1. Common RF Analysis Methods
RF measurement methodology can generally be divided into three major categories – spectral analysis, vector analysis, and network analysis. Spectrum analyzers, which provide basic measurement capabilities, are the most popular type of RF instrument in many general-purpose applications. Specifically, using a spectrum analyzer, the user can view power-vs-frequency information, and can sometimes demodulate analog formats such as AM, FM, and PM. Vector instruments include vector or real-time signal analyzers. These instruments analyze broadband waveforms, and capture time, frequency, and power information from signals of interest. Network analyzers are typically used for making S-parameter measurements, often performed on RF components.
More specialized pieces of RF instrumentation called "test sets" make complex protocol or standard-specific measurements specific to Bluetooth, GSM, or 802.11 wireless networking. Though expensive, these instruments are designed to make all the necessary measurements for a particular standard “out of the box.” They accelerate development of test routines, but are typically slower than general-purpose instruments. Because of this fact, they are well suited to design and development, but present cost challenges in manufacturing test environments.
All of these instruments – spectrum analyzers, vector analyzers, network analyzers, and test sets – offer different functionality to the end user. However, all are generally built on either a scalar or a vector architecture, each of which has its advantages and disadvantages. A scalar architecture is typically less expensive to build and offers performance advantages in noise floor and phase noise. Still, because it is narrowband in nature, a scalar architecture is not well suited to analyzing the broadband signals that are becoming common in today's marketplace. In addition, a scalar architecture gives only a 2-dimensional (power-vs-frequency) view of the signals being acquired, and is typically slower than vector-based architectures.
2. The Evolution of Communications Technologies
Though coding schemes and spectral efficiency measures may affect data rates, over the past decade bandwidth has become an increasingly important consideration. The demand for greater bandwidth capacity has forced communications systems to shift from narrowband to broadband approaches that entail higher data requirements. This evolution is illustrated by communications technologies.
For many years, AM/FM radio and voice telephony were among the most common communications applications. These systems have low data rates and require limited amounts of bandwidth – for example, 8 kHz for voice telephony and 200 kHz for FM radio. However, we are now moving toward 3G cellular systems that use 5 MHz of spectrum. Wireless networking technologies like Bluetooth and IEE 802.11b can occupy 80 MHz of spectrum and use 1 and 22 MHz channels, respectively. These wider bandwidths are coupled with digital modulation formats such as QPSK, FSK, GMSK, and QAM to achieve the high rates these applications demand.
A number of today’s communication technologies have existed for years, but were limited almost exclusively to military and defense applications. Many modulation schemes, wideband transmitters, spread-spectrum capabilities, and RF transmission and reception originated in military applications. Today, as a result of massive research and development investment and the tremendous increase in the semiconductor price/performance ratio, sophisticated RF capabilities are deployed in many commercial applications. For example, RF capabilities are now embedded in consumer products from phones to appliance remote controls to automobile remote-entry key fobs. Broadband applications such as TV systems, satellite, and cable modems also use RF capabilities.
3. Improving RF Measurements through Vector Analysis
To accurately capture and characterize broadband signals, it is necessary to change from narrowband measurement equipment to broadband vector instruments. Using vector instruments with a real-time bandwidth equal to or wider than the bandwidth of the transmitter, you can ensure capture of all signals or interest from your device under test (DUT).
Though typically more expensive than scalar instruments, vector instruments provide faster measurements and more complex signal analysis and generation. Specifically, vector instruments use wider filters than narrowband instruments such as spectrum analyzers. Because this width reduces the number of times the filter must be retuned, a vector instrument can sweep across the frequency spectrum more quickly than a scalar one. With a vector architecture, you can also generate complex signals such as the modulated waveforms used in most communications system.
When choosing a vector instrument to measure a broadband signal, it is necessary to consider the bandwidth of your DUT as well as all other measurement factors. For example, you may be interested in analyzing digital satellite radio signals that have only a 4 MHz bandwidth. At the same time, you may also need to measure adjacent-channel power to ensure that the transmitter meets government specifications for leakage into channels used by other service providers. For this measurement, the vector instrument should have a real-time bandwidth at least three times that of the device.
In addition to capturing broadband signals, vector instruments deliver other key advantages to your measurement application. When performing spectral sweeps or other measurements that span a large frequency range, the wide real-time bandwidth of a vector analyzer can dramatically improve test time. For instance, the new PXI-5660 RF Signal Analyzer from National Instruments features a 20 MHz real-time bandwidth and delivers measurement throughput advantages from 30 to 200 times that of traditional instrumentation.
For any general-purpose spectrum acquisition, vector instrumentation enables faster acquisition and measurement times than scalar instrumentation. Vector instruments capture phase, amplitude, and frequency information, while traditional instrumentation typically cannot. You can use this vector capability to simultaneously capture and display frequency and time information, which is necessary for performing joint time-frequency analysis and displaying 3D spectrograms or waterfall plots. Finally, with vector instruments, you can use phase information with frequency information in I/Q or modulation analysis to elicit a more detailed view of the signals under analysis. These additional benefits make a vector instrument much more powerful and flexible than traditional narrrowband spectral analysis.