Wireless Waveform Creation Software for Arbitrary Waveform Generators
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AWGs can be used in RF applications in different ways depending on the test requirements and hardware capabilities.
Author(s):
Joan Mercadé - Arbitrary Resources S.L.
Industry:
Products:
The Challenge:
Providing a simple to use, easy to interface software solution for high quality, spurious-free wireless signal generation using high-performance, multiple architecture Arbitrary Waveform Generators
The Solution:
Using the unique combination of the user interface creation tools and signal processing and display capabilities provided by the LabWindows/CVI development environment and the interface-independent VISA I/O library and IVI drivers has resulted in a compact but powerful wireless signal generation software solution for all kind of users.
"The NI PXI-5422 incorporates parallel digital outputs that convert them into pattern generators. This capability is especially useful in testing DSP-based RF architectures such as software-defined radio (SDR)."
Arbitrary waveform generators (AWG) are among the most powerful and flexible tools available to any test engineer. Their unique capability to generate any signal within the reach of their basic specifications makes them the instrument of choice when real-life signals are required.
As in other test and measurement devices, wider, deeper, and faster explain the evolution of AWG performance. AWGs with high sampling speed, high vertical resolution, and good linearity are widely available in a variety of formats such as benchtop, GPIB/USB/Ethernet controlled instruments, and PCI/PXI cards such as the National Instruments PXI-5422. This increasing performance is helping engineers to better address traditional applications and at the same time, it opens new application areas such as RF/wireless testing. Using AWGs in such an application opens the way to unique capabilities such as creating a virtually unlimited number of simultaneous, dissimilar carriers and extremely high symbol rates.
Applying AWGs to RF
AWGs are not new to RF/wireless engineers. They have been used for years to generate complex waveforms, especially baseband (I-Q) or relatively low-frequency IF signals. The current AWG’ performance makes feasible generating RF signals directly at the carrier frequency instead of using a complementary quadrature modulator or up-converter. These signals can be used directly in many tests at both ends of the signal path. Virtually any distortion, linear and non-linear, can be added to the signal. Although some RF tests require high-quality, low-distortion signals, many others use real-world impairments. AWGs can easily implement multipath, predistortion, or modulation impairments, even simultaneously.
AWGs are general-purpose tools that can be used in many applications besides RF/wireless test what results in more flexible and multi-purpose test systems. Some high-end AWGs, such as the NI PXI-5422, also incorporate parallel digital outputs that convert them into pattern generators. This capability is especially useful in testing DSP-based RF architectures such as software-defined radio (SDR).
Making it work with Modula(r)
Real-life multicarrier, digitally modulated signals involving many symbols results in long record length waveforms and require tens of millions of complex calculations that may need a long compilation time. Number-crunching time is important when the application requires a high degree of interactivity or the signal contents depend on previous test results or device under test data.
Calculation time is not always an issue. In some cases, the required signals are calculated once and stored in an internal non-volatile memory in the target AWG. If this is not the case and waveforms must be calculated all the time, fast computers and efficient software packages are required. The Modula(r) software package from Arbitrary Resources is an example. It uses algorithms that do not require exact calculations at the baseband level while maintaining calculation noise beyond the requirements for ideal better-than-16-bit vertical resolution AWGs. As many test set-ups require playing back the signal longer than allowed by the waveform memory available at the target AWG the only way to operate is by looping the signal. Keeping the integrity of the signal at all levels (carrier, symbol, base-band filtering, and channel coding) is paramount is order to make the test work properly. It is necessary then to maintain the continuity of the signal between the end and the beginning of the waveform to avoid any impairment resulting from the “wrap-around” artifact that could ruin the test results. The Modula(r) software package take care of automatically adjusting all the parameters for any number of dissimilar carriers so there are neither wrap around artifacts nor undesired side effects such as spectral growth, high EVM values, or even unlocked DUTs. Given that many mid-range and high-end AWGs supports memory segmentation and the definition of complex sequences, Modula(r) can also calculate those segments to be continuous too and avoid any problem. This strategy is extremely useful to create a more realistic behavior and specific impairment sequences that could not be accommodated in the available waveform memory.
As Modula(r) is designed to be used by a vast range of users and AWG platforms, it supports most modulation schemes, from ASK up to OFDM, and can be applied to any carrier frequency, symbol rate, and target instrument’ sampling rate and record length while keeping the capability of calculating, even simultaneously, baseband I and Q components and IF/RF waveforms. Although it implements most of the industry standards regarding modulation schemes, baseband filters, and data sequences, it provides with tools to create user-defined constellations and symbol mappings and import externally defined data, baseband filters, symbol lists, and even I and Q waveforms.
LabWindows/CVI and VISA are the key enablers for Modula(r)
Developing Modula(r) has been a challenge as a new set of algorithms specifically tailored to the needs of AWGs in RF applications had to de created. The availability of a rich set and easy-to-use mathematical and signal-processing function libraries in LabWindows/CVI provides the tools necessary to concentrate on solving the problem instead of just coding the math. Those can do what much more expensive math tools can only deliver to specialists. Complex functions required in this application as convolution, re-sampling, and interpolation can be performed with jus a few lines of code. Although Modula(r) is a waveform creation tool, most users require some analysis and visualization capabilities in order to validate the signals and/or compare their characteristics against those at the device under test. The graph controls, array processing, and statistical analysis functions in LabWindows/CVI are used to easily calculate parameters such as the Peak-to-Average-Power-Ratio (PAPR) and to display graphs such as Constellation Diagrams, CCDF (Complementary Cumulative Distribution Function), Eye Diagrams, and others.
Modern AWGs use a variety of interfaces to communicate with Controllers. Besides the traditional GPIB interface, some bench top instruments can use Ethernet (with a variety of TCP/IP protocols on top of it), RS-232, and USB. Card based AWGs can be connected to a variety of buses such as PCI, including Compact PCI and PXI, and VME/VXI. As Modula(r) is an open tool designed to be used with a large variety of instruments, the control strategy is extremely important. Coding the commands for each instrument and using different APIs for every interface and protocol would make the process of adding the support for an instrument painful, even when using a compatible instrument driver. The standard VISA API makes communication with instruments interface independent and the availability of IVI (Interchangeable Virtual Instrument) driver makes control independent of the specific instrument command set. As Modula(r) deals basically with waveforms transfers to the target instruments, adding a new IVI compatible instruments basically requires including just some information about the basic hardware characteristics of each device such as the maximum sampling speed, total record length and, in some cases, the granularity of the record length as some instruments require it to be a multiple of a given number.
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