Spectrum analyzers come in all sizes. A few consist of a PC-plug-in card plus software, but most are stand-alone instruments–some with a built-in PC. The price range is much greater than for most other instruments: beginning at less than $1,000 and traveling into the $60,000 region. Of course, the capabilities are commensurate with price.
Spectrum analyzer applications range from finding resonances in mechanical structures to verifying performance of satellite communications systems. Since the frequencies span anywhere from subhertz to several kilohertz for one to the gigahertz range for the other, instrument implementations and measurement techniques required to analyze these signals differ substantially.
At the low-frequency end, most modern spectrum analyzers use the Fast Fourier Transform (FFT) technique. FFT provides an almost instantaneous time-domain to frequency-domain transform. FFT analyzers not only produce the frequency spectrum of a time-varying signal but also provide phase relationship information.
There is one shortcoming. Only signals containing frequency components less than 100 kHz can be handled by today’s FFT analyzers. Still they are the instrument-of-choice for mechanical, vibration or acoustic analysis.
At higher frequencies–RF through microwave, a swept measurement technique usually is used. A resonant circuit is tuned successively over a frequency range of interest and the magnitude of individual signal components are displayed one at a time.
If a slow sweep speed is employed, transient signals which occur while the resonant circuit is not tuned to their frequency may be missed. A fast sweep speed is more desirable.
For many EMC test applications, a very wide frequency range must be scanned and many measurements must be taken. For these, not only fast sweep speed, but also a capability to gather precise data quickly, is important.
Since EMC compliance is now mandatory for many commercial and all export products, more engineers must be familiar with EMC testing which, in turn, requires competent usage of spectrum analyzers. To ease their task, many of the newer instruments are tailored to applications, such as EMC testing, or can become application-specific by using measurement personality software.
Along with ease of operation, new technological demands are being made, primarily to satisfy communications measurement needs. Many new communications systems are being developed in response to the growing interests in mobile applications. These systems employ complex modulation schemes and use and radiate minimum power. Consequently, they require sensitive receivers, and testing them requires greater capabilities than even yesterday’s high-performance spectrum analyzers provide.
New Peaks of Performance
High-performance RF and microwave spectrum analyzers have long featured superb sensitivity, low thermal and phase noise, low distortion, extensive dynamic range, and high measurement accuracy. But to satisfy new measurement demands, spectrum analyzer designers are devising techniques to achieve even higher performance, such as intrinsic noise of -145 dB/Hz at 26.5 GHz, zero-span sweep times of 2 µs, and high frequency-measurement accuracy over wide ranges without relying on a counter.
The sensitivity of a spectrum analyzer is primarily determined by the inherent noise level of the local oscillator (LO) and the first mixer. The higher the noise level is in the first mixer, the greater is the loss in sensitivity–an effect referred to as mixer loss.
The frequency range that an LO can generate using only a single resonant circuit is quite limited. So to achieve a cost-effective implementation, most microwave spectrum analyzers use the LO fundamental for the lowest band and the second, third or the nth LO harmonics to cover the higher microwave bands.
Unfortunately, when LO harmonics are mixed, the mixer loss increases proportionally with n. “Theoretically, the sensitivity at 26 GHz (n = 4) is 12 dB lower than at 4 GHz (n = 1),” said Hans Funk, Business Development Manager at Wandel & Goltermann. “In practice, the reduction in sensitivity due to frequency-dependent losses at high frequencies is even greater.”
When only the LO fundamental is used for frequency conversion, high sensitivity can be achieved even at the highest bands. Spectrum analyzers operating on this principle, such as SNA-23 and SNA-33 from Wandel & Goltermann, exhibit intrinsic noise levels below -145 dBm/Hz, even at 26.5 GHz.
The greater complexity and higher component count required to implement the fundamental mixing method entails additional costs–but provides many advantages. “For example, very small signals, which would go undetected if harmonic mixing was used, can be measured with a fundamental mixing,” said Mr. Funk. “Also, signals which are only discernible with a narrow resolution bandwidth when harmonic mixing is used can easily be captured with a considerably faster (factor of >10) sweep time.”
Capturing signals is only the first step. Measuring their amplitudes and frequencies with required accuracy is equally important. Precise amplitude measurements are relatively easy to obtain with a high-performance analyzer since most of these analyzers feature a very flat frequency response over each band and many apply stored correction factors.
High frequency accuracy is not as simple. Early spectrum analyzers used voltage-controlled local oscillators which were easily implementable but never very precise. Most modern spectrum analyzers use synthesizers that produce highly accurate discrete frequencies which can be increased and decreased in small steps. To produce a continuous sweep over a desired frequency range, synthesizers are usually used in conjunction with phase-locked loop oscillators.
To increase sweep speed, the lock-and-roll synthesis method is often used. The LO phase-locked loop is locked to a precise reference frequency, usually at the center of the frequency span of interest. While scanning is being performed, the loop is opened so that the LO, driven by a sawtooth voltage, can sweep over the desired frequency span.
With this method, high-frequency accuracy can only be guaranteed in the locked state; in this case, at the center frequency. Considerably larger errors, up to several percent of the frequency span, may occur at other values along the frequency axis. As a result, an internal frequency counter usually is provided for more accurate measurements.1
The lock-and-roll implementation is being supplemented or superseded by several new techniques. For instance, digital techniques are being used in conjunction with conventional microwave LO designs.
“The Advantest R3272 is the first spectrum analyzer in its price/performance range to use direct digital synthesis (DDS) which ensures the highest level of frequency accuracy throughout the measurement range,” said Bob Buxton, Product Marketing Manager, RF and Wireless Test at Tektronix. “DDS simplifies the measurement task and saves time by reducing the need to use a frequency counter for accurate measurements.”
Another implementation that supersedes the lock-and-roll technique is the sweepable-phase-locked-synthesizer (SPLS) principle developed by Wandel & Goltermann. “With this principle, the synthesizer loop is always closed,” explained Mr. Funk. “The LO is not voltage controlled but set by digitally controlling the fractional-n-divider that determines the frequency in the appropriate loop (Figure 1). This means that all points on the frequency axis have exactly the same accuracy as the reference frequency; for example, 1 x 10-8.
“Using this technique, not only is the frequency accuracy increased, but operation is also simplified. Frequency counting is no longer necessary for accurate frequency measurements. Spectral lines of interest are denoted with a marker and frequency and amplitude are displayed directly on the screen with high accuracy,” he continued.
“A signal-to-noise ratio of a few dB is sufficient. Time consuming measurements don’t have to be repeated for each spectral line as all lines of a spectrum can be measured accurately with a single sweep,” Mr. Funk concluded.
Communications Test Demands
Requirements of the communications industry are impacting almost the entire spectrum-analyzer gamut, from economy to high-end implementations. “The range of our newest spectrum analyzer extends to 1 GHz, double the bandwidth of its predecessor,” said Fred Katz, Chief Engineer at Hameg.
“This increase has essentially been driven by the need for expanded communications, especially cellular telephones and pocket pagers. New state legislation has upped CATV test requirements beyond 500 MHz and, as two-way cable communication is introduced, analyzer bandwidth requirements will climb still higher,” he said.
Spectrum-conserving and bandwidth-efficient access and modulation schemes are also posing special test challenges. Among the technologies already deployed are time division multiple access (TDMA) systems such as North American digital cellular, the Japanese personal digital cellular and personal handy phone systems, and the European global system for mobile communication and digital cordless systems.
To provide the needed test facilities, the latest spectrum analyzers include fast sweeps in zero span, to capture and view transmitted bursts; and gated sweeps, to see the modulation spectrum of burst transmission undisturbed by switching transients, remarked Mr. Buxton. “For power measurements, algorithms for the calculation of average power during a burst as well as burst-to-burst averaging have been implemented,” he added.
“For checking digital modulation, it also has been necessary to build in digital demodulation and vector signal-analysis facilities, such as phase and frequency error measurements, and the I/Q vector and constellation displays,” Mr. Buxton concluded.
Code division multiple access (CDMA) is the most recent modulation technique affecting spectrum analyzers. CDMA exhibits a noise-like spectrum which is processed differently by the spectrum analyzer than sinusoidal spectra types.
To deal with these signals, special measurement techniques are required. To assist engineers in making these measurements, some companies, have recently issued useful application notes.2
Some of today’s spectrum analyzers may perform required CDMA measurements easily. For instance, the Tektronix RS3261C/D with Option 1K, furnished with resident firmware, readily determines the characteristics of CDMA signals. The portable Hewlett-Packard HP 8590 E may be loaded with the HP 85725 CDMA measurement personality card to perform measurements according to EIA/TIA standards.
But TDMA, CDMA and other modulation schemes come in a variety of flavors, and many specifications are in a state of flux. Many of the newest spectrum analyzers feature a modular construction for upgrading and PCMCIA slots to accept software upgrades or special measurement personalities.
“In response to the rapid development of a variety of wireless communications formats throughout the world, we have developed nearly 20 measurement personalities (software) to help our customers design and test wireless communications products, such as cellular telephones, and meet their time-to-market goals,” said Larry Ligon, Marketing Engineer at Hewlett-Packard.
“Our engineers also participate on standards committees to ensure that we understand the regulations. This helps us to design test equipment that simplifies the rigors of meeting these regulations. Most tests are one-button, pass/fail so manufacturers can concentrate on making products and not on the intricate details of spectrum analysis,” Mr. Ligon concluded.
References
1. Funk, H., “What You Should Know Before You Purchase a Microwave Spectrum Analyzer,” Memo, Wandel & Goltermann.
2. “Complex Modulation Signal Measurements Using the Spectrum Analyzer,” Technical Note 11/94, Tektronix.
Spectrum Analyzer Products
1.6-GHz Spectrum Analyzer
Implemented as PC/AT Plug-In
The 9052 Spectrum Analyzer features synthesized tuning, a dynamic range >80 dB, a sensitivity of better than -120 dBm and a frequency range from 100 kHz to 1.6 GHz. PC facilities provide overlays, extensive marker functions, trace math, hard-copy plots, automatic test sequences and remote operation. Multiple phase-locked loops and direct digital synthesis assure fast frequency stepping, low phase noise and accuracy. A temperature-stabilized crystal oscillator provides a precise reference. Nine trace registers temporarily store any of the four screen traces. $7,500. Morrow Technologies Corp., (813) 531-4000.
FFT Analyzer Combines
Three Functions
The CF-1200 FFT Analyzer is a combination vibration meter, waveform monitor and data collector. It operates continuously for 8 h on 4 AA batteries or by using an optional AC adapter. It can be calibrated to work with a variety of transducers, including vibration, acoustic and electrical. $3,750. Ono Sokki Technology, Inc., (708) 627-9700.
Automated EMC Analyzer
Speeds Precompliance Testing
The HP 8590EM Series Analyzers, covering 9 kHz to 1.8, 2.9, 6.5, 12.8 or 22 GHz, have two operational modes: EMI analysis and spectrum analysis. Automated features include setup keys linking predefined CISPR parameters, antenna/amplifier/cable corrections, limit lines and predefined routines that automatically measure all signals over specified frequency ranges. Swept-tuned receivers ensure that intermittent signals are not overlooked, and continuously adjustable spans optimize signal evaluation. Swept measurements are performed on multiple signals and up to 230 signals can be saved. From $16,300. Hewlett-Packard Co., (800) 452-4844.
High-Performance Spectrum
Analyzer Is Portable
The SNA-33 Spectrum Analyzer provides coaxial measurements from 20 Hz to 26.5 GHz. An intrinsic noise floor below -145 dBm/Hz at 26.5 GHz and a 90-dB dynamic range are achieved with minimal filtering by employing fundamental frequency mixing. A swept-phase-lock-synthesizer implementation enables crystal oscillator frequency accuracy to be transferred to all frequencies during swept measurements. The amplitude range extends from -115 dBm to +30 dBm with 0.1-dB resolution and ±2.7-dB accuracy. The resolution bandwidth ranges from 1 Hz to 10 MHz. Frequency accuracy is 0.01 ppm. $50,825. Wandel & Goltermann Inc., (800) 924-3773.
FFT Analyzer Has
Frequency Range to 100 kHz
The SR760/SR770 FFT Analyzers offer a 90-dB dynamic range, a 476-µHz to 100-kHz frequency span and a 100-kHz real-time bandwidth. Analysis functions, such as THD, PSD, octave, band and sideband analysis, are menu driven. Traces, limit and data tables and instrument setup files can be stored on 3.5″ disk or accessed through RS-232 or GPIB interfaces. The SR770 also includes a low-distortion (-80 dBc) synchronized source to generate frequency response measurements accurate to 0.05 dB. SR760: $4,750; SR770: $6,500. Stanford Research Systems, Inc., (408) 744-9040.
Analyzer Has 4-Digit
Frequency/Marker Display
The SA-505 Spectrum Analyzer has a frequency range from 0.5 MHz to 500 MHz. It accommodates input amplitudes from -100 dBm to +13 dBm and has an 80-dB display range. A 4-digit readout indicates center or marker frequency. Amplitude measurements are performed in the zero scan mode while tuned to a fixed frequency. The analyzer may be used in conjunction with the PA102 preamplifier, AB900/AL100 biconical/log-periodic antennas or the L1100 LISN to perform EMI tests. $1,170. Com-Power Corp., (714) 528-8800.
Multichannel Signal
Analyzer Is PC-Based
The DP420 FFT Analyzer has a DC to 20-kHz frequency range, 4,000-line baseband resolution and an 80-dB dynamic range. Anti-alias filters provide programmable cutoff frequencies matched to analog bandwidth. The modular system, configurable up to 16 channels, consists of the PC-AT plug-in DSP mother board (DP320), a single-input/single-output daughter board (DP340) and a dual-input-channel daughter board (DP360). The DP420 combines a GUI, high-speed DSP hardware and signal processing software. Data Physics Corp., (408) 371-7100.
FFT Analyzer Contains 386,
Upgradable to Pentium
The RS-100 Real-Time Spectrum Analyzers cover 8 µHz to 100 kHz; feature an 80-dB dynamic range, >10 kHz real-time bandwidth, and nonvolatile memory to 8 MB; and use individual DSP chips per channel. Data on the 10.4″ electroluminescent display or an external VGA monitor can be analyzed in 400-line, 1/3 and full octaves. Zooms up to 32X for a 12,800-line frequency spectrum can be applied. A 386 CPU along with 4 MB of RAM, a 250-MB hard drive, and a 1.44-MB 3.5″ floppy are included. Single-channel: $8,995; dual-channel: $9,995; four-channel: $15,995. Rockland Scientific, (201) 984-1900.
Frequency Analysis Software
Eliminates Power-of-2 Restriction
The SM-FA Frequency Analysis Software for WindowsR Version 3.0 contains new features. The FFT window width may be selected with prime factorial including an even combination of 2, 3, 5 and/or 7, up to 16,384 data points. This eliminates the common power-of-2 restriction and provides more than 300 FFT window-width options. Ten new windowing types are included for a total of 23. All FFT functions, channels and window types are listed in one dialogue box, and frequency domain calculations are set up by mouse. $495. HEM Data Corp., (800) 436-4330.
Lightweight Spectrum Analyzer
Redefines Measurement Versatility
The Advantest R3272 Spectrum Analyzer covers 9 kHz to 26.5 GHz. It utilizes DDS, ensuring accuracy throughout the range and saving time by reducing the need to use a frequency counter. A 100-dB display dynamic range and 300-Hz to 5-MHz resolution facilitate complex signal evaluation. Up to four measurement windows can be displayed simultaneously on the 256-color TFT LCD. Measurements are performed automatically. Two PCMCIA interfaces are provided. Application-specific measurement routines are stored in ROM. $28,990. Tektronix, Inc., (503) 627-5757.
Copyright 1995 Nelson Publishing Inc.
May 1995