Time-correlated analysis of signals found in embedded designs

Author : Guido Schulze, Rohde & Schwarz

15 December 2016

The staggering need for cost-­efficient and powerful communications and control electronics for ­industry, motor vehicles and the entertainment and smart home sector is driving the integration of electronic circuits.

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These advanced embedded designs integrate a variety of functional units and technologies. The processor, power management, digital communications interfaces, local program memory, data memory and sensors all operate in the smallest of spaces. The next integration step is radio modules. The variety of signal waveforms is quite large, ranging from RF radio signals, analogue signals from sensors or protocol-coded signals from the control interfaces.

This complexity represents a challenge for developers because highly integrated designs are significantly more prone to mutual interference. Undesirable interactions must be eliminated with an exact time reference at the system level.

Embedded designs represent the greatest T&M challenge in development and service today. These demanding measurement tasks require intelligent solutions such as those offered by the R&S RTO2000 oscilloscope. Its toolset includes functions for time, frequency, logic and protocol analysis – a variety that in the past required several single-purpose test instruments.

Multidomain functionality for integration tests

Low-noise frontends and high-resolution A/D converters allow the R&S RTO2000 analogue input channels to perform accurate measurements in the time domain across a large dynamic range. Users benefit from reliable results, whether performing easy voltage level checks over time or specialised measurements such as jitter analyses on clock or data signals or power analyses on switched-mode power supplies. 

The 16 digital channels extend the oscilloscope’s test resources, e. g. to precisely measure the logical level (high, low) on digital interfaces over time. Timing errors in parallel interfaces are quickly detected.

The many tools for analysing protocol-based serial interfaces provide a broad spectrum of trigger and decoding options for a variety of standards, including I2C, SPI, USB and ­Ethernet. The R&S RTO2000 allows both analogue and digital channels to be used for protocol decoding. The oscilloscope uses its hardware-assisted protocol triggering to quickly trigger on details such as addresses or data.

Even in situations where spectrum analysers are the first choice for precise measurements on radio interfaces, the R&S RTO2000 is suitable for acquiring radio signals thanks to the high dynamic range of its analogue channels. When testing at the system level, the channels deliver a precise time correlation to the other functional units in embedded designs. 

Fig. 2 shows the variety of measurement options in an Internet of Things (IoT) application with a Wi-Fi radio module. Channel 1 (yellow) acquires the Wi-Fi signal and displays it in the time domain. However, the signal waveform is not clearly recognisable until it is viewed in the spectrum (Math4). Channel 3 (orange) shows how the radio activity affects current consumption. The timing of the USB interface control commands is also visible. The R&S RTO-K60 option decodes the signals acquired on channels 2 and 4 (green and blue) into readable USB data.

Analysis of smaller currents 

Once the initial functional tests on the electronic design are completed, circuit optimisation starts. For mobile applications, minimising current consumption is paramount. This requires a measurement instrument that can resolve low currents down into the 1 mA range while also correlating the timing of current changes to switching activities, e. g. when transmitting radio sequences or entering power save mode.

The dynamic range and sensitivity of its analogue input channels make the R&S RTO2000 ideal for measuring low voltages and currents. The R&S RT-ZC30 option is a sensitive current probe that can measure currents down to 1 mA at 120 MHz bandwidth. In HD mode, dynamic variations as small as 100 µA can be resolved.

Using an analogue channel to perform current measurements provides a fixed time reference to the other measurement signals. Fig. 3 shows an example of a current probe in­channel 3 (orange) measuring a current of 1.7 mA during a sleep sequence. The current consumption is correlated with the radio signal output on channel 1 (yellow) and the system activity at the UART interface. During the sleep sequence, the module does not transmit any radio signals, but it receives regular paging signals from the base station. The current consumption briefly increases to 105 mA and the module transmits a UART-coded communications signal on the clear-to-send (CTS) line, which is acquired with a digital channel.

Enhanced debugging in the spectrum

The FFT-based spectrum analysis function on all R&S RTO2000 analogue input channels opens up additional possibilities, e.g. analysing radio signals, EMI debugging to find interferers in the spectrum or spectral analysis of power supplies. In contrast to conventional FFT implementations in oscilloscopes, the R&S RTO2000 achieves resolution and display speed with its digital downconversion (DDC), in which the FFT calculation can be limited to a selected frequency range.

User-friendly functions such as automated measurements, peak lists, max. hold detectors and mask tests support debugging in the spectrum. One unique characteristic is the spectrogram, which visualises the changes in frequency components over time.

Zone trigger in the time and frequency domains

The zone trigger can be used to graphically differentiate between events in the time and frequency domains. Up to eight zones of any shape can be defined and logically linked as trigger conditions. A trigger is initiated when test signals intersect defined zones or when those zones are not touched. This makes it possible to detect interferers in the spectrum during EMI debugging or to separate read and write cycles in memory controllers. 

Fig. 1: Multidomain application in a state-of-the-art embedded design: analogue measurements in the time domain, measurements in the spectrum as well as protocol and logic analysis.

Fig. 2: Example of a multidomain application – IoT ­module with Wi-Fi radio ­module, battery-operated power supply and USB interface.

Fig. 3: Measurement of the current consumption of an embedded design in sleep mode. The base station remains in contact with the GSM radio module via paging (short current pulses).

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