Fig 1. Occupied bandwidth is the frequency band bandwidth that contains a specified percentage of the total power of the signal. This image shows an occupied bandwidth of 15 MHz.
Fig 2. Peak search determines the levels and frequencies of peaks in a specified band. This image shows an example of locating multiple peaks above a specific threshold.
Fig 3. Adjacent channel power (ACP) measures the way a particular channel and its two adjacent channels distribute power. This image illustrates a typical ACP measurement and the center frequency, bandwidth, and spacing that describe the channels.
The use of RF and wireless technology has become pervasive in our everyday lives around the globe. However, it is still a relatively niche (and highly sought after) discipline in the engineering world. Many have been wondering for years when RF and wireless were going to hit their tipping point in the engineering community with regards to the percentage of those who speak RF and those who don’t.
I recall making a declaration at a keynote in Germany in 2008 that RF instrumentation soon would be as common as digital multi-meters (DMMs) in most test systems. I also challenged the audience of test engineers to begin learning the fundamentals of RF and wireless measurements and communication systems to be ready for their first encounter with this rapidly growing domain.
This advice for today’s test engineers remains more important than ever. The RF and wireless arena is the fastest growing and most lively of nearly all disciplines at the moment with no shortage of customer demand for products in the visible future.
All of the big RF and wireless tradeshows, including European Microwave Week, the International Microwave Symposium, and Mobile World Congress, are busting at the seams with attendees, exhibitors, and an endless stream of mind-blowing technologies to demonstrate. Demand for engineers with RF and wireless expertise is also soaring as the number of related job opportunities increases without a wide distribution of RF knowledge prevalent in the test engineering community.
Do You RF?
If you’re a test engineer with minimal or no RF and wireless background, you should declare RF and wireless your next personal growth frontier for 2012. This can simply mean focusing your continuing education on RF and wireless or even pursuing an advanced degree or job change. One thing is for certain—any time you invest to grow your knowledge in this area will not be for naught.
If you’re already fluent in RF and wireless concepts and measurements, focus on understanding the next generation of modular, software-defined RF, and microwave instrumentation (ni.com/rf) and how it can decrease your test times by three to 10 times and dramatically increase the flexibility of your RF test and measurement systems.
Below is a brief RF and wireless primer to help you embark on your next personal frontier in test and measurement along with a few additional resources to help you along the way.
What Is RF?
RF and wireless have been around for more than a century with Alexander Popov and Sir Oliver Lodge laying the groundwork for Guglielmo Marconi’s wireless radio developments in the early 20th century. In December 1901, Marconi performed his most prominent experiment, successfully transmitting Morse code from Cornwall, England, to St John’s, Canada.
RF itself has become synonymous with wireless and high-frequency signals, describing anything from AM radio between 535 kHz and 1605 kHz to computer local-area networks (LANs) at 2.4 GHz. However, RF has traditionally defined frequencies from a few kilohertz to roughly 1 GHz. If one considers microwave frequencies as RF, this range extends to 300 GHz.
RF measurement methodology generally can 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 you can view power-versus-frequency information and can sometimes demodulate analog formats, such as amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).
Vector instruments include vector or real-time signal analyzers and generators. These instruments analyze and generate broadband waveforms and capture time, frequency, phase, and power information from signals of interest. These instruments are much more powerful than spectrum analyzers and offer excellent modulation control and signal analysis.
Network analyzers, on the other hand, are typically used for making S-parameter measurements and other characterization measurements on RF or high-frequency components. They correlate both the generation and analysis on multiple channels but at a much higher price than spectrum analyzers and vector signal generators/analyzers.
Spectral Measurements
Several common spectral measurements in RF and communications systems include power in band, occupied bandwidth, peak search, and adjacent channel power. Power in band measures the total power within any specified frequency range or band. It is characterized by:
where X is the input power spectrum from a specified band, fl is the low bound of the frequency band, and fh is the high bound of the frequency band. The low and high bounds of this band can be determined from the center frequency.
Occupied bandwidth is a measurement of the frequency band bandwidth that contains a specified percentage of the total power of the signal. Occupied bandwidth is the inverse of power in band.
For a specified percentage B, the upper and lower limits of the frequency band are the frequencies within which the total power is found. For example, if B is 99, then the occupied bandwidth is the bandwidth that contains 99% of the total power of the signal. Figure 1 shows a calculated occupied bandwidth of 15 MHz.
A spectral peak search algorithm determines the levels and frequencies of peaks in a specified band. The algorithm uses interpolation to precisely locate frequency peaks in the amplitude or power spectrum in any units or scaling. You also can specify whether to locate a single maximum peak or multiple peaks that exceed a specified threshold. Figure 2 shows an example of locating multiple peaks above a specific threshold.
Adjacent channel power (ACP) measures the way a particular channel and its two adjacent channels distribute power. This measurement is performed by calculating the total power in the channel and the total power in the surrounding upper and lower channels. Figure 3 illustrates a typical ACP measurement and the center frequency, bandwidth, and spacing that describe the channels.
Many technologies allocate adjacent channels for information distribution from different providers, such as cell phones, TV, radio, and cable. In these and other applications, it is important that transmission from one channel does not cross over to an adjacent channel, which noticeably degrades the quality in the other channel.
Depending on the technology standard you’re measuring, different criteria exist for adjacent channel power measurements. For example, the code division multiple access (CDMA) wireless standard requires transmissions to fit within a 4.096-MHz bandwidth. Moreover, adjacent channel power, measured at 5-MHz offsets, must be at least 70 dB below the in-channel average power.
Learn More
I encourage you to continue on your new journey to learn more about RF and wireless. National Instruments offers a vast array of complimentary tutorials and fundamentals content to help you learn at your own pace (http://zone.ni.com/devzone/cda/tut/p/id/3992). Many local colleges and universities offer introductory and advanced courses you might consider. Given the rapid pace of technology and adoption in this field, I can assure you it will be time well invested.