PXI-Based RF Record/Playback System

Author(s):
Lance Butler - B&B Technologies, an NTS division

Industry:
RF/Communications, Machines/Mechanics

Products:
LabVIEW, PXI/CompactPCI,

The Challenge:
Developing a cost-effective, portable system to record RF transmissions in the field and to play them back in the lab for simulated RF testing.

The Solution:
Using the PXI platform and NI modular instruments combined with Conduant’s StreamStor technology, we can record 20 MHz wide signals at up to 2.7 GHz center frequencies for more than 4 hours. We then can play these signals back in the lab.

Many of our customers face the same problem when developing RF receiving devices such as satellite radios. They need to test their radios in real-world environments but face several difficulties in bringing their products to the field for testing. A major problem is that the environment is constantly changing due to seasonal factors such as wind, rain or snow or even foliage on trees. The environment also changes with the construction of buildings and installation of new sources.

Another important consideration is the logistics involved with bringing large quantities of product into the field for testing. Particularly in a production environment, the thought of testing product in the field is daunting with the logistics alone.

PXI to the Rescue

Our PXI system solves all of these problems by simulating a real-world environment in the lab or on the production floor. Test personnel can now travel to any given environment just once to record a signal and then play that signal under a controlled environment in the lab to an unlimited number of units under test (UUTs).

For example, a company testing satellite radio might record a set of files in the Austin metro area periodically through the year to capture effects of various seasons, weather effects, and areas of town. They might also record under the same circumstances in other areas. These files can then be reliably played back repeatedly so that they can test different designs with the exact same signal in order to compare the strengths and weaknesses of the designs.

The open architecture nature of PXI is the key to making this system a reality. The system is comprised of NI instruments that take advantage of the company’s strength in RF and modular instruments   together with Conduant hardware that makes use of their specialization in extremely fast data storage and playback. PXI allows us to combine the strengths of both of these companies and create a solution that would be impractical for a single entity alone.

How it Works

RF input signals are routed through the NI PXI-5690 if needed to increase the signal levels so that they are suitable for recording. The signal is then run through the NI PXI-5600 for down conversion to an appropriate frequency. The NI PXI-5122 then digitizes the signal and routes the digital data through a DMA channel. The Conduant StreamStor card then simultaneously funnels the data onto four hard drives.

With this combination, the system records data for more than four hours without interruption. While additional drive arrays can be purchased for additional record time, we found that this time is sufficient for most applications.

For the playback process, signals are essentially routed in the reverse order through playback hardware. The StreamStor card feeds data from the drives to the DMA channel on the NI PXI-5671. The data is then played through a digital to analog converter (DAC), upconverted to the recorded frequency, and sent to the UUT.

A utility further extends the capabilities of this system by allowing files to be moved to or from the drives. In the former case, this allows files to be ported into any analysis package where they can be analyzed or modified. The latter case allows for synthesized signals to be played to simulate various effects. As an example, a user may record a given signal and then modify it to simulate periodic signal strength loss. This new file can then be played to test the effects of the change versus the original file.

Flexibility of PXI for Many Options

The most common systems involve a record system in a portable configuration that typically includes a shock resistant shippable container and a rack mount playback system for the lab. However, thanks to the modular nature of PXI and the power of LabVIEW, other configurations have been developed for various needs.

When the source signal is readily available at the lab, but the requirement for testing multiple UUTs with the same signal still exists the two systems can be combined in one chassis. This version uses an 18-slot PXI chassis to house components of both the record and playback system and uses the same LabVIEW software as the other systems.

If two channels are required to be synchronously recorded, additional hardware can be added. In this case the RTSI bus inherent in PXI offers tight synchronization of the two channels. The two channels can come from different sources for diversity testing, or they can be set for different frequencies. The two-channel playback version of this system maintains the synchronicity throughout the process.

PXI also allows for the integration of GPS position data into the recording. This can be useful when terrain features are of particular interest.

Because LabVIEW already has many of the necessary tools built in or available as toolkits and PXI drivers are easy to develop, this system represents a significant cost savings compared to any other method. The cost of the initial system including development is considerably lower than the cost of other products which have less functionality.

For more information, contact:

Timothy R. Brooks

NTS

5200 Pasadena Ave NE

Albuquerque, NM 87113

Tel: 505-345-9499

E-mail: tim.brooks@ntscorp.com

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