The past decade has seen dramatic progress in the design of the spectrum analyzer as communications technology quickly rushed forward to stay apace of user needs. What is the channel power? The adjacent channel power? The modulation quality? The spectrum analyzer can give you the answer.
The list of spectrum-analyzer applications reads like a directory of new-product development. With the proper choice of instruments, you can evaluate second- and third-generation communications equipment, determine settling time for oscillators and synthesizers, or measure phase. You can monitor CATV return-paths, analyze networks, or locate microwave data-link faults.
Some analyzers let you study transitional signals. Bursting signals such as time-division multiple access (TDMA) use phase-locked loops for frequency stability. The frequency change during key-on and key-off is critical and can be evaluated by spectrum analysis.
That’s not all. With the right choice of instruments, you can measure spread-spectrum channel power, study intermodulation, evaluate modulation quality, or verify code performance on code-division multiple access (CDMA) and wideband CDMA (WCDMA) systems. This versatile instrument also measures spectrum, time, and modulation characteristics on a mobile digital radio transmitter, locates faults on a radar system, evaluates EMC, does surveillance, debugs software/hardware interfaces, or performs signature analysis.
Although such a wide range of capabilities suits the spectrum analyzer to many markets and applications, communications technology is the one market that stands above the others in the opinion of Larry Ligon, product manager at Agilent Technologies. “The designers and testers of new communications systems must have the ability to do very detailed spectrum evaluation as their products are developed and manufactured,” he said.
There is no argument from Dell Covington, the RF/field marketing director at Anritsu. “Rapidly advancing wireless communications technologies have greatly increased the demand for signal analysis in the 20- to 40-GHz part of the spectrum,” he said. “This includes applications such as local multipoint distribution service, broadband wireless access systems, multimedia mobile access communications systems, and high-speed wireless local area networks (LANs). The need for validation of CDMA and WCDMA links creates the need for new spectrum analysis equipment.”
Whatever your application may be, the spectrum analyzer has the answer. More accurately, it has the answer if you can set it up properly and interpret the results correctly. The operator interface is becoming more friendly as designers learn how to harness the power of the ever-present microprocessor to help you get the most from this complex instrument.
Comparisons
In choosing a spectrum analyzer, first determine what performance you really need. There is no need to use an instrument that is overpowered for the application. But comparing spectrum-analyzer performance parameters is difficult. There are a large number of models on the market, and two manufacturers, Agilent Technologies and Tektronix, have a stable full. How do you make a logical comparison?
Traditional or Real-Time?
Two very different types of spectrum analyzers are on the market: the traditional spectrum analyzer and the real-time spectrum analyzer or Fourier analyzer.
The traditional swept-frequency spectrum analyzer is a narrowband receiver that can be tuned rapidly and repeatedly over a portion of the spectrum. The heterodyned analog output, routed through a relatively narrow bandpass filter, represents the instantaneous amplitude of the signal in that part of the spectrum.
The sequence of such outputs created by each sweep forms a plot of amplitude over the spectrum being scanned. Besides being displayed, the result generally can be output as a hard copy. Resolution is determined by the bandpass filter selection. Figure 1 (see June 2000 issue of EE) represents the traditional spectrum-analysis process.
Typically, the signal of interest is analyzed for you, using more processing power to ensure less probability of error than with a simple display.
Within the traditional category are RF and optical analyzers. The concept is the same, but the input is RF spectrum in one case and optical spectrum in the other.
The real-time spectrum analyzer also is a wideband receiver. However, unlike the traditional instrument, this unit uses a fixed-frequency local oscillator as shown in Figure 2 (see June 2000 issue of EE) and captures a block of frequencies at one time.
The heterodyned spectrum is amplified and passed through a wideband filter, typically 100 Hz to 5 MHz. All the frequencies in the passband are presented to a high-speed analog-to-digital converter (ADC) simultaneously. The balance of the signal path is digital, with filtering and fast-Fourier-transform (FFT) analysis and display under computer control. The intermediate-frequency signal is filtered at about one-half of the ADC rate to satisfy Nyquist sampling criteria.
Since both amplitude and phase can be measured, the real-time instrument analyzes network properties very effectively. Both periodic and random transient events are captured and evaluated.
Comparing the traditional and real-time spectrum analyzers, the top frequency coverage of the traditional unit for RF applications is about 40 GHz, with even higher frequencies possible when using an external receiver and down-converter. The optical unit covers dense wavelength-division multiplex (DWDM) and ultraDWDM (UDWDM) systems. The real-time unit is limited to approximately 3 GHz now although this is increasing.
The traditional instrument for RF has a typical minimum resolution of 1 kHz; however, the popular new digital filtering technique lowers this to 10 Hz. You can find optical resolutions of 10 pm. The typical real-time unit has a much greater resolution, possibly down to 20 mHz for vibration analysis.
The traditional instrument analyzes amplitude vs frequency on periodic events. A real-time instrument can look at both amplitude and phase vs frequency, making it possible to analyze random as well as periodic phenomena.
Typical applications for the traditional spectrum analyzer include EMC, spectral analysis, production tests on RF equipment, and remote-site tests. The real-time analyzer is used for acoustical studies, modal analysis, and studies of rotating machinery. It puts out results 20 to 100 times faster than the traditional analyzer and is less expensive.
Special Factors
“It is extremely important to evaluate your task carefully before choosing an analyzer,” said Stuart Creed, business development manager at Tektronix. “Carefully defining channel power measurements may bring a 10-fold increase in speed and yet reduce the price by 50%.”
Mr. Covington of Anritsu reminded us to evaluate the resolution bandwidth, the noise floor, the dynamic range, and the video bandwidth. “The unit should be designed to be easily interfaced to other equipment. Also, the instrument must have some standard type of storage medium for transfer, such as a floppy disk or a PCMCIA card,” he explained.
Do you need to do channel power measurements quickly? Take a look at the latest technology, and you may find a 10:1 improvement over what was available a couple of years ago, according to Mr. Creed. The adjacent channel power ratio (ACPR) can be measured in less than 100 ms.
Look at the price/performance ratio carefully warned Wes Stamper, the product marketing manager at IFR Systems. “You can pay from $6,000 to $50,000 for a spectrum analyzer. Instruments at both ends of the scale have great specifications, but there are significant improvements as prices increase. You must decide what performance you need, and then find that capability at a price you can afford,” he said.
Greg VonRehder, an applications engineer at B+K Precision, reminded us that all too often low-cost spectrum analyzers are unstable, drift excessively, have low dynamic range, can’t handle high-level input signals, and lack features such as a tracking generator and an audio demodulator. The industry, he said, has the opportunity to overcome these deficiencies and offer high-scale performance at an affordable price.
Simplified Operation
After all the operational factors are considered, can you set up a spectrum analyzer and interpret the results? Simplification of the operator interface is a real challenge. The best instrument in the world is not very useful unless you can set it up to analyze your signal, then interpret and display the results in a meaningful format.
Let’s face it: The spectrum analyzer has never been an easy instrument to use. Why? Doesn’t it just analyze spectrums? Yes, but the spectrums that we want to analyze usually are quite complex.
Armstrong with his FM development in the 1930s didn’t need spectrum analyzers. However, if he had used these instruments, the setup would have been easy because he was generating simple RF carriers. It’s the complexity of our 21st century carrier modulation that makes this instrument so valuable to us. But that same complexity often makes it difficult to use a spectrum analyzer.
The operating problem is two-fold. You must set the controls correctly, of course. Recognize, however, that you can’t make some measurements directly even if the settings are perfect. This becomes more pronounced with every advance in the complexity of the spectrum you want to analyze.
Finding some of those evasive answers involves making a sequence of measurements and calculations, then letting the smart instrument manipulate the data to get the signal analysis you need. For this reason, spectrum-analyzer manufacturers have been hard at work trying to anticipate your needs and give you help to do the job.
According to the descriptions of today’s spectrum analyzers, instrument designers already have made significant progress toward simplification. It is much easier to use an analyzer now than it was five years ago. You can set up the instrument to look at your complex spectrum and give you results that are understandable. The analysis can be stored on board for later perusal, printed out in a useful format, and transferred to a PC for further handling, all under front-panel control.
As a matter of fact, putting a PC in the laboratory-based instrument simplifies operation by allowing the designer to focus on measurement hardware. “You benefit from familiar off-the-shelf displays, printer support, and network cards,” Mr. Creed of Tektronix noted. “Data can easily be shared with other analysis packages, too.”
Setup
Some improvements that make setup more efficient are incorporating menus with easy-to-understand selections and arranging test data, scale factors, and setup parameters in a logical manner. Other helps include displaying setups, traces, and graticules in contrasting colors for easy identification.
Some models allow you to tailor operation to your requirements using frequently used functions prominently located. This may include channel power measurement, adjacent-channel power measurement, occupied bandwidth determination, and tables of peaks and harmonics.
To simplify your setup, you can have one-keystroke recall of the basic functions and frequently used help tools. The PC also lets you store today’s setups on disk for fast recall and get on-screen help in interpreting setup functions.
Analysis and Interpretation
Designers have made it easier for you to see, interpret, and store data. Some laboratory instruments use a large color screen to provide good resolution and provide a VGA port for connecting an external color monitor. You can expand the color display modes to include spectrum, waterfall, spectrogram, and digital modulation analysis screens. Other features enable you to display constellation and vector diagrams as well as frequency, phase, magnitude, and in-phase and quadrature data vs time.
Do you want a movable marker to help you analyze the modulation at any point? You can have it. Also, you may define limits and let the instrument provide pass/fail messages.
Add a frequency mask trigger to capture random signals such as intermittent spurious emissions. If you aren’t interested in the period between transmission bursts, with the frequency mask trigger you can capture burst-on periods to optimize the use of the memory for spectral, time, or modulation analysis.
Graphical definition of the frequency trigger mask causes the trigger condition of special interest to be generated by the correct event in the frequency domain. This happens no matter what’s going on at other periods within the displayed span. This is especially helpful when you are interested in capturing signals arising from intermittent spurious events or spectral regrowth events.
A zoom capability lets you select and analyze the unique signal of interest in a multisignal environment. AM/FM demodulation and a speaker expand the capability to identify signals.
When you use a portable analyzer, results can be stored on a floppy disk or PCMCIA card for transfer to a PC for local analysis. Formatted test results can be sent to a printer port for report generation. Some analyzers time-tag printer and disk data so you can identify relative or absolute times of occurrence. And yes, you can interface to other equipment via RS-232, IEEE 488, or VXI/PXI using standard language and protocols.
The laboratory-based analyzer may be built around a Pentium chip, large RAM, a hard drive, a removable disk, and Windows software. Likely, it will communicate with other systems via ISA and PCI buses with IEEE 488, Centronics, and SCSI ports. As a result, the same instrument that analyzes spectrums also serves as your system controller.
“The trend is to put the power of a PC into the laboratory-based spectrum analyzer,” Mr. Stamper of IFR Systems noted. “This gives you the power of a desktop computer for spectrum analysis and lets you attach it to a network, wireless modem, and printer.”
What about operating software for such an instrument? It is built into the high-performance spectrum analyzer. As Mr. Ligon of Agilent Technologies explained, “You need a familiar-looking screen, and Bill Gates has taken care of that for us.”
The Near Future
Design engineers are monitoring your needs. What will they do to enhance next year’s products? Mr. Covington of Anritsu noted, “DSP advances have had a major impact on modulation techniques. Si-Ge technology in RF components gives better speed and resolution.”
High-speed digital architecture is the key to advances in this field. Fast CPUs enhance the user interface, and major advances in DSP technologies have molded the spectrum analyzer into an invaluable test instrument. Added to all that increased complexity is the continued effort to simplify the setup of these powerful instruments and the analysis of their displays.
Return to EE Home Page
Published by EE-Evaluation Engineering
All contents © 2000 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.
June 2000