Getting the most out of your mixed signal oscilloscope

Mixed signal oscilloscopes have evolved over time to meet changing requirements. This article details some popular applications of the technology and provides measurement tips and tricks for getting the most out of mixed signal oscilloscopes.

A mixed signal oscilloscope (MSO) is a ubiquitous term in the oscilloscope market today. An MSO is a 2- or 4-channel oscilloscope with 8 or 16 (rarely more) logic analysis channels built in for timing analysis.

The main benefits of this integration are time correlation between signals on the analog and digital channels of the oscilloscope and more powerful triggering between the two. 

MSOs — a brief history

If the 1980s were the decade of the microprocessor, the 1990s were the decade of the microcontroller. Miniaturisation and digitisation of electronics was happening, and 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, with high resolution and fast waveform update rate. Logic analysers traditionally had 64 or more digital lanes with ultradeep memory, timing analysis, state analysis and full customisation of the user interface. They were useful for debugging and designing even the most complex FPGAs, ASICs and microprocessors. But the needs of engineers were changing, and with that, the tools changed too.

Microcontrollers of the time were usually 8- or 16-bit devices, and the swing from parallel to serial communications was already happening. Logic analysers were a tool of last resort — often way overpowered for the task at hand. People who weren’t power users often struggled to set up and use the device. Engineers needed an easy-to-use and portable logic analyser with a familiar interface. The solution was the Hewlett Packard 54620A, a 16-channel timing-only logic analyser in a 54645A oscilloscope’s housing, with an oscilloscope user interface. This product was successful with engineers who needed simple timing analysis, without the other 95% of capabilities in comparatively complex and expensive logic analysers.

Target applications

Engineers were, for the first time ever, able to make analog and digital acquisitions simultaneously with fast update rate. This was, and still is, important for designers who are conscious of transients and physical layer phenomena in digital design. Let’s take a look at some popular applications for mixed signal oscilloscopes today.

Being able to trigger on the digital channels is the most powerful capability of MSOs. Triggering on a specific condition — eg, a memory write to certain address, and then viewing the signal integrity on the data bus under that target condition — is important because data dependent inter-signal interference, crosstalk and jitter all impact the ability to transfer data over the data bus.

There are times when a specific target condition creates the perfect storm for inadequate signal integrity. In other situations, it is more than just a specific state of the operation that creates a signal integrity or timing issue, but rather the transition from one state to another where the issue arises. Signal integrity cannot be easily viewed on digital channels alone, so having access to four 8-bit channels to view physical phenomena with a fast refresh rate is critical.

Observing basic timing relationships on control signals and data buses is another key capability of an MSO. Although not nearly as accurate as analog scope channels, the 16 digital inputs do allow a view of basic timing relationships across related input signals, with low channel-to-channel skew and very tight correlation between channel samples. This is because the 16 channels are all derived from the same timebase sampling process. The use of symbol names and bus representation views also allows an intuitive view of state machine condition reporting or data bus values to validate or debug digital systems.

Another helpful use of digital input channels 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. This can be accomplished through observing the target condition via the digital input channels to see a state machine condition, see command/control signal condition, or data values.

Measurement tips and tricks

Autoscale is generally an engineer’s best friend on the bench, especially when you haven’t picked up a scope in a while. Keysight InfiniiVision X-Series scopes have an autoscale that, when pressed, will monitor all analog and digital channels for activity, and turn them on for you if signals are present. If your digital channels have somewhat constant activity, a simple autoscale can get your screen looking much cleaner. See below for an example of before and after an autoscale after plugging in all your channels. This will only work if your signals being probed have activity. If you are preparing to monitor a single shot event, you’ll need to set the channel scaling yourself.

Figure 1.

Next, let’s quickly discuss the fundamental differences between traditional logic analysers (LA) and MSOs. First, an MSO does not provide any state analysis, or definition of states within a system. MSOs provide purely timing analysis. There is a pseudo state mode in which the scope can decode the digital channels and read out a bus value on screen, but that’s it. You can see it on the bottom of the below screenshot, as x14. Second, 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. This means you need to be conscious of your sample rate on the digital channels. 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, as exemplified in the above images. The scope has been set up to decode 50 ms of data, and thus the sample rate has lowered to 2.5 MSa/s on the digital channels to accommodate. This means there are 400 ns gaps between samples. The uncertainly between samples is shown as a filled in box, to let you know that the real transition could have occurred anywhere within that time period.

Figure 2.

Another tip is that you can define channels individually to two separate buses that can be decoded simultaneously on screen. In the below example on the left, we’ve defined two 4-lane buses as B1 and B2, being decoded in real time on the bottom of the display. B1 is assigned digital channels 0–3, and B2 is assigned digital channels 4–7. In the below example 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 if needed (eg, B1 = channels 0–11, B2 = channels 4–15).

Figure 3.

Finally, there are some neat math functions built into the scope to better visualise the bus. Below 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 if you wish to make measurements on the output, as shown above, where we are measuring the frequency of the output, as well as max and min based on the below factors of 1 mV per code (eg, as an 8-bit bus, the simulated output range would be 0–256 mV).

Figure 4.

There is also a state analysis function that is similar to the timing chart, except instead of looking for changes on the bus, it uses a separate clock line to visualise changes on the simulated output.

Conclusion

Due to the popularity of serial protocols over parallel buses, logic analysers of the past have largely fallen out of favour with engineers, who are more likely to reach for an MSO-enabled oscilloscope to make their measurements. MSO technology is offered by nearly every vendor, on a wide range of scopes from a few thousand dollars to over two hundred thousand dollars. 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.

Top image credit: ©iStockphoto.com/Krzysztof Zmij