Sampler Extended Real Time Oscilloscope (SXRTO)

DSO Fundamentals

In a Digital Storage Oscilloscope (DSO), waveforms are sampled (digitized) as analogue signals. Sampling is achieved by capturing a portion of the input waveform which is converted to a digital representation of that signal, which is then stored in memory. The number of bits of sampling resolution determines the voltage or vertical accuracy and the number of samples taken of the input waveform determine the timing or horizontal accuracy. 

Real-Time Sampling

Real-time sampling is where the oscilloscope samples the whole waveform in a single acquisition. This makes this sampling mode ideal for capturing single-shot transient events and non-repetitive waveforms. If the sample rate is not high enough, then the high frequency components of the signal will be incorrectly represented as indistinguishable artefacts known as aliasing. Nyquist states that the sample rate should be at least twice that of the highest frequency component of the signal. This means that for an oscilloscope with a real-time sample rate of 500 MS/s, the highest frequency component that can be captured accurately is 250 MHz. Or to put it another way, a 5 GHz bandwidth oscilloscope would need a minimum real-time sample rate of 10 GS/s to represent the highest frequency component without aliasing.

Equivalent-Time Sampling

Equivalent-time sampling takes advantage of the reality that most high-bandwidth waveforms are repetitive. This means that samples can be made over multiple acquisitions of the waveform with one or more samples being taken during each acquisition. This allows the oscilloscope to make acquisitions of waveforms, which contain frequency components many times higher than that of the real-time sample rate of the oscilloscope. In addition, the lower sample rate used means that equivalent-time sampling will typically offer higher vertical resolution than the extremely high real-time sample rates used at such frequencies.

SXRTO operation

At low timebase speeds an SXRTO makes many samples per trigger and behaves as a real-time digitiser that acquires the whole waveform from a single trigger.

Conversely as the timebase speed is increased the SXRTO switches to Equivalent-Time Sampling mode. Successive samples are made irrespective of the trigger position and are displayed relative to the trigger position. Although these samples are successive, they are made randomly with respect to the trigger position, hence the name.

As the samples are acquired randomly with respect to the trigger position, it is possible to display the sampled waveform prior to and including the trigger position, which allows engineers to view the waveform leading up to the trigger condition. The ability to capture and view pre-trigger information is a valuable aid in fault analysis, allowing the design engineer to view the cause of a fault condition.

An SXRTO such as the PicoScope 9404-05, with 12-bit vertical/voltage resolution, 1 ps horizontal/timing resolution and 5 GHz bandwidth is ideal for making precision measurements of repetitive signals and eye diagrams without the cost of a typical high-performance DSO or the measurement compromises of a sampling oscilloscope.