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Characterise vector response of broadband instruments

Posted: 18 Mar 2014  Print Version  Bookmark and Share

Keywords:Frequency spectrum allocation  3GPP  wireless systems interoperability  Vector signal generators  LTE 

Frequency spectrum allocation for wireless communications is exceeded by demand and regulators are tackling this issue by re-releasing 'white space' and frequency blocks previously used for analogue services. In turn, the specification standard bodies such as 3GPP [1] define performance metrics and measurement configurations for wireless system. Their objective is to guarantee compliance with frequency allocation and to ensure interoperability of wireless systems as nobody wants products that do not work together or cause interference. The challenge for designers of both RF frontends [2] and digital circuits is to accommodate the wider range of frequency channels and achieve acceptable performance. Measurement is about commerce so the instrument manufacturers that support the development and manufacturing processes face similar issues of performance demands at acceptable cost. Engineering is the art of the achievable and so this is a dynamic process with considerable interaction between these players.

Let us briefly introduce properties of the main measurement instrumentation used in R&D of wireless systems. Vector signal generators (VSG) currently work with carrier frequencies up to 30GHz or more and have a complex modulation bandwidth (in-phase and quadrature) of up to about 2GHz. A vector signal analysers (VSA) is the instrument of choice to analyse modern broadband communication standards, such as LTE. Commercially available instruments have demodulation bandwidth of up to 300MHz operate over a similar frequency range as the VSG.

In order to check instruments specifications, it must be sent to an independent or manufacturers calibration laboratory where parameters are measured with uncertainty traceable to basic SI units. Traceability means an unbroken chain of comparisons relating an instruments measurements to a known standard. This is well established for parameters such as RF power, however, traceability is difficult to achieve for dynamic measurements such as the vector response of a VSA or error parameters of modulated signals, where no accredited method exists yet. This especially holds for error vector magnitude (EVM), which is one of the key parameters used to define the performance of wireless communication equipment and circuits. Manufacturers of measurement instruments use slightly different methods to calibrate their own instruments. Moreover, the calibration usually forms a closed loop, e.g., for calibration of a VSG, one needs a calibrated VSA and vice versa. These methods are clearly not traceable to standards of a higher precision.

In order to overcome these drawbacks, a manufacturer-independent calibration method has been developed by a consortium of national metrology institutes in the framework of the EMRP IND16 Ultrafast joint research project [3]. In this article we will describe a method for achieving manufacturer independent traceable characterisation of VSAs vector response and for EVM measurement.

Traceable measurement of EVM
A traceable approach to EVM measurement is to use a well characterized sampling device such as an oscilloscope to sample the signal directly at the carrier frequency or a VSA to sample the signal at the intermediate frequency or in the base band. The second step is a signal processing and EVM calculation using a validated method, which means the use of external software where all computational steps and errors are open to analysis. In the framework of current project [3], freely available software is being developed for the EVM and uncertainty calculation of basic modulation schemes. A digital real-time oscilloscope (DRTO) is attractive for measuring the waveform as it can acquire over long epochs and provides a high degree of oversampling, compared with a VSA. A digital sampling oscilloscope (DSO) can provide the calibration link to the primary standard (electro-optic sampling system, EOS) but is not suitable for measuring communication waveforms for a variety of reasons. However, they can be used with a VSG to calibrate a VSA. The drawbacks of the DRTO are low vertical resolution, typically less than 8 bits, and the volume of data (10 ms at 5 GSa/sec is a 50 Mb file). This is mitigated to some extent by the temporal oversampling. Also, the multiple A/D converters within these instruments give rise to some non-linear behaviour and a complicated error structure. Measuring at certain frequencies introduces spur components within the measurement bandwidth. In general this can be mitigated by multiple measurements.

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