What do developers need from today’s mixed signal test equipment?

4 mins read

The more tasks an embedded system is to accomplish, the more complex it becomes and the greater the variety and number of interfaces between its analogue, digital and rf component parts.

These days, it is not uncommon for a system design to combine 1bit signals, clocked and unclocked parallel data buses and serial data buses, with the task further complicated by the move to higher speeds. So, whilst an automotive developer may be contending with FlexRay and MOST, in addition to CAN and LIN, a communications network designer could be dealing with Infiniband and MIPI. At the same time, many consumer device designers are designing DDR3, PCI Express and/or USB 3.0 interfaces. To manage this growing complexity, developers need to analyse all interfaces at different levels of abstraction. Analogue waveforms are scrutinised for: signal amplitude and timing parameters, to evaluate timing margins; signal integrity characterisation, such as crosstalk, ground bounce, overshoot, ringing, fault and metastability transitions detection; power and current measurements. Digital signals are subjected to complex analysis and debug, evaluation of logic states and setup hold time violations, with developers needing to create logic triggers to isolate complex system events. Meanwhile, it is common for tests to be made of protocol compliance and interoperability. Analogue and digital signals often need to be compared synchronously in order to give time correlated comparisons. For example, when designing an a/d converter, it is necessary to match the analogue and digital domains, whilst during protocol decode, it is useful to be able to trace the source of wrong commands sent over the bus by checking the analogue signal. The usual mode for tackling this requires complex test setups with multiple instruments, each of which is operated differently and can take a long time to set up. A converged test system has become a 'must have' these days in order to meet demand for mixed and multiple protocol verification and compliance testing.

Finding errors in the time domain Timing resolution is another important 'must have', as there are many timing considerations to take into account when combining synchronous and asynchronous circuit designs. Adequate timing resolution allows an oscilloscope to analyse events in the digital domain with the required precision to reliably detect various signal integrity issues, including narrow glitches caused by timing errors. However, timing resolution on its own is not enough to capture glitches and anomalies. A 'scope with a fast update rate/acquisition rate is critical.

A digital 'scope samples the measured signal for a defined period and then stores the data. In a second step, it processes these samples and displays the waveform. During this period, the oscilloscope is 'blind' to the measurement signal. When conventional digital 'scopes operate at their maximum sampling rate, this blind time can exceed 99.5% of the overall acquisition time. To combat blind time, 'scope users will often increase the time base in order to improve the probability of capturing an elusive event. However, increasing the time base can result in a shorter blind time ratio as the longer record length results in a reduced acquisition rate and a slower waveform update rate. The Rohde & Schwarz RTO-B1 mixed signal option equips its RTO scope with 16 digital channels, each with a sampling rate of 5Gsample/s, in addition to the 10Gsample/s analogue channels. For the digital channels, this translates to a resolution of 200ps over an acquisition depth of 200Msamples, which means that even events that occur long after the trigger point are displayed with high time precision. The mixed signal option's high speed chip is so fast that the 'scope can display up to 200,000 waveform/s, irrespective of the number of signals being analysed. Other common causes of error in serial circuits include clock instability and crosstalk. The former can be characterised using jitter and timing analysis, accomplished by triggering on timing conditions such as a glitch or pulse width. An oscilloscope's trigger has two main applications: to ensure a stable display; and to display specific signal characteristics accurately. To display and analyse a signal effectively, trigger functionality needs to be highly sensitive. The digital trigger system on the RTO makes it possible to pinpoint accurately glitches that may last for as little as 50ps and their origin, whilst an adjustable trigger hysteresis allows users to optimise the trigger sensitivity to the signal characteristics. While the use of differential signals in a system largely minimises emi and crosstalk errors, a 'scope still needs a wide dynamic range if it is to capture these very low power signals. Many 'scopes aren't sensitive enough to register this kind of signalling and developers risk seeing the noise of the 'scope itself, rather than signals they are looking to acquire. Some 'scopes use software simply to spread the signal amplitude across the screen and therefore use only a small part of the a/d converter's range. In contrast, R&S' RTO operates down to the full 1mV/div sensitivity using activated input amplifiers that take advantage of the a/d converter's full dynamic range. In addition, its inherent low noise means that it does not need to be further reduced by decreasing the input bandwidth, allowing the full bandwidth to be used for making accurate measurements in all sensitivity ranges. Tracking errors in the frequency domain With a large number of baseband and rf signals, it is useful to be able to synchronise frequency domain analysis with the time domain. Oscilloscopes normally take the time domain samples and perform a fast Fourier transfer (FFT) – a complex mathematical formula that extracts frequency information from the time domain signal – to produce the spectrum. While conventional oscilloscopes perform an FFT on a whole time domain capture/data, the RTO uses a digital down converter to focus in on the user's choice of centre frequency and span, greatly reducing the amount of data to be processed. In addition, the FFT calculation is performed by a dedicated fpga, making the FFT function much faster than other oscilloscopes available. Returning to the R&S RTO-B1 option, a new feature is the ability to perform FFTs on parallel data buses. This can be particularly useful when analysing the performance of a/d converters. The sinusoidal input tone to the converter is digitised by the device and the output signal is fed to the oscilloscope's digital channels as a parallel bus signal. It is difficult to detect interference in the time domain, but it will be clearly visible in the frequency domain. This ability to display digital bus signal spectra greatly simplifies the analysis of typical mixed signal designs. An oscilloscope with basic logic and protocol analyser functionality is an ideal tool for hardware developers investigating signal integrity and for software developers when analysing the content of signals. A single user interface and synchronised visualisation of analogue waveforms, digital signals and protocol details in one instrument will allow users to concentrate on the complex process of debugging their embedded system, rather than spending precious time on test set up. Jithu Abraham is regional product manager at Rohde & Schwarz UK.