By Guido Schulze, product manager for oscilloscopes at Rohde & Schwarz in Munich.
The Internet of Things (IoT) is becoming an important driver of innovations in the electronics industry. Intelligent IoT modules communicate between industrial facilities, machines, Hi-Fi devices as well as between household appliances and mobile phones. They bring together multiple technologies into the smallest of spaces and typically include a radio module. This complexity can become a real challenge for developers of integrated circuitry. Highly sensitive oscilloscopes with multi-domain capability are extremely helpful during optimisation and commissioning of these components.
When debugging IoT modules, all module functions must be tested, as well as the interactions between individual functions and components. A multi-domain oscilloscope is needed in order to perform comprehensive measurements using only one test instrument. The latest generation of ‘scopes can be used to test all the module’s sensor and control signals, along with its integrated data processing, radio module and power supply. Multi-domain capability enables time, frequency, protocol and logic analyses and establishes all time references. Via the oscilloscope’s analogue input channels, the user simultaneously sees the signal in the time and frequency domain, and if desired, the spectrogram. This makes it possible to perform debugging on the functional system level.
Measurements on a GSM IoT module
The “Cinterion BGS2” GSM module from Gemalto (Figure 1) represents a good example of complex embedded wireless design for machine-to-machine (M2M) applications. It connects a GSM radio module to a baseband processor, to the power supply management, to various serial interfaces for modems, inter-integrated circuits (I2C) and multipurpose (GPIO), as well as to the clock source, flash memory, a converter and an audio interface.
Maximizing battery runtime – sleep mode
Like most IoT modules, the GSM board from Gemalto is designed for autonomous remote operation using a long-life battery for its power supply. At minimal current drain, (Figure 2/3) the module will provide data via the radio interface for years. Characterising the module’s power usage is therefore an important part of commissioning and optimisation. Key measurements include the dynamic response of the power supply during data transmission and when operating state changes.
Current of the IoT module is measured using the most sensitive current probe available, capable of resolving currents less than 1 mA at a bandwidth of 120 MHz, with a maximum current of 5 A. Precise current measurements are possible in conjunction with the unusually low-noise input stages of the oscilloscope. To prevent false results, the current probe has to be demagnetized before measuring the current. Also the current probe and measurement channel need to be auto-zeroed to ensure that the lowest currents are measured accurately.
Radio antenna signals are captured via a near-field probe connected to the analogue input channel on the oscilloscope. Radio signals from the module are viewed as an analogue signal in both time and frequency domains via a fast Fourier transform (FFT). A further oscilloscope channel is connected to the power supply via an active probe. Digital channels (MSO) then capture the communications at the modem interface. Individual UART serial bus signals are decoded with the RTO-K1 option.
Some power supply measurements require more detailed anaysis. For example, this module exhibits a dynamic transition from sleep mode with very low current of 1 -2 mA, to an operational state with currents greater than 1 A. Switching to 16-bit high definition mode, adjustable low-pass filters are applied to the signal after the A/D converter, enabling exceptionally high resolution. Signal details in the mA range can be analysed over a large vertical measurement range.
Current and voltage in transmit mode
Current and voltage waveforms can be analysed during radio operation in order to reveal additional sources of interference and power-reduction options. For example, the voltage dip during high current draw from transmit sequences is especially critical (Figure 4). Falling below the lower voltage limit may cause the IoT module to be switched off automatically.
The IoT module’s power supply also has to maintain minimum voltage through voltage dips, ripples and peaks. Radio signal quality is also degraded by noise and spectral interference in the power supply. To help achieve this, the Gemalto IoT module has an internal power management controller, along with low-dropout voltage regulators (LDOs) and DC-DC downconverters that together ensure a stable power supply for the GSM module and SIM card. The power management controller also controls on/off switching operations. The module monitors the voltage via an integrated A/D converter, which can determine voltage values at intervals down to 0.5 s. This is ample for normal operation, but too inaccurate for debugging and optimisation of the power supply during commissioning.
Instead, voltage is measured using the oscilloscope and a single-ended active probe that has a separate offset setting, set to quiescent potential during the measurement. A fine vertical scaling allows users to zoom in on the power supply details, in particular the noise characteristics. Spectral interferers are easy to detect with user-friendly FFT functions. The spectrogram even makes analysis of the frequency components possible over a longer time period; faults are quickly detected in the graphical spectrogram display.
Recently introduced functions like the zone trigger provide further useful insights into power supply characteristics. For example, with a mask defined in the 890 MHz to 910 MHz range, triggers can be initiated only when a transmit pulse is detected in this mask. The current and voltage waveforms can be later correlated with the transmit pulses using the oscilloscope’s history player.
Debugging at system level – from radio signal to modem signal
Typical IoT modules employ an embedded design approach, where all functions are integrated into an extremely small footprint. It is therefore important to characterise the interference between function blocks. A test tool is needed that acquires data at the various interfaces with a time correlation and then performs analysis. This is where users can benefit from multi-domain functionality.
For example, when an IoT module is contacted via a GSM connection, the RF signal as well as the voltage and current supply are measured via the analogue channels. Digital channels record the subsequent communications between the IoT module and UART interfaces. Protocol decoding makes it possible to read the ” ‘R’ ‘I’ ‘N’ ‘G’ ” in ASCII code on the ring line and, thanks to the fixed time correlation between the signals, the temporal sequence for data acquisition, processing and communications can be analysed. Faults passing through the system can easily be detected and battery lifetime optimisation is supported by correlating all activities with the corresponding current drain.
This (Figure 5) shows clearly how highly sensitive oscilloscopes with multi-domain capability can prove extremely helpful during optimisation and commissioning of IoT components.