and find out the relationship between bandwidth,
sampling rate and storage depth.
3.1.Sampling principle
According to Nyquist sampling principle, in order to restore the analog signal without
distortion, the sampling frequency should be more than twice of the highest frequency in
the analog signal spectrum(Fs>2Fmax)The higher the sampling rate, the closer the
waveform recovered later is to the original signal, but the system requirements are higher,
the conversion circuit must have a faster conversion speed.
3.1.1.Waveform aliasing
If sample can’t meet (Fs>2Fmax), The waveform frequency is lower than the actual signal
frequency when the sampled data is reconstructed.The most common aliasing is jitter on
fast edges.
The following two measures can avoid aliasing:
1.Increase the sampling frequency to more than twice the maximum signal frequency;
2.Introducing a low pass filter or raising its parameters. The low - pass filter is usually
called anti - aliasing filter, Anti-aliasing filter can limit the bandwidth of the signal and make
it satisfy the condition of sampling theorem. This is possible in theory, but impossible in
practice. Because the filter cannot completely filter out signals above the Nyquist
frequency, so,there is always some "small" energy beyond the bandwidth required by the
sampling theorem. However anti-aliasing filters can make these energies small enough to
be negligible.
3.1.2.Waveform distortion
Due to the lack of some waveform details caused by too low sampling rate, the waveform
displayed by oscilloscope sampling is quite different from the actual waveform.
3.1.3.Waveform loss
Because the sampling rate is too low, the waveform during the reconstruction of the
sampled data does not reflect all the actual signals.
