1. IntroductionThe National Instruments PXI-5660 2.7 GHz RF signal analyzer extends the speed and flexibility of virtual instrumentation into the GHz frequency range. Based on the latest advances in computer, communications, and software-defined radio technologies, the PXI-5660 provides RF measurements up to 2.7 GHz with throughput from 10 to 200 times greater than traditional instrumentation.
To address a broad range of industries, including consumer, communications, defense, and aerospace, the NI PXI-5660 uses a highly modular architecture based on a broadband 9 kHz to 2.7 GHz RF digitizer combined with the powerful Spectral Measurements Toolkit (SMT) software. The modular RF digitizer hardware enables precise signal acquisition with 80 dB dynamic range and ±50 ppb frequency accuracy, while the SMT software provides the full range of standard RF power, frequency, spectral, and vector measurements.
The revolutionary virtual instrumentation architecture of the PXI-5660 takes customers well beyond the fixed functionality of traditional vendor-defined instrumentation because they can now define unique applications using the advanced signal processing, visualization, and Internet capabilities of National Instruments LabVIEW and LabWindows/CVI. By fitting all of this functionality into a very compact 3U PXI package, the RF Signal Analyzer is a cost-effective solution for diverse applications ranging from component characterization in R&D to the remote monitoring of RF navigation systems deployed in the field.
2. Benefits of Virtual Instrumentation
The NI PXI-5660 uses leading computing and communications technology to deliver superior measurement performance, speed, and flexibility. NI uses the latest analog-to-digital converters (ADCs), digital downconversion ASICs, and ultraminiature acoustic resonator filters and oscillators throughout the RF Signal Analyzer. In addition, by building on the open PXI platform and NI's industry-standard LabVIEW and LabWindows/CVI software, the RF Signal Analyzer reflects the advantages of standard Microsoft and Intel technology.
The virtual instrumentation architecture and commercial technology advantages of the NI PXI-5660 deliver many benefits. Thanks to the low power and high integration of the PXI platform, we were able to fit complete vector RF analyzer hardware to in a very compact 3U module – a dramatic reduction in size over traditional instrumentation. Thanks to the use of high-volume commercial technology components, the PXI-5660 provides customers a very cost-effective solution that meets the reduced budgets facing today's R&D and manufacturing departments. Lastly, by combining a wideband real-time architecture with onboard processing with the speed of the PXI platform, the PXI-5660 accelerates RF measurements by a factor of 10 to 200.
These hardware advances are enhanced by the flexibility of virtual instrumentation. Today’s test systems are moving towards higher frequencies, wider bandwidths, and transmission schemes that require complex and rapidly changing measurements. Trying to build a measurement system that can meet today's needs while retaining the flexibility to meet future demands can be difficult – even more so when cost is thrown into the equation. Because the software layer defines the measurements in virtual instrumentation, it is easy to reconfigure underlying measurement system hardware in response to new application requirements.
For example, the 3D spectrogram shown below combines the hardware vector capture ability of the PXI-5660 with the flexible software processing of LabVIEW and the SMT to give users a unique view of their signal of interest. In the figure below, the modulation signal (a sine wave) is clearly visible in the complex FM signal spectrogram.
Figure 1. Visualize RF 3D Spectrograms with the PXI-5660
This is just one scenario where the ability of virtual instrumentation to quickly reconfigure hardware adds significant value to your application. Instead of limited, vendor-defined applications, you can use the PXI-5660 for any measurement defined in the versatile software environment of LabVIEW or a C programming environment. Together with the SMT software package, the NI PXI-5660 offers a complete set of tools to analyze, measure, and display signals of interest.
The SMT delivered with the RF Signal Analyzer makes it easy to make common RF power and frequency measurements – many of these measurements are application functions that can be customized in a few minutes and run to perform the desired measurement. Using the flexible software analysis capabilities, the user can quickly add new measurements such as 3D spectrograms, phase noise, or carrier-to-noise measurements to the application. For example, the I/Q data available from the SMT can be used in algorithms to perform analog and digital demodulation. By defining your measurements in LabVIEW, you can define new measurements in software as your needs change. Along with the modularity afforded by the PXI platform, this flexibility ensures your measurement solution is "future proof."
3. Product Architecture
The PXI-5660 has a modular architecture consisting of a broadband RF digitizer and the SMT software package. The RF digitizer is a combination of two hardware modules – a broadband RF downconverter and a high spectral purity IF digitizer. The downconverter module includes an onboard ultrahigh-stability oven-controlled crystal oscillator (OCXO) to provide the outstanding frequency accuracy and stability necessary for today's automated applications.
The downconverter acquires signals between 9 kHz and 2.7 GHz with a 20 MHz real-time bandwidth. These signals are frequency-translated to center around 15 MHz, and the translated signal is then output to a fixed IF band between 5 and 25 MHz, which is fed directly into the digitizer. The digitizer module provides 80 dB of spurious-free dynamic range through its 14-bit ADC and also contains a digital downconversion ASIC that provides hardware acceleration capabilities.
All of the acquired data – in both the time and frequency domain – is transferred directly to LabVIEW or a C programming environment for analysis and measurement using the most complete set of software tools available. The SMT contains highly optimized zoom FFT algorithms that offer general frequency-domain analysis, as well as measurement algorithms for computing common spectral measurements such as peak power and frequency, in-band power, and occupied bandwidth. The SMT includes additional common measurement functions to reduce test development time. The SMT outputs the acquired time-domain data for use in I/Q and complex modulation applications.
4. Hardware Architecture
The broadband capability of the downconverter is useful for two major reasons. First, it improves spectral analysis by dramatically increasing measurement throughput. Because it acquires 20 MHz "blocks" of spectrum, PXI-5660 acquisition time is much faster than that of traditional instrumentation. Second, because of the broadband capability, the PXI-5660 captures both frequency and phase data. This information can be used to make I/Q and modulation measurements in software. By using state-of-the-art technology such as ultraminiaturized thin-film-resonator acoustic filters and YIG oscillators, the RF downconverter and IF digitizer modules are both packaged in compact 3U PXI modules.
Figure 2. Downconverter Architecture
The downconverter first applies user-selectable attenuation to any signals up to 2.7 GHz, which are then fed into an upconversion stage. The resulting signals are fed through an acoustic resonator filter for image rejection before entering a multistage downconversion block. The downconverted signals are output in an IF frequency band from 5 to 25 MHz. The downconverter uses a highly stable oven-controlled oscillator that drives the rest of the system clocks and offers ±50 ppb frequency accuracy. To help fit into the compact PXI package, a high-performance miniature YIG-based oscillator is used to generate the LO signal for the high-frequency upconverter stage. Careful frequency planning and the multistage architecture of the PXI-5660 ensure that spurious responses are kept well below the dynamic range of the instrument.
The digitizer module features a fast ADC and onboard hardware processing capabilities. The digitizer is built around a 14-bit, 64 MS/s ADC and a digital downconversion ASIC. This digital downconversion chip performs real-time decimation and downconversion to baseband of any signals with a span of up to 1.25 MHz, ideal for capturing many cellular and other communications channels. The downconversion ASIC also generates complex I/Q data from the spectrum. The digitizer incorporates the patented high-speed NI MiniMITE ASIC for data transfer. The MiniMITE handles DMA data transfers to the host PC, freeing the host CPU to perform advanced analysis, visualization, and communication tasks.
Figure 3. Digitizer Architecture
5. Software Architecture
The SMT uses NI's world-class zoom FFT algorithms to convert the acquired time-domain data into frequency-domain data. The SMT provides common measurements such as zoom power spectrum, peak power and frequency, in-band power, adjacent-channel power, and occupied bandwidth on top of these FFT algorithms. It also provides both continuous and block zoom FFT processing. Continuous-zoom FFT processing is useful for applications where real-time spectral analysis is required; block-zoom FFT processing is most useful as a postprocessing method. When more complex analysis is called for, the SMT can provide magnitude-phase or I/Q data for use in modulation processing; complete analog modulation functions are included.
Figure 4. Software Architecture
NI LabVIEW provides powerful tools designed specifically for extracting useful information from any set of acquired data, analyzing measurements, and processing signals. LabVIEW offers functions that you can easily incorporate into your applications in order to add inline analysis and decision-making capabilities. Available functions include mathematics libraries, advanced signal processing tools, measurement analysis functions such as FFT and power spectrum, signal generation, digital filters, and curve fitting. In addition, National Instruments offers a series of toolkits that extend the analysis capabilities of LabVIEW for more specialized applications, such as sound and vibration analysis, order analysis, and digital signal processing. By building analysis capabilities directly into your application and eliminating the need for postacquisition analysis, you obtain results more quickly.
These myriad analysis functions feed a diverse set of visualization and presentation tools, with which you can view your data in many ways as well as import and export data to other popular software programs such as Microsoft Excel and databases for storing large data sets.
LabVIEW provides enterprise connectivity to the Internet, DLLs, Microsoft Office applications, and databases; and it offers DataSocket, TCP/IP, and a host of other communications protocols. LabVIEW also provides you with a vast set of signal processing tools ranging from wavelets and joint-time frequency analysis to a host of various filters such as FIR, Bessel, Butterworth, and Chebyshev. It is possible to add array and waveform-based measurements to your applications, including power and FFT spectra, power spectral density, and amplitude and phase spectra.
LabVIEW not only takes your local measurements, but also gives you the Web and other connectivity options to share your data with colleagues, customers, and suppliers around the world. Using Microsoft’s ActiveX technology, you can connect to local and remote databases and effectively manage your larger applications. LabVIEW also gives you Internet functionality to publish documents and front panels, a built-in Web server, and libraries to add e-mail, CGI programming, and FTP capabilities to your application.
6. System Performance
The PXI-5660 offers more than 80 dB of intermodulation dynamic range, useful for demanding measurement applications in communications and other areas where complex signals must be displayed. The PXI-5660 also has a typical noise density of less than -140 dBm/Hz, which provides a wide range of signal acquisition. The graph below is a second and third-order distortion plot showing the dynamic range available from the PXI-5660.
Figure 7. Distortion Plot of the PXI-5660 RF Signal Analyzer
The PXI-5660 has an ultrahigh-stability timebase with its OCXO, which provides frequency stability of ±20 ppb and frequency accuracy of ±50 ppb. This performance creates an environment for highly stable, precise frequency measurements. The SMT also contributes to the high frequency accuracy of the PXI-5660 because it contains a patent-pending spectrum-peak-search algorithm that can locate spectral peaks that lie between spectral bins. The SMT uses a curve-fitting algorithm on three points around each detected peak to estimate true frequency and amplitude. If the peak frequency falls exactly on one of the FFT bins, then you can accurately determine the peak frequency using a simple threshold peak detection technique. However, if the peak frequency does not fall on a bin, then this simple technique introduces error. The SMT algorithm takes into account the shape of the window, applies a curve fit to three data points around each detected peak, and then calculates true frequency and amplitude. The amplitude/frequency estimation method works best on averaged power spectra because the averaging reduces the noise and provides a more consistent measurement.There are significant advantages to making RF measurements on the PXI
platform, the most dramatic of which is the increase in measurement throughput. The PXI data bus is based on the PCI bus, which offers data transfer rates orders of magnitude faster than traditional instrumentation. Using the PXI platform also centralizes all the processing capability of a measurement system. Traditional measurement systems are built around a number of stand-alone instruments, each with its own onboard processor. They are then connected to a PC whose processor handles the control aspects of the system. With PXI, the same processor both controls the acquisition hardware and performs measurement analysis on the acquired data, thus simplifying test development and improving efficiency.
The PXI-5660 takes advantage of the integration benefits of the open, multivendor PXI platform. Many different types of PXI modules can reside in the same chassis. The chassis provides for tight timing and triggering integration between all of the various PXI modules. For example, DC/transient current measurements and RF switching are commonly used in many RF test/characterization setups. There are a variety of DMM, RF switching and relay control modules available in PXI to develop an integrated mixed-signal test system.
One specific benefit of the PXI-5660 is its ability to share the onboard high-performance OCXO-based reference clock with all the other PXI modules present in the system. The PXI-5660 can drive its reference clock to the PXI backplane, to which other modules can synchronize to create a fully synchronized PXI-based measurement system. Trigger signals can be shared across the backplane in a similar fashion.
The flexibility of LabVIEW-based software makes it easy to integrate spectrum and modulation measurements within a single measurement framework. The SMT simplifies development by including common measurements – such as adjacent channel power, in-band power, occupied bandwidth, and a 3D spectrogram for joint time-frequency analysis – in application functions and basic modulation examples. In addition, with access to all the raw data and the myriad analysis tools available in LabVIEW, it is easy to incorporate custom measurements that may be unavailable in traditional instrumentation.
8. Measurement Throughput
One of the most critical economic benefits of the NI PXI-5660 architecture is its measurement throughput. By delivering throughput up to 200 times that of traditional instrumentation, the PXI-5660 cuts design and production test debug time and manufacturing cost. There are many features that contribute to this throughput advantage. With vector-based architecture, the PXI-5660 captures a wide band of spectrum simultaneously, which leads to faster acquisition times than traditional instrumentation. The ASIC digital downconversion chip on the digitizer acts as a hardware accelerator when acquiring spans less than or equal to 1.25 MHz because of downconversion and decimation. In addition, the NI-patented MiniMITE chip gives the digitizer DMA capability to transfer data directly to host PC memory through the PXI bus, a 32-bit, 133 Mbytes/s data bus based on the PCI standard. This optimized data transfer removes a long latency present in traditional instrumentation systems.
The SMT also plays a significant part in the throughput advantages of the NI PXI-5660. The FFT algorithms underlying the SMT are some of the fastest in the world. The SMT zoom FFT and power spectrum algorithms perform calculations corresponding to the frequency span of interest – not the entire frequency spectrum – thereby increasing overall measurement throughput.
Engineers can use the power of LabVIEW and its integration with the PXI-5660 to increase measurement throughput further by performing multiple measurements on a single spectrum acquisition. Once a digitized block of time-domain data is converted to frequency-domain data, several measurements can be made on the same block of frequency data because LabVIEW takes spectrum measurements without altering the spectrum data. In contrast, traditional instrumentation typically requires a separate acquisition for each measurement.
Figure 9. Acquisition Speed Comparison: Spectrum Analyzer vs. NI PXI-5660
The PXI-5660 has been employed in systems for EMC testing, cable modems, and wireless applications. One National Instruments partner has developed a test system for a two-way wireless e-mail device using the PXI-5660 integrated with other National Instruments tools, reducing the time needed to test the transmitter on the device. Another customer has taken advantage of the small size and flexible programming of the PXI-5660 to develop a modulation receiver for an onboard aircraft position-tracking system. This customer was able to quickly add demodulation measurements to the system using the programming power of LabVIEW and additional routines from National Instruments. When the application required a new series of warning LEDs, it was easy to add this capability within the same analysis and measurement program using available National Instruments data acquisition hardware.
Applications for the NI PXI-5660 RF Signal Analyzer include:
- RF IC and component characterization
- Wireless consumer technologies such as 802.11 and Bluetooth
- EMC compliance testing
- Mobile, satellite, and point-to-point communications systems
- Signal surveillance and monitoring
- Analog/digital television and cable systems
- Avionics system test, monitoring, and field maintenance
- Academic research and teaching labs
Several of these technologies – cellular, 802.11b, Bluetooth – are excellent examples of wireless functionality horizontally integrated into existing devices. This diversity of devices and applications places increasing value on a measurement solution that can quickly be reconfigured from one application to the next as needs evolve.
In conclusion, the PXI-5660 RF Signal Analyzer offers significant advances in measurement throughput, flexibility, and ease of customization for RF measurements. The PXI-5660 takes advantage of its vector architecture to deliver superior performance and the PXI platform to provide flexible modularity and excellent throughput for end user applications.
Because the NI PXI-5660 fits into an open, industry-standard platform, it can be combined with existing and future measurement modules from many companies to build a complete, flexible measurement solution. Additionally, the NI PXI-5660 harnesses the power of National Instruments LabVIEW software to deliver customizable, easy-to-use measurements that can be combined with powerful visualization, report generation, and Internet connectivity.