In order to digitize analog waveforms there are two common methods used in digital storage oscilloscope.
Real-Time Sampling (RTS)
Equivalent-Time Sampling (ETS)
The first method samples the measured signal in real-time. During this method, the oscilloscopes acquires the signal within one trigger event. The waveform is basically captured at a defined sample rate and can be immediately displayed after a minimum of samples is available. This mode can be used to measure repetitive as well as non-repetitive signals.
The ETS method requires more than one trigger event in order to fully restore a captured waveform . ETS enables a digital oscilloscope to capture signals with a much higher effective sample rate than the actual sample rate of the built-in ADCs. Subsequently, the oscilloscope will then be able to measure and display signals with a higher frequency.
The disadvantages of this method is the fact that the measured signal has to be a periodic signal. This means that the shape and amplitude of the waveform has to repeat itself at a certain point in time. And these repetitions have to continue until the oscilloscope is able to correctly reconstruct and display the measured waveform. Signals measured by the real-time sampling method does not have this restriction. However, most signals generated by man-made machines are indeed repetitive waveforms . Another restriction is that during ETS, it is not possible to take single shots of non-repetitive events like glitches or spikes. Figure 1 shows a non-repetitive signal captured with an oscilloscope running in ETS mode.
A similar effect can be observed using the OpenLab oscilloscope. For example, if the oscilloscope captures in ETS mode and the user disables the triggering, the display will get distorted. The result can be seen in figure 2.
The reason for this is similar to the effect described by the Tektronix article . ETS depends very much on repetitive trigger events. The acquisition will not work probably if the limitations of ETS are ignored. ETS acquires signals by taking samples across multiple trigger events. This scans the signal step by step and enables an accurate representation of the captured waveform. This process is illustrated in figure 3.
By accumulating the taken samples of the periodic waveform, the original signal can be reconstructed and displayed on the oscilloscope screen. The first sample is taken at time t. The next sample will be taken at the same time t but delayed by a constant value delta t. The upcoming sample will be taken at t + (2 * delta t) and so on. This method needs a reference point which synchronises the time when the ADC has to capture a sample. This reference point is, depended on the implementation of the ETS mode, given by the trigger system or an internal clock.
A detailed description of how the ETS mode works is given by the upcoming sub-chapters. ETS can be implemented in several ways each with their own advantages and disadvantages. This wiki will introduce the two most common ETS methods:
TEKTRONIX: Real-Time Versus Equivalent-Time Sampling. [Online] tek.com
TEKTRONIX: XYZs of Oscilloscopes. Primer, Tektronix, January 2016.
KOSTA ILIC, E. M.: Equivalent Time Sampling for High-Speed Repetitive Signals Using E Series Boards and NI-DAQ Software. Techn. Rep., National Instruments, June 1995.