Mixed Signal Oscilloscope Explained

An MSO (mixed signal oscilloscope) is a hybrid test device that seamlessly combines some of the measurement capabilities found in a logic analyzer with the measurement capabilities found in a DSO (digital storage oscilloscope), such as trigger holdoff, autoscale, probe/channel de-skew and infinitepersistence on digital and analog channels — in one compact instrument. An MSO allows users to view multiple time-aligned digital and analog waveforms on a single display.

Compared to a full-fledged logic analyzer, an MSO does not have large number of digital acquisition channels. It also lacks most of the advanced digital measurement functions offered by logic analyzers. Despite this, an MSO still offers some unique advantages over both logic analyzers and traditional oscilloscopes.

One of its primary advantages is its use model. An MSO can be used in almost the same manner that users use an oscilloscope. Because of the complexity and time needed to learn or relearn a logic analyzer, design and test engineers generally avoid using one. Another reason why engineers avoid a logic analyzer is the much longer time needed to set one up to make measurements. Moreover, the logic analyzer’s advanced measurement capabilities add complexity and are commonly regarded as an overkill for most of MCU- (microcontrollers) and DSP-based (digital signal processors) designs.

As one of R&D’s most commonly utilized test instruments, embedded hardware design engineers must have a basic knowledge on how to operate an oscilloscope to achieve signal-quality and timing measurements. However, the measurements of dual-channel and four-channel oscilloscope are insufficient to test and monitor critical timing interactions between digital and analog signals. This is the time when an MSO proves to be of much help.

Since an MSO offers “just enough” logic analyzer measurement capability, minus the complexity, it is usually regarded as the appropriate tool for debugging embedded designs. With the use-model similar to that of an oscilloscope, an MSO can also be thought of as a multi-channel oscilloscope with some channels (logic/digital) offering low-resolution measurements (one bit), while several channels (analog) offers lots of vertical resolution, usually eight-bits.

Unlike a loosely-tethered two-box, mixed-signal measurement solution, a highly integrated MSO is user-friendly, delivers fast waveform update rates while operating more like an oscilloscope and not like a logic analyzer.

Waveform update is one of the important characteristics of an oscilloscope since it directly impacts the instrument’s usability. With sluggish response limiting the usability, users may find it frustrating to operate an unresponsive and slow oscilloscope. This applies not only to DSOs but to MSOs as well. Thus, oscilloscope vendors should ensure that waveform update rate is not compromised when they create an MSO by porting logic acquisition channels into a DSO. Otherwise, this would lead to sacrificing the use-model of traditional oscilloscope. Moreover, mixed-signal measurement solutions that are based on external logic pods and/or two-box solutions connected through an external communication bus like USB are generally difficult to use and unresponsive. On the other hand, MSOs that are based on highly integrated hardware architecture are easier to use and more responsive.