Mixed Signal Oscilloscope, MSO, Tutorial

- the mixed signal oscilloscope, MSO forms a good bridge between scope and logic analyser technology.

The mixed signal oscilloscope, MSO, provides additional functionality over some other forms of oscilloscope.

Essentially the MSO is a combination of an oscilloscope with a logic analyser, and as such it is able to provide functionality which is particularly useful in a number of applications.

MSO capabilities

The aim of the mixed signal oscilloscope is to provide the functionality of a digital oscilloscope along with that of a basic logic analyser.

The logic analyser performance is normally not that of the top-line analysers, but will normally include 16 or 32 channels and functions such as parallel / serial bus protocols decoding and triggering.

The MSO scope channels operate in the same way as a traditional scope - the instrument can be used purely as an oscilloscope if required. The MSO treats the oscilloscope and logic channels in different ways:

  • Oscilloscope channels:   These are full analogue channels that use a analogue to digital converter, ADC, to convert the analogue inputs into a digital format. This information is then processed so that the waveform is displayed showing the analogue aspects of the signal
  • Logic channels:   The logic channels on the mixed signal oscilloscope are converted straight into a digital format. The signals then appear on the screen as a high or low level, and no analogue information is present. There are more of these logic channels to enable full visibility of the logic on a circuit, but they are much easier to implement because they do not require full analogue to digital conversion. If the analogue aspects of a digital signal are required to be viewed, then this can be connected to one of the analogue channels.

One of the key advantages of the Mixed Signal Oscilloscope, MSO is that only one unit is required for performing both oscilloscope and logic analyser based tests. When investigating logic circuits including those with FPGAs and ASICs, often the full capability of a logic analyser is not needed. Instead a combination of both scope and analyser is needed.

As such the MSO logic channels provide sufficient logic analysis for many applications, but combine this with an excellent oscilloscope to give a test instrument that is sufficient for most occasions.

Comparison of logic analyser and MSO

The MSO, mixed signal oscilloscope provides an excellent solution for many applications, but when choosing whether to proceed with one, there are some decisions to be made.

A comparison of typical capabilities is given below.

Parameter Mixed Signal Oscilloscope Logic Analyzer
Channel count 16 / 32 64 to 204 +
Triggering Single event on both analogue and digital channels Advanced sequential capabilities
Timing analysis Yes Yes
State analysis Yes: resulting from inclusion of separate channels for clocks No; no provision for clock input

There are many aspects on which decisions can be made, but these points highlight some of the major areas that are of interest to those using MSOs and logic analysers.

By Ian Poole

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