Digital Storage Oscilloscope

Digital Storage Oscilloscope

We live in a digital world.  This revolution has been going on for the past two decades.  The digital audio Compact Disc was probably the first all-digital product introduced to the world market.   That was back in 1982.  Since then great advancements have been made in practically every electronic product on the face of planet earth!  The digital oscilloscope is no exception.  There are digital sampling oscilloscopes and digital storage oscilloscope.  They are used for purposes similar to the older analog scopes or in addition to them for investigating digital pulses and waveforms.

A Digital Storage Oscilloscope has the advantage of being able to store a measured signal or screen shot in digital form and hold it for later retrieval.

In fact, the great advantage of digitizing the signal under test is that it can be stored for very long periods of time and be brought up later on in its exact form that it was recorded in.  Well, almost exact.  Well, even exact in terms of digital information theory.  Ever heard of the Nyquist criteria?   It has, in this context, to do with the sampling frequency used to convert the incoming signal under test to its digital counterpart.  The Nyquist rule is that so long as the sampling frequency is at least twice the frequency of the signal being tested it will allow storing of digital codes that can allow for accurate reproduction compared to the original.  The entire world of “analog to digital conversion” or ADC converters rest on this theorem.

In reality, while the theory holds,  it is found that in practice, due to the imperfections in filters and electronic processing, it is more likely to find 3 or 4 or even up to 10 samples per wave period in order to get a more perfect representation of the original.  Hence, the sampling frequency becomes an important aspect of the precision of a sampling and data storage oscilloscope.

In the example above (which came from TiePie Engineering, thank you!) it shows three different levels of sampling for a given sine wave input.   What this means is that a more perfect sampling – the one on the right – which has about 10 samples per cycle – also requires storage of more data points.   Hence the storage requirement for the signal with a sampling frequency of 10x the test signal vs one with the Nyquist frequency is 5 greater.  But, digital storage media is getting less costly as the days go by.  This is due to Moore’s Law – a subject for another discussion on another day.

Other concerns for the sampling process is anti-aliasing.   This phenomenon occurs when the sample is taken by the ADC processor.  It produces a sum and difference output.  Hence when a signal of say 1000 Hz is sampled at the rate of 1500 Hz it can produce results that are not only 1000 Hz and 1500 Hz but also 2500 Hz and 500 Hz.   These aliasing products must be dealt with in the filters and electronic processing so as to ignore the aliasing distortions.

The Digital Sampling Oscilloscope, a special case of the digital storage oscilloscope, must have all the other features such as time-base sweep for the viewing panel and input voltage adjustments to size the signal to match the internal amplification characteristics.   Thus, a sampling oscilloscope may look very much like a conventional scope on the outside but the inside is quite different and its user will find it a little more complex to use but of great value in being able to store the sampled information.

Here is what the block diagram of the main parts of a digital storage oscilloscope might look like

The above is compliments of the Electrical Engineering Department of the University of Pennsylvania.  Thank you.  The digitizer and clock are the components that sample the test signal.   The number of samples taken by the digitizer is controlled by the clock or sampling frequency.  The details of the actual circuits used is well beyond the scope of this shortened dissertation!

Below is what an actual, commercial sampling and storage oscilloscope might look like.

This picture is compliments of and represents a small size nearly portable type instrument which is self-contained.

Of course, digital storage and sampling oscilloscopes may come in the form of a self-contained product as shown above, or as a USB based instrument that interfaces with a pc, or any other arrangement where there is an interface to some other display or recording device.

Most oscilloscopes in the marketplace today are utilizing digital processing in some way or another.  They are often designed for a specific technical target area and meet the needs of a special group or group of users.  As a result there are a very large number of brands and models.   For one who must select the  instrument of use for the activity anticipated it becomes a daunting task.  You can study other postings by this author for insight into these matters.

Thank you for reading this article on Digital Storage Oscilloscope. If you have any questions about the digital storage oscilloscope please ask in the comment box below!