New developments in mixed-signal oscilloscopes

31 December 2009

The Yokogawa DLM6000 mixed-signal oscilloscope
The Yokogawa DLM6000 mixed-signal oscilloscope

With more engineers needing to work in a mixed-signal environment, instruments are being called on to provide digital performance to match analogue. Clive Davis looks at how the balance is being reached.

The worldwide boom in consumer electronics has placed an increased emphasis on the need for sophisticated designs which combine both analogue and digital technology. Such projects are characterised by short development cycles, market pressures for low cost products, and the need for devices that combine flexibility and low power consumption. The circuits involved in these designs increasingly include advanced components such as Flash CPUs, FPGAs, CPLDs and Flash memories, and the debugging of these circuits involves engineers looking at analogue as well as digital characteristics of signals.

The tools of choice to carry out debugging and analysis in mixed analogue and digital signal environments have been the oscilloscope to look at the analogue aspects and the logic analyser to look at multi-channel logic signals. However, as logic speeds increase and signal-integrity issues increasingly relate to analogue effects on logic signals, so there is an increased need for a single measurement tool that can look at both signal environments.

The mixed-signal oscilloscope
Test and measurement equipment manufacturers have responded to this challenge by developing instruments known as ‘mixed signal oscilloscopes’, which combine a digital oscilloscope for looking at analogue waveforms with digital inputs that allow logic channels to be viewed at the same time. However, to date these instruments have suffered from some significant limitations, since the main emphasis has been on the analogue side. In particular, existing mixed-signal oscilloscopes have been equipped with only a limited number of logic inputs as standard (typically 16), which has resulted in a poor display for logic signals, no analysis features for logic signals, and a slow display refresh rate.

Such instruments are clearly becoming inadequate for today’s digital consumer electronics and automotive market sectors, where the demands of high-performance multimedia applications and the networking environment mean that 16-bit embedded processors are rapidly being succeeded by 32-bit devices. Even domestic appliances are moving towards the use of 16-bit processors. At the same time, the number of I/O ports on embedded processors is increasing - to dozens in many cases. In such circumstances, even 16 logic channels in a measuring instrument are not enough to analyse address and data bus signals, or to analyse a number of I/O ports on an embedded processor. Moreover, engineers familiar with logic analysers want to have full bus display and state analysis capabilities, along with a fast refresh rate on the logic display.

A further challenge is presented by the increased resolution of modern analogue/digital and digital/analogue convertors, typically 10 to 12 bits for video and imaging applications and 20 bits or more for audio applications. Again, 16 bits are not enough to analyse the data bus together with the control bus of the A/D or D/A convertor.

A response to these challenges is provided by mixed-signal oscilloscopes that combine high-performance analogue channels with 32 bits of logic input. Such instruments, in which the logic channels are sampled simultaneously with the analogue channels and at the same maximum speed, are ideally suited to the needs of users who wish to analyse logic signals as well as analogue waveforms. They also address the needs of engineers who need to analyse multi-channel logic signals and require features such as bus display and state analysis, search and zoom capability, a powerful range of triggers and digital/analogue calculations on logic signals.

Meeting users’ requirements
Extensive user research on the requirements of oscilloscope users has established that the following key factors play a major part in the choice of an oscilloscope:
Ease of use, with particular emphasis on the user interface
Waveform acquisition and characterisation
Detection of glitches and anomalies
Signal enhancement
Mixed-signal capabilities for embedded applications
Additional analysis capabilities for serial-bus and power applications
Ease of integration with other test equipment or production ATE systems.

These requirements have led to the development of a new generation of mixed-signal oscilloscopes (Fig.1) featuring bandwidths up to 1 GHz, memory of 6.25M points per channel, an intuitive graphical user interface and a number of advanced analysis features. The user interface incorporates a new physical layout with backlit buttons, on-screen visual elements including graduated menus, and innovative controls including a four-direction selector button and a ‘jog shuttle’ control.

For analysing modern embedded systems, a fast acquisition update rate is required: up to 25,000 waveforms per second in some instruments. Further insight into signal behaviour is provided by combining the high refresh rate with a history memory that allows the user to recall and display previously acquired data from up to 2000 screens’ worth of past waveforms. This ability to redisplay and subsequently analyse data offers significant benefits in troubleshooting and analysis. The refresh rate of up to 25,000 waveforms per second is not compromised by changes in the bus display mode. Some oscilloscopes also feature an ‘N single’ mode, which allows up to 1600 waveforms to be captured at the smallest dead time on the market of 400 ns: equivalent to 2.5M waveform acquisitions per second.

Other features that are finding their way into modern mixed-signal oscilloscopes include the ability to search the instrument’s memory for desired waveforms and zoom in to observe these waveforms in detail. In addition to searching based on criteria such as signal edge, pulse, and multichannel state, the memory can be searched by serial or parallel waveform patterns and waveform parameters. Users can quickly find desired waveform data in memory, enlarge the area with the zoom function, and scroll the data.

Signal enhancement capabilities include new IIR and FIR bandwidth filtering, a high-resolution mode, averaging, and real-time maths channels. Further enhancement is provided by dual-window zoom capabilities, which allow the user to simultaneously zoom in on two areas. Two individual zoom factors and positions can be set with independent timescales and displayed simultaneously. Using an automatic scroll function, it is possible to automatically scroll waveforms captured in long memory and change the position of the zoom areas.

Trigger functions
As oscilloscopes evolve, so their trigger functions become more powerful. This includes the ability to trigger conditions using a logic signal as the source. Various trigger conditions can be combined to capture only the desired signals.

When examining signals within embedded systems, it is sometimes useful to assign signals into groups. For example, 32-bit logic signals may be split into up to five groups, with no limit to the number of bits allowed in each group. Groups can then be assigned using a graphical interface for flexible and easy settings. Even in cases such as where a reconfigurable device’s pin assignments have been changed, the corresponding adjustments can be made by simply by changing the mapping of the groups. Analysis such as bus display, state display, and digital/analogue conversion can then be executed on a group-by-group basis.

Advanced logic analysis capabilities, all of which benefit from the instrument’s ability to sample the logic signals simultaneously with the analogue channels and at the same maximum speed, include a virtual D/A feature that enables the instrument to calculate the analogue signal from the logic information and display the analogue signal on screen. This allows the MSO to be used to characterise the performance of devices such as an A/D or D/A convertors: for example, phase shift or distortion (using FFT processing) can be measured. It is also possible to create logic symbol definition files and display symbolic values in the bus display mode in addition to hex or binary forms.

Bus and power analysis
Engineers are increasingly being called upon to carry out serial bus analysis on I2C, SPI, CAN or LIN bus systems, and oscilloscopes are increasingly being equipped with dedicated triggers for these bus types. These functions make it easy to discriminate between partial software failures and physical-layer waveform problems when troubleshooting systems by observing the physical-layer characteristics of signals. Using a dual-window zoom function, an oscilloscope can simultaneously analyse and display waveforms from buses running at different speeds.

Another part of the measurement tool set that the oscilloscope offers to the embedded engineer is the ability to make automatic waveform parameter measurements such as maximum, minimum, peak-to-peak, pulse width, period, frequency, rise time, fall time, and duty ratio. It is also possible to calculate the statistics of waveform parameters, such as the average, maximum, minimum and standard deviation, over multiple cycles within an acquisition or over multiple acquisitions.

Another key area where test and measurement plays a crucial part is the use of mask test functions that can be used for automatic parameter measurements in eye-pattern analysis or for evaluating signal integrity of data communication. Here, the ability to calculate parameters based on the eye pattern formed by the crossings of two or more waveforms can provide additional insight.

Automated waveform computations combined with statistical analysis are also invaluable for tests on power supplies, particularly in the light of new compliance requirements for power quality and harmonic behaviour laid down in standards such as EN61000-3-2.

Clive Davis works for Yokogawa Europe – Test & Measurement

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