Advanced testing for SDRs
01 May 2007
More than ever, RF technologies are being developed for adaptation. Nowhere is this more evident than with Software Defined Radios (SDRs), which enable software to control communications parameters
SDR delivers more flexibility than conventional RF technologies, although it does introduce a host of new problems. One of the most significant implications is that the hardware in a robust SDR design requires extensive adaptability and high performance over a wide range of operating parameters in order to answer the demands of the software. SDRs are now being used in a number of areas, including 3G wireless basestations and user equipment, military radios, land mobile and satellite transceivers.
This level of flexibility and the amount of variables an SDR must be able to accommodate complicate design requirements and introduce the need for new testing methodologies. In addition to network control of the operating frequency, more advanced SDRs allow control of modulation scheme, frequency hopping patterns, power levels, coding schemes and data rates.
The dynamic generation of radio frequency waveforms through digital signal processing (DSP) and the integration of digital and RF circuits, often on the same IC, also create issues. This added complexity presents not only RF design challenges, but also changes the nature of RF testing. The performance of SDR transmitters must be verified with measurements that are beyond the traditional RF transmitter conformance tests.
Error location
These transmitters also make use of their inherent intelligence and flexibility to adapt themselves to current conditions and requirements. These complex software- controlled changes commonly cause glitches, intermittent interference, pulse aberrations, digital-to-RF couplings and software- dependent phase errors. Addressing these new transients and problems requires SDR system designers to analyse and characterise their system simultaneously in both the time and frequency domains. As system parameters change over time, anomalous signal events and non-linear device behaviour can be readily discovered with DPX, showing a live RF signal representation.
Performing frequency selective triggering is necessary to pinpoint the instant a transient event occurs. Performing multiple domain and time- correlated analysis is required to determine the specific cause of each problem.
Transceiver testing
Verifying the performance of a typical SDR transceiver requires an integrated testing strategy that correlates measurements taken at different points along the transmit/receive chain. For example, an intermittent signal can be captured by the Frequency Mask Trigger (FMT) of a pre- eminent Real-Time Signal Analyser (RTSA). The RTSA uses the frequency mask violation to trigger a logic analyser and oscilloscope, allowing the user to look at the digital and analogue properties of the associated signals. Using this approach, the designer can determine if something in either the logic circuitry or analogue control voltages correlates to the frequency domain violation.
Beyond Steady State
SDR testing includes traditional transmitter testing. Each of the different possible configurations for the radio must conform to traditional specifications such as occupied bandwidth, channel power and adjacent channel power. For systems with time division duplexing or time division multiplexing, there are timing requirements such as rise time and fall time. For frequency hopping systems, there can be both frequency and time domain specifications related to the hopping PLL system. Unlike a conventional transceiver, the SDR device must pass these tests under a much wider variety of operating modes.
Modulation quality measurements are also a significant element of conformance testing. For digitally modulated signals, these usually include Error Vector Magnitude (EVM) or correlated power (RHO) measurements. Moreover, SDR designs that support analogue modes must pass analogue modulation conformance tests. Modulation quality is both a conformance measurement as well as a system performance issue. Poor EVM reduces data rate, clarity of voice transmissions and transmit range. The EVM measurement also provides insight into potential transmitter problems. Conformance testing alone is not sufficient for ensuring a SDR works properly. In order to achieve network flexibility, each SDR device has to change significant operating parameters over time to keep up with the network demands and all changes are implemented by software controlling transceiver hardware. It is essential to have a tool to help capture possible RF glitches, transients and other anomalies and device behaviour changing over time. Determining which component has caused the problem can be a significant task as well, and a thorough troubleshooting strategy is required.
Leading RTSAs offer powerful capabilities for SDR troubleshooting. Firstly, it is important to discover the existence of a problem in the physical layer. These transient events can occur very quickly, and today’s RTSAs give designers the ability to observe them in the frequency domain as they change over time. Using the RTSA to discover anomalous signal behaviour, the user triggers, captures and analyses the associated signals in multiple, time- correlated domains. This ability to extend beyond pure conformance testing is necessary for dynamic signal characterisation and troubleshooting.
Frequency hopping and transmitter testing
Frequency hopping is used in many systems, including those that are software defined, to avoid detection, jamming and interference and to improve performance in an environment with multi-path and fading. Frequency hopping spreads the information over a wide range of frequencies. This makes systems more robust because frequency- dependent errors, such as interference, result in loss of only a fraction of the data. In addition to the common measurements of hop timing, frequency settling time and amplitude settling time, there are several other measurements that can be used to troubleshoot hopping radios using a RTSA. Hopping involves the interaction of frequency, time and modulation domains. The ability to show many domains in a correlated fashion can be an invaluable tool in troubleshooting SDR devices.
Taking a digital phosphor display of a Bluetooth device hopping as an example, digital phosphor display technology, traditionally used in advanced oscilloscopes, has been applied to the RF domain and is now employed by select RTSAs. In allowing users to view live RF signals for the first time, digital phosphor technology provides unmatched insight into RF signal behaviour.
The RTSA’s spectrogram shows the frequency behaviour of a Bluetooth signal over time. There is spectral energy around these hops and it is possible that the transmitter may be interfering with neighbouring devices. It is important that the instrument used to capture the frequency hopping has a wide enough real-time bandwidth to capture a large partition of the hopping sequence bandwidth, as well as the frequency splatter that can occur around it. Implementation challenge While Bluetooth is not necessarily implemented using a software radio, it provides a good example of the challenges that arise when trying to implement a frequency hopping system. For most frequency hopping systems, it is important to be able to measure each of the hopping frequencies. For this measurement, the instrument being used to measure the hopping sequence must cover the entire hopping range.
In another example, the RSA is used to troubleshoot an infrequent signal that is difficult to detect. This could be the result of a frequency-switching transient, which can also result in an even larger phase transient. This could be caused by improper control of a PLL circuit to change to a particular frequency. Once a glitch has been identified using digital phosphor technology, the FMT of select RTSAs can reliably capture the signal for in- depth analysis.
The frequency mask is user-defined and can be drawn to best capture the signal. In the example of a Bluetooth frequency hop, the user is able to define the mask to trigger on a specific frequency hop, rather than the change in power level. The digital phosphor display demonstrates that the signal hops to roughly 3MHz above the signal of interest.
JONATHAN MEES is EMEA market development manager, RF Test, Tektronix
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