Using PXI and RF Products to Create the URT RF Signal Record and Playback System
"When you’re dealing with the most prestigious automakers in the world, quality is of the utmost concern. The high-throughput RF streaming capabilities of NI PXI technology has made it efficient for us to record and play back all common analog and broadcast signals in use today, including AM/FM, GPS and HD Radio"
- Sophie Gigliotti,
Developing a cost-effective, complex, portable system that captures live RF signals as they are, in the field without any demodulation or alterations for later playback in the laboratory in order to reproduce the real-life environment and have a device in the lab to react as much as possible like if it was in the field.
Using the NI PXI platform and NI RF instruments we have developed a sturdy and reliable system that meets the requirements of a record and playback system usable in a field environment. The Averna URTTM (Universal Receiver Tester) RF Record & Playback System leverages the PXI express technologies into a complete turnkey solution for recording RF signals such as radio, video, GPS, wireless, and others.
With the trend in RF receivers to offer more and more RF related features embedded in an integrated device, the need for field testing is becoming more important then ever as this allows validation of the RF devices’ behaviour under various conditions such as: physically degraded signals, interference signals, receiver in motion, unpredictable content, particular implementation of an RF standard in a specific country, products designed in one area of the world for another one (live signals not available where the product is designed). Field testing also allows the receivers to be exposed to real life impairments such as: large-scale fading, Rayleigh fading with multipath, adjacent channel interference, co-channel interference, and impulse noise.
Car navigation systems with built-in TV, radio tuners, GPS, and traffic message services; smart phones with GPS and radio features; and radios supporting several broadcast protocols are just a few examples of these all-in-one RF receivers.
Despite the benefits, field testing is still costly, time consuming, and not repeatable. As such, RF receivers’ manufacturers often limit their field tests to only a few geographic areas, for short periods of time.
As an alternative to using channel simulators based on mathematical models, and spending hours on field testing, a record and playback system may be used to capture real-life RF signals in the field for later playback in the laboratory.
Considerations for choosing the development platform
When choosing the development platform for the application, we had to take into consideration the specifications that a record & playback system must meet. These include wide bandwidth to acquire specific signals entirely such as the complete FM band (20MHz), DVB signals (8MHz) and GPS signals (5MHz) and a good dynamic range to capture interference signals, weak signals in the presence of a large one, etc. Also the system needs to be capable of acquiring data for a longer period of time (3 hours, the volume of data depends on the sample rate) in order to capture a realistic sample of real world conditions. Furthermore, attributes such as minimal signal degradation and the ability to acquire and broadcast multiple signals simultaneously are important as well. Signal frequencies up to 3.3 GHz have to be captured for later playback or post-processing.
The ease of use of the solution is also important as in that case technicians can also perform the tests and there is no need for senior engineers.
We chose the build our system using the NI PXI platform with modular RF instrumentation. The new PXI express bus provides the industry’s highest bandwidth which allows to stream RF data from and to disk at 200 Mb/s. Also PXI provides a dedicated synchronization bus which allows recording and broadcasting of multiple RF signals simultaneously. National Instruments’ modular RF Instrumentation offers up to 50 MHz of instantaneous bandwidth for signals up to 6.6 GHz. Tight integration with other PXI instrumentation and external RF instrumentation as well as the ruggedness of the NI PXI platform where other decision factors.
We have developed the application using NI LabVIEW Graphical Programming. Its ease of use and the built-in tools and available toolkits (NI Modulation Toolkit) have helped us reducing development time significantly.
Implementation of the Averna RF Record & Playback System
The system is incorporated in an 8-slot or 18-slot NI PXI chassis to house components of both the record and playback system. In case that the recording is performed in standard cars or vans instead of specially shielded vehicle, the PXI chassis is enclosed in a shielded box designed specifically for mobile recording applications, with a DC to AC power converter, to avoid contamination of the recordings by the system noise.
The recording of the RF signals is performed by an NI RF downconverter which captures real-life RF signals along with their impairments and converts them into baseband signals. We have used a NI vector signal analyzer to transform the captured baseband signals into 16 bytes IQ samples. These samples are then stored into an external storage device using a SATA connection, or are stored within an in-chassis storage unit using the PXI Express bus. RF signals of up to 20MHz of bandwidth, located anywhere between 500 kHz and 3.3 GHz can be captured for later playback or post-processing analysis.
The recorder system is also equipped with a preamplifier, with manual or Automatic Gain Control (AGC), to ensure the dynamic range is optimized for the input power. It also comes with a built-in Low Noise Amplifier (LNA) and can be configured in various ways to accommodate passive or active antennas as well as external components. To complement the RF recordings, GPS coordinates and video images of the recording location may be captured by a multimedia laptop which stores the data as NMEA messages and compressed MPEG 2 streams.
For playback, an NI vector signal generator uses the stored IQ samples and generates baseband signals which in turn are converted into RF signals and impairments using a NI upconverter. The multimedia laptop displays simultaneously the position from the NMEA messages and video streams to complement the RF recordings.
With applications such antenna diversity or multiple signal combinations such as GPS with TMC, recording and playback of multiple channels is gaining importance. Using tight synchronization technology between several vector signal analyzers or generators, accurate recording and playback of multiple channels simultaneously is now possible with the Averna RF Record & Playback System.
"When you’re dealing with the most prestigious automakers in the world, quality is of the utmost concern,” said Hans-Joachim Tepper, Test Engineer at IAV. (IAV is a German company, a leading provider of engineering services to the automotive industry worldwide.) “The high-throughput RF streaming capabilities of NI PXI technology has made it efficient for us to record and play back all common analog and broadcast signals in use today, including AM/FM, GPS and HD Radio™ (IBOC), as well as a stunning range of impairments. The URT Record and Playback System by Averna was instrumental in packaging those capabilities, and giving us the repeatability we needed to meet the exacting standards of our customers. We are confident that this solution will help us to significantly improve our long-term test efficiency."
The RF Record & Playback System of Averna is a new, reliable, and user-friendly alternative to field testing of RF receivers, as it captures real, live RF signals. Time spent on drive tests is significantly reduced, taking only as long as needed to record all the required signals and impairments at the various desired locations and paths. With the readily available recorded signals as references, greater emphasis on in-lab analysis is possible, allowing manufacturers to benefit from accurate, reliable, and repeatable performance testing, while reducing both testing costs and time.
With NI PXI and NI LabVIEW the system is based upon an open and flexible platform that allows upgrades and modifications for any future requirements such as new standards and protocols.
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