PDF1. Introduction
In many spectral monitoring and signal intelligence applications, you may need to perform communications data processing continually in real time. For one common technique, you monitor the RF spectrum for unanticipated transmissions by comparing an acquired RF spectrum against an expected frequency mask. The traditional swept-spectrum approach performs this comparison as frequently as the measurement system can acquire and process the data, but the processing limitation creates periods where the RF band of interest is not monitored. It is probable that packet-based communication schemes such as GSM or WiMAX could evade detection because there may not be a transmission when the analyzer is sweeping their frequencies.
You can perform data processing by streaming baseband I/Q data over the PCI Express bus from a baseband digitizer back to its host controller. Modern multicore processors such as the one on the NI PXIe-8106 embedded controller can scale, window, Fourier transform, and compare this data against a frequency mask at rates greater than 2.5 MS/s, effectively monitoring up to 10 MHz of RF bandwidth. For applications that require greater bandwidths or in which the controller CPU is required for other functions, field-programmable gate arrays (FPGAs) can perform the same processing at even higher rates. For instance, the NI PXIe-5641R IF transceiver can perform eight simultaneous, complex, 1024-point fast Fourier transforms (FFTs) at an average rate of more than 50,000 FFTs per second. This is sufficient to continually monitor 20 MHz of real-time bandwidth. You can stream from a device to the host and back to another device with DMA, which does not significantly tax the bus or CPU. This is therefore a viable architecture for baseband data processing. These techniques combined with other PXI features – GPS synchronization and triggering, integration with other instruments, and a small form factor – make PXI an ideal solution for spectral monitoring and signal intelligence.
2. FPGA-Enabled Spectral Monitoring and Signal Intelligence System
You can continually monitor and record the RF spectrum by pairing the NI PXIe-5641R RIO IF transceiver with the NI PXI-5600 downconverter and NI 8260 high-speed data storage module. With this combination, you can create an FPGA-enabled spectral monitoring or signal intelligence system on the PXI platform covering frequencies from 250 kHz to 2.7 GHz with 20 MHz of real-time bandwidth. The NI PXIe-5641R features two independent 14-bit, 100 MS/s analog-to-digital converters (ADCs) with built-in 20 MHz bandwidth digital downconverters. This architecture efficiently delivers I/Q data to the Virtex-5 SX95T FPGA on the module for analysis, demodulation, and signal processing tasks using the NI LabVIEW FPGA Module. For continuous logging, you can stream 20 MHz of RF bandwidth (100 MB/s) per FPGA module to a RAID-based hard disk such as the NI 8260 for several hours. With the configurability of LabVIEW and LabVIEW FPGA, you can program the NI PXIe-5641R for real-time signal analysis and event detection in hardware. This section explores the system components of this FPGA-enabled spectral monitoring and signal intelligence system. Figure 1 shows the individual hardware components inside the NI PXIe-1075 chassis.
Figure 1. PXI-Based Spectral Monitoring and Signal Intelligence System
To view a spectral monitoring application, visit the Spectral Monitoring Demonstration page.
Transceiver
The NI PXIe-5641R contains a 20 MHz bandwidth intermediate frequency (IF) transceiver that interfaces with a DSP-optimized Xilinx Virtex-5 SX95T FPGA. This FPGA, programmable using the LabVIEW FPGA Module, is capable of performing complex demodulation and signal processing in hardware at high rates with low-latency added benefits for applications that require quick reaction times and high input signal bandwidth. The NI PXIe-5641R features two IF inputs and outputs for interfacing with analog upconverters and downconverters.
Figure 2. NI PXIe-5641R RIO IF Transceiver
Downconverter
The NI PXI-5600 accepts RF signals from 9 kHz to 2.7 GHz with up to 20 MHz bandwidth and then downconverts them to 15 MHz IF signals, providing excellent integration with the NI PXIe-5641R. Its superheterodyne architecture ensures that out-of-band signals do not compromise input dynamic range. Select up to two downconverters to independently tune to distinct RF frequencies through each of the two NI PXIe-5641 IF inputs. The PXI-5600 is a compact, two-slot, 3U PXI module.
Figure 3. NI PXI-5600 Downconverter
High-Speed Data Storage
The NI 8260 is a four-drive RAID. This in-chassis, high-speed data storage module has a total capacity of 1 TB and is capable of recording 20 MHz of RF bandwidth for more than 2.5 hours. With the NI 8260, you have a reliable and rugged but compact solution for in-field spectral monitoring and signal recording applications. Using LabVIEW VIs, you can play back the recorded spectrum for post-acquisition analysis.
Figure 4. NI 8260 Four-Drive RAID
PXI Controller
The NI PXIe-8106 is a high-performance PXI Express controller that you can use to complement the fixed-point signal processing on your FPGA with floating-point processing on a high-speed dual-core processor. The PXI controller is the CPU of the PXI system. With PXI modularity, you can separate the RF circuitry from the processing capability of the instrument. CPUs evolve much faster than RF circuitry technology, so if a new processor or advanced transducer stage becomes available, you need only to change the system element that has improved rather than buy an entirely new instrument.
Figure 5. NI PXIe-8106 Controller
National Instruments provides dual-core and quad-core controllers in PXI and PXI Express form factors.
Software
The NI PXIe-5641R is programmable using the LabVIEW FPGA Module. LabVIEW FPGA is distinctly suited for FPGA programming because it clearly represents parallelism and data flow into FPGAs. With the LabVIEW FPGA Module, you can create custom measurements and embed your digital signal processing algorithms, demodulation schemes, and control hardware without low-level hardware description languages or board-level design.
LabVIEW software provides a unique and easy-to-use graphical programming environment ideal for multicore processor programming. With this software, you can create parallel algorithms that take advantage of multicore processors. Moreover, PXI Express modular instruments enhance this benefit because they take advantage of the high data transfer rates possible with the PCI Express bus. Two specific applications that benefit from multicore processors and PXI Express instruments are multichannel signal analysis applications such as spectral monitoring and signal intelligence applications.
Figure 6. System Software Components
VIs created in LabVIEW FPGA, called FPGA VIs, run on the FPGA of the NI PXIe-5641R. VIs created in LabVIEW, called Host VIs, run on the host. FPGA VIs and Host VIs communicate in the background and transfer data to one another using DMA, which does not significantly task the CPU because data is written directly to RAM. You can program the NI PXIe-5641R to demodulate or decode baseband signals and transfer the resultant data to the host. Once the data is on the host, you can use LabVIEW to analyze and present it.
Extend the built-in capabilities of LabVIEW with powerful host-side tools (VIs) for analyzing, processing, and visualizing your data using the LabVIEW toolkits described in the next sections.
NI Modulation Toolkit
Add signal generation, analysis, visualization, and processing of standard or custom digital and analog modulation formats. With this toolkit, you can rapidly develop custom applications for research, design, characterization, validation, and test of communications systems and components that modulate or demodulate signals.
NI Spectral Measurements Toolkit
Take advantage of flexible host-side spectral measurements functions to measure parameters such as in-band power, adjacent-channel power, and occupied bandwidth.
NI LabVIEW Advanced Signal Processing Toolkit
Access powerful time-frequency analysis functions and create digital filters that run on the host or on the FPGA using the LabVIEW Digital Filter Design Toolkit, which is included with this toolkit. Visit the NI LabVIEW Advanced Signal Processing Toolkit page to learn more.
3. Additional Resources
Reconfigurable I/O Intermediate Frequency Transceivers
NI PXIe-5641R Product In-Depth
Spectral Monitoring with the NI PXIe-5641R
Spectral Monitoring Demonstration
Software-Defined Radio (SDR) System
Introduction to Programming Using Asynchronous Wires
NI PXIe-5641R RIO IF Transceiver Example: Using the IF Transceiver with an NI PXI-5600 Downconverter
