Spectrum Analyzers Answer Call For Speed

The easiest way to envision a spectrum analyzer is to begin with an oscilloscope that plots magnitude versus time. Then swap frequency for time—and voilà—a spectrum analyzer!

A gross simplification perhaps, but no one questions that the spectrum analyzer has long been the instrument of choice for measuring modulation, distortion, and noise. With wireless growth severely limiting bandwidth, measuring the spectral characteristics of the modulation is vital in both the design and test phases of a new product.

Distortion measurements in-clude intermodulation, harmonics, and spurious emissions. Measuring distortion is particularly critical in receivers and transmitters to prevent crosstalk. So an essential attribute for any spectrum analyzer is that noise generated within the instrument be extremely low.

Introduced in the late 1970s, the first high-performance spectrum analyzers were developed with an eye on fast, automated measurements. The same holds true today. Throughput is critical in spectrum-analyzer measurements. To improve throughput, a spectrum analyzer must exhibit faster measurement speed and better accuracy. Today's spectrum-analyzer measurement speeds are several times quicker than previous-generation speeds. Moreover, spurious emissions testing now takes a fraction of the time.

Accuracy is another salient issue. Greater accuracy means closer margins and lower uncertainty. With closer margins, fewer good devices are rejected, thereby improving yield.

Agilent Technologies' newest spectrum analyzers reflect this need for speed. The company just introduced two new members in its Performance Spectrum Analyzer (PSA) family. The E4448A enables users to design and test integrated systems, as well as perform depot-level testing at up to 50 GHz. Meanwhile, the E4446A is a high-performance 44-GHz spectrum analyzer with an intuitive interface and one-button measurement capability.

Both instruments have enhanced amplitude accuracy and speed. Amplitude accuracy is 2.4 dB, and speed is much higher than in Agilent's previous analyzer, the 8564. For example, a 1-GHz sweep for spurious signals, which previously required up to 200 seconds, now takes just 11 seconds.

Speed is very important because most developers have to look for out-of-band spurious signals, performing a search for spurs for every function that a device can perform. Consider how numerous wideband CDMA systems run four or five channels at a time. So, one must look for unwanted intermodulation and harmonic products that are spurious in each channel.

Both Agilent spectrum analyzers meet the latest requirements of popular communications formats—1xEV-DO, W-CDMA, cdma2000, GSM/EDGE, CDMA, and NADC/PDC. Gaining in importance, the 1xEV-DO format handles the data that rides on cdma-2000, enabling data transmission plus voice.

Limit lines can be adjusted to indicate a pass or fail line, or to trigger an alarm that alerts the user if a signal is out of bounds. An amplitude correction factor lets the user compensate for gain and loss due to the presence of a cable, an external amplifier, or an antenna. In fact, users can stack up to four different correction factors to provide an overall correction, turning each on or off. Agilent also provides an oscilloscope with spectral analysis capabilities (see "FFT Brings Spectrum Analysis To The Scope," p. 76).

A Performer In A Small Package: Packing a lot of performance in a small 4.9-lb package, including battery, Anritsu's MS2711B portable spectrum analyzer enables users to identify and solve many of today's wireless communication systems problems (Fig. 1). Just 10 by 7 by 2.4 in., this unusually compact spectrum analyzer is intended for engineers and technicians performing measurements on complex, digital wireless signals in the 100-kHz to 3-GHz range.

The MS2711B is well suited for users installing, maintaining, and troubleshooting modern wireless communication systems, which many times transmit just slightly above the system noise level. Accordingly, it can measure signal levels down to ­115 dBm.

Options consist of a preamplifier and a tracking generator. With the preamplifier, users can solve RF system-related problems, such as coverage and interference analysis. The tracking generator is suitable for tuning cavity filters and adjusting repeater gain and antenna response.

Dedicated single-button measurements, like channel power, adjacent channel power ratio, and occupied bandwidth, let Anritsu's spectrum analyzer measure the distortion level and channel power performance of transmitters in AMPS, TDMA, CDMA, and GSM basestations. The device can conduct measurements of spurious signal components. Plus, a field-strength mode handles the measurement of propagation and coverage. It also pinpoints electromagnetic leakage in broadcast systems.

The MS2711B comprises several new enhancements that make testing cellular, DCS/PCS, paging, WLAN (IEEE 802.11, 802.11b, and Bluetooth), and other communications systems easier. A dynamic attenuation capability automatically tracks the input signal level and adjusts the reference level to protect the spectrum analyzer from a high RF signal level. A preamplifier is activated whenever the applied RF signal level is low, thereby improving measurement accuracy.

In addition, the device features a multilingual user interface with on-screen menus and messages in six languages. Though nominally a 50-Ω instrument, an interface permits measurements of 75-Ω systems without sacrificing accuracy.

2000-Point Resolution: Hameg's HM5012 and HM5014 spectrum analyzers create a usable frequency range of 150 kHz to 1.05 GHz. Resolution bandwidths are 9 kHz, 120 kHz, and 1 MHz, and the amplitude range spans ­100 dBm to +13 dBm. The digital signal display operates in real time and is resolved with up to 2000 points over the entire screen. All selected frequency settings and marker results also are displayed on-screen.

These spectrum analyzers offer extensive measurement capabilities, like amplitude indication in Peak and Average modes. For precise evaluation of the signals, a marker provides a readout of amplitude and frequency on-screen. Also, newly acquired signals can be compared with the content of the reference storage. A save/recall function enables storage of complicated and frequently used equipment settings.

Both analyzers come with an RS-232 interface for PC communication and printout. Model HM5014 includes a tracking generator, a handy tool for evaluating the frequency characteristics of four-terminal devices, such as filters. The optional HZ70 optoisolator with fiber-optic cable is available to isolate the spectrum analyzer from interference effects and ground loops.

Fulfilling 2.5G And 3G Requirements: With a frequency range of 20 Hz to 26.5 GHz, the new Rhode & Schwarz FSU26 microwave spectrum analyzer is well suited for a number of environments, like the manufacture and support of mobile radio, military/satellite communications, radar/EW, microwave links, and microwave components (Fig. 2). The FSU26 also is ideal for 2.5G and 3G component development and amplifier production test.

A significant extension of the FSU family, this product exhibits enhanced dynamic resolution, very low phase noise, and a broad digital resolution bandwidth (RBW). For instance, RBW is 10 Hz to 100 kHz, and phase noise is ­155 dBc at a 10-MHz offset. Moreover, the FSU26 boosts the family's performance into a frequency range that meets the needs of emerging 3G mobile technologies and satellite applications. For example, W-CDMA standards call for spurious emissions measurements of up to 12.75 GHz.

Sold and supported in the United States by Tektronix, the FSU26 is designed to increase productivity in both design and manufacturing environments. To that end, the analyzer features an enhanced GPIB transfer speed and fast-Fourier-transform (FFT) based filters.

High Performance At A Low Price: Selling for $2375, the Instek GSP-810 delivers a lot of performance for the money. It employs a fully digital, phase-locked loop design and covers the 150-kHz to 1-GHz range. The GSP-810 exhibits a noise-floor performance of ­95 dBm at 30 kHz with a typical ­100 dBm over its operating range. Furthermore, it grants input protection of 30 dBm and 25 V dc.

Options consist of a power meter, tracking generator, and remote-control software. Nine save/recall memories and two markers are available for absolute and relative measurements. An RS-232 interface and companion software transfers trace information from the GSP-810 to a PC. Spurious noise is ­60 dBc, or better.

Distance To Fault: An un-usual feature of IFR Systems' 2399 spectrum analyzer is its Distance to Fault (DTF) option. It enables test and field engineers to locate faults in feeder cables at basestations. By employing a return-loss bridge in conjunction with the 2399, users can measure the return loss of a coaxial feeder. Based upon cable characteristics that are resident in the 2399 analyzer, the DTF can locate faults along the cable responsible for the high return loss.

This product suits a variety of applications. It covers 9 kHz to 2.9 GHz and hosts a fast processor and a memory capable of storing up to 1000 screen traces and 2000 operational states. A marker system can simultaneously display up to nine markers, along with a marker table that shows the frequency and level of each marker selected. This means that multiple signals can be evaluated at once.

The 2399 provides delta markers, peak search, peak trace, 1/delta, marker track, marker-to-center, and marker-to-reference capabilities. Interfaces for IEEE-488, RS-232C, printers, and active probes are built in. The instrument employs a color TFT LCD display that's easy to read, even in high ambient light conditions, and it weighs less than 21 lbs (9.4 kg).

Not all spectrum analyzers target communications applications. For instance, Stanford Research Systems supplies FFT analyzers that are de-signed expressly for modal analysis, machine diagnostics, vibration testing, control systems design, and acoustic measurements. The SR760 and SR770 are both single-channel, 100-kHz FFT spectrum analyzers with a dynamic range of 90 dB and real-time bandwidth of 100 kHz.

With its low-distortion synthesized source, SR770 users can measure transfer functions of electronic and mechanical systems. In acoustic and noise-measurement applications, the SR760 computes the 15- and 30-band 1/3-octave spectra in common use. A-weighting compensation is available for octave measurements.

The spectrum, power-spectral density, and input-time record can be displayed in a variety of convenient linear and logarithmic units, such as V, V_rms, dBV, dBV_rms, or in user-defined "engineering units."

The magnitude and phase, as well as the real and imaginary parts of complex signals, can also be displayed. Several window functions are provided, such as Hanning, flattop, uniform, and Blackman-Harris. Users just select the appropriate function to optimize in-band amplitude accuracy, or to minimize out-of-band side lobes.

The SR770 version has a low-distortion (­80 dBc) source that generates sine waves, two-tone signals, white and pink noise, and chirps. These make frequency-response measurements to 100 kHz with 0.05-dB precision possible. Standard features on both units span total harmonic distortion; 1/3-octave, band, and sideband analysis; GO/NO-GO testing; and post-acquisition math.

Plus, Stanford Research Systems manufactures a two-channel spectrum analyzer, the SR785. It supplies a real-time bandwidth of 102.4 kHz, a dy-namic range of 90 dB, and 8 Mbytes of memory. A 32-Mbyte memory is optional, while a DOS disk drive, printer port, and RS-232 and GPIB computer interfaces are standard.