Under the bonnet of a mixed-signal oscilloscope

To understand the benefits of using a mixed-signal oscilloscope it is helpful to see how it performs time-correlated acquisition and analysis of digital and analogue signals, writes Wolfgang Herbordt.

The more tasks that embedded systems take on, the more complex these systems become. At the same time, both the variety and number of interfaces between digital and analogue components are increasing.

A typical design might use 1 bit signals, clocked and unclocked parallel data buses and serial data buses, standardised or proprietary transmission formats and a variety of data rates.

Anyone who wants to get a handle on the increasing complexity has to analyse all of these interfaces at different levels of abstraction. This usually requires complex test setups with multiple instruments: analogue waveforms are studied using an oscilloscope, digital signals with a logic analyser and transmission protocols with a protocol analyser.

A mixed-signal oscilloscope provides digital channels for analysing digital states and protocol details in addition to the analogue channels.

A benefit is that users can analyse the circuits at the various levels of abstraction with only one instrument and therefore only one user interface. 

Two-phase principle: acquisition and analysis

The functioning of a digital oscilloscope can be split into two sequential phases, the acquisition phase followed by the analysis phase. During the acquisition phase, the sampled test signals are saved to a data memory.

The acquisition phase is characterised by the sampling frequency, the acquisition depth and the trigger options.

During the analysis phase, the acquired waveforms are analysed and output over the user interface; i.e. to the instrument screen or to files. The data analysis functions on a digital oscilloscope include the Zoom, Test, Cursor, Math and Search functions.

A mixed-signal oscilloscope will use this two-phase principle for both the analogue and digital channels. The instrument must continue to serve as a conventional oscilloscope and the functionality of the analogue and digital channels must integrate cleanly. The many channels and the resulting range of setting options make a simple and clear user interface even more important.

To look at time synchronisation in a mixed-signal scope we see that analogue and digital channels are acquired synchronously on the same instrument so that analogue waveforms, digital signals and protocol details are time-correlated and can be analysed in one location.

A delay compensation between the analogue and the digital channels is needed for synchronous signal acquisition.

This delay compensation can take place between the digital channel probe boxes and the analogue probe connectors inside the instrument. As long as the delay between the analogue probes and the probe tips on the digital channels is not important to the user, then no further settings are needed.

Time resolution and acquisition cycle

A high time resolution for both the analogue and digital channels is a preferred feature because the events within the digital signals are analysed with a high degree of temporal accuracy and narrow glitches can be detected.

In this way even it is important that when digital channels are used as the trigger source, the trigger time is determined with a high degree of accuracy, ensuring that waveform jitter is minimal during visualisation.

For example the R&S RTO scope with B1 mixed-signal option has a sampling frequency of 5Gsample/s for the 16 digital channels, as compared to 10Gsample/s for the analogue channels. The resulting time resolution for the digital channels is 200ps. 

If the time resolution in the analogue channels exceeds that in the digital channels, as is the case at a sampling rate of 10Gsample/s or during interpolation, sample & hold interpolation is used to adjust the digital channels to the sampling rate of the analogue channels. Joint analysis of analogue waveforms and digital signals is thereby ensured.

It is also important to consider the acquisition depth of per digital channel for acquiring long data sequences from serial buses. A bit rate of 400Mbit/s and a sampling frequency of 5Gsample/s, for example, results in an acquisition depth of 16Mbit.
Oscilloscope trigger types for which a single amplitude threshold (i.e. the threshold for the logical transition) is sufficient are frequently offered for the digital channels.

Measurement speed

A typical challenge in the design of digital oscilloscopes is the reduction of its “blind time”. This is the time during which no data acquisition takes place and therefore potentially interesting events are not seen. How can the blind time be reduced so that rare events are detected more quickly?

The blind time is shortened by optimising the analysis phase.

Oscilloscope design can address this with dedicated Asics in which data acquisition and analysis take place simultaneously. The result is an instrument speed of up to one million visualised waveforms per second.

Similarly in mixed-signal mode, signal processing takes place within an FPGA over the entire process, from acquisition and triggering to visualisation, cursor functions and measurements. Analysis is performed in parallel for all 16 digital channels. This is done at a speed of up to 200 000 visualised waveforms per second.

Typcially, a screen dump takes place every 30ms to match the visual perception of the human eye. Between two screen dumps, the oscilloscope’s hardware-superimposes the waveforms from the analogue channels in order to display all waveforms on the screen. The scope’s mixed-signal option also uses this display method for the digital channels.

Bus signals, on the other hand, are not superimposed because they include data content from multiple combined binary signals. To allow analysis of bus signals, the user can adapt the display format to the bus format. A distinction is made between unclocked and clocked data buses. With unclocked data buses, the logic state is determined for each sampling period. With clocked data buses, it is determined only fo r valid edges. Display is in bus format, in table format, or as an analog waveform in binary, hexadecimal, decimal and fractional number formats.

Signal analysis

Critical factors for the effective and efficient study of waveforms are the number and quality of the analysis functions provided by the oscilloscope. In particular, automated amplitude and time measurements, including their statistical analysis, math functions and cursor functions are provided. For digital channels, only time measurements and the associated statistical analyses are used. Maths functions are reduced to logic operations for binary signals.

For example, in mixed-signal mode the maths signal can be any logical combination of all 16 digital channels. This is also used as the source signal for measurement functions. Cursor functions can be used on binary signals, on bus signals and on logically combined digital signals.

Mixed-signal oscilloscopes expand the basic oscilloscope functionality to include logic and protocol analyser elements. Users benefit from the simplified test setups, uniform operation and synchronous visualisation of analogue waveforms, digital signals and protocol details within a single instrument. 

Author is Dr. Wolfgang Herbordt works in oscilloscope design at Rohde & Schwarz.


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