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What is the difference between an equivalent time sampling oscilloscope and a real-time oscilloscope? Application Note In the past, deciding between an equivalent time sampling oscilloscope and a real time oscilloscope was a matter of determining your bandwidth requirements; but w
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  What is the difference between an equivalent time sampling oscilloscope and a real-time oscilloscope? Application Note In the past, deciding between an equivalent time sampling oscilloscope and a real time oscilloscope was a matter of determining your bandwidth requirements; but with today’s high performance instruments that distinction isn’t as clear. This document will discuss how each type of scope samples the incoming waveform and explain the trigger requirements. A summary detailing the advantages of each scope is provided at the end.  2 Real-time oscilloscope A real-time oscilloscope, sometimes called a “single-shot” scope, captures an entire waveform on each trigger event. Put another way, this means that a large number of data points are captured in one continuous record. To better understand this type of data acquisition, imagine it as an extremely fast analog-to-digital converter (ADC) in which the sample rate determines the sample spacing and the memory depth determines the number of points that will be displayed. In order to capture any waveform, the ADC sampling rate needs to be significantly faster than the frequency of the incoming waveform. This sample rate, which can be as fast as 80 GSa/s, determines the bandwidth which currently extends to 63 GHz.The real-time scope can be triggered on a feature of the data itself, and often a trigger will occur when the incoming waveform’s amplitude reaches a certain threshold. It is at this point that the scope starts converting the analog waveform to digital data points at a rate asynchronous and very much unrelated to the input waveform’s data rate. That conversion rate, known as the sampling rate, is typically derived from an internal clock signal. The scope samples the amplitude of the input waveform, stores that value in memory, and continues to the next sample as illustrated in Figure 1. The trigger’s main job is to provide a horizontal time reference point for the incoming data. Input signalSample clockTrigger signalReconstructedwaveform t s t d Figure 1 – Waveform acquisition using a real-time oscilloscope Triggering the real-time oscilloscopeReal-time oscilloscope as an ADC  3 Equivalent time sampling oscilloscope An equivalent time sampling oscilloscope, sometimes simply called a “sampling scope,” measures only the instantaneous amplitude of the waveform at the sampling instant. In contrast to the real-time scope, the input signal is only sampled once per trigger. The next time the scope is triggered, a small delay is added and another sample is taken. The intended number of samples determines the resulting number of cycles needed to reproduce the waveform. The measurement bandwidth is determined by the frequency response of the sampler which currently can extend beyond 80 GHz.The triggering and subsequent sampling of an equivalent time sampling oscilloscope is different from a real-time oscilloscope in some very tangible ways. Most importantly, the equivalent time sampling scope needs an explicit trigger in order to operate, and this trigger needs to be synchronous with the input data. Typically this trigger is provided by the user but, in some cases, the trigger can be obtained using a hardware clock recovery module. The sampling works as follows: A trigger event initiates the acquisition of the first sample, then the scope rearms and waits for another trigger event. The rearm time is approximately 25 µs. The next trigger event initiates the second acquisition and adds a precise incremental delay before sampling the second data point. This incremental delay time is determined by the time base setting and the number of sample points. This process, which is illustrated in Figure 2, is repeated until the entire waveform is acquired. Input Signal SequentialDelay   Trigger SamplingTrigger Level Re-Arm Time Trigger SignalReconstructedWaveform Figure 2 - Waveform acquisition using an equivalent time sampling oscilloscope One sample per cycleSampling methodology  4 There are two ways to trigger an equivalent time sampling oscilloscope and each results in a different data viewing format; either a bit stream or an eye diagram. Viewing the individual bits in the signal allows the user to see the pattern dependencies in the system, but does not allow for high resolution with large numbers of bits. In order to view a bit stream, the trigger must only pulse once during the period of the input pattern and must be at the same relative location in the bit pattern for each event. The input signal is then sampled and upon the next trigger event, the incremental delay is added and the bit steam is sampled until the entire waveform is acquired. In order to view a bit stream on an equivalent time scope, you must have a repetitive waveform; otherwise a real-time scope is needed. The triggering process to display a bit stream waveform is shown in Figure 3. Triggering the equivalent time sampling scope PRBSPatterntriggerReconstructedwaveform Sequential delay =Full screen sweep timeNumber of trace points Trigger pointSampling point 2 4  - 1 = 15 bits2 4  - 1 = 15 bitsRe-arm timeSequential delay Figure 3 - Sampling Process for a Bit Steam Pattern Waveform Equivalent time sampling oscilloscope (Continuation)
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