An MSO, or Mixed Signal Oscilloscope, is an analogue oscilloscope with logic analysis channels. The main benefits of this integration are time correlation between the analog and digital channels and more powerful triggering between the two.
In this article, we will discuss why the MSO was created, popular applications of the technology and some measurement tips and tricks for getting the most out of MSOs.
Brief history of MSO
The 1990’s were the decade of the microcontroller. Miniaturisation and digitisation of electronics was happening fast. Classically, a design engineer had two key tools on their bench: an oscilloscope, and a logic analyser. These were purpose-built tools with amazing performance and capabilities. Oscilloscopes were unmatched in showing signal quality and integrity at the physical layer. Logic analysers had 64+ digital lanes with ultra-deep memory, timing and state analysis. They were useful for debugging the most complex silicon, but the needs of engineers were changing.
Microcontrollers of the time were usually 8 or 16 bit devices, and the swing from parallel to serial communications was happening. Logic analysers were a tool of last resort – often overpowered, users struggled to set up and use them. Engineers needed a simpler portable logic analyser. Hewlett Packard’s solution was the 54620A, a 16 channel timing-only logic analyser in the housing and user interface of an oscilloscope. This product was successful with engineers who needed simple timing analysis but not a complex and expensive logic analyser.
Combining the 54620A with a DSO bore the 54645D – the industry’s first mixed-signal oscilloscope. Engineers were now able to make analog and digital acquisitions simultaneously, important for designers who are conscious of transients and physical layer phenomena in digital design.
Being able to trigger on the digital channels and triggering on a specific condition - e.g. a memory write, and then view the signal integrity on the data bus - is important because signal interference such as ISI, crosstalk and jitter all impact the ability to transfer data over the bus. There are times when a specific target condition creates the perfect storm for signals. In other situations the transition from one state to another is where the issue arises. Signal integrity cannot be easily viewed on digital channels, so having access analogue channels to view physical phenomena with a fast refresh rate is critical.
Observing timing relationships on control signals and data buses is another key capability of an MSO, allowing a timing view of relationships across signals with very tight correlation between channel samples.
The use of symbol names and bus representation views also allow an intuitive view of data bus values to validate or debug digital systems.
Another helpful use of digital inputs in an MSO is the ability to trigger the oscilloscope on a signal integrity issue detected on an analog input channel, and then observe the condition of the target system when that problem happens.
Measurement tips and tricks
Autoscale is often an engineer’s best friend! Keysight’s scopes have an autoscale that monitors all analog and digital channels for activity, and turn them on if signals are present. If digital channels have somewhat constant activity, a simple autoscale can get the screen looking much cleaner. See below for an example of before and after an autoscale after plugging in all your channels.
The two fundamental differences between logic analysers (LA) and MSOs are important to remember: Firstly, an MSO does not provide any state analysis, or definition of states; MSOs provide purely timing analysis. Secondly, the digital channels of an MSO share a timebase with the analog channels. This means the digital channels are constantly being sampled and saved into acquisition memory, not just at transitions like a dedicated LA. So any transitions that occur between samples will be reported as happening at the next sample period, and any glitches that occur within a sample period may not be recorded. This can be an issue when you are viewing very long traces of data.
Another tip is that you can define channels individually to two separate buses that can be decoded simultaneously. For example on the left, we define two four-lane buses as B1 and B2: B1 is assigned digital channels 0-3, and B2 is assigned digital channels 4-7. On the right, we’ve only defined one bus, B1, and assigned digital channels 0-7. Each bus can be between 1 and 16 channels, and there can be overlap of channels if needed.
Finally, the math functions built into the scope enable to better visualise the bus. Above is the timing chart function, assigned to Bus1 as channels 0-7. The chart will be visualised as a quasi-DAC, with a simulated analog output based on the state of the bus when a channel transitions. Scaling factors and units can be assigned to make measurements: on the above we are measuring the frequency of the output, as well as max and min based on 1 mV per code (e.g. an 8 bit bus output range of 0-256 mV).
Due to the popularity of serial protocols over parallel buses, logic analysers have largely fallen out of favour with engineers, who are more likely to reach for an MSO to make measurements. In the decades following HP’s innovative 54645D, MSO technology is now offered on a wide range of scopes. The ability to cross trigger between analog and digital channels, as well as decode and view states on screen, are powerful features that engineers leverage to design and debug today’s most challenging mixed signal designs.
A full range of Keysight Mixed Signal Oscilloscopes is available from RS.
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