News and New Products

RF-vector-signal generator combines high throughput, low phase noise

By Dan Strassberg, Contributing Technical Editor -- EDN, 10/6/2009

Keithley Instruments has upgraded its RF-vector-signal-generator line with new capabilities that reduce signal-generation times and enhance signal quality. In contrast with competitive generators, which typically force users to choose between best signal quality and maximum test throughput, the Model 2920A combines both capabilities in the same instrument. Patent-pending technology allows the instrument to support both frequency- and amplitude-switching rates that, according to the manufacturer, are significantly faster than those of competitive systems. The device is cost-effective for both production testing and design verification. The instrument, which provides signal-generation-bandwidth options to 80 MHz and frequency ranges from 10 MHz to either 4 or 6 GHz, sells for $17,850 and $23,460, respectively. It builds on the manufacturer’s Series 2900 signal-generator line and expands the line’s applications for testing wireless devices to more than 14 wide-bandwidth wireless-telecom standards that use complex modulation to achieve high throughput. The instrument performs quick and thorough calibration and testing of components, such as amplifiers, filters, and wireless receivers, over their full performance range.

Several major advances allow the unit to provide substantially higher test throughput than did earlier signal-generation products: A patent-pending technique, which ensures fast settling of phase-locked-loop synthesizers, allows the instrument to tune in 300 μsec or less to new frequencies in a DUT’s (device under test's) operating band; competitive products typically require 600 μsec or longer. The instrument provides electronic attenuation and a fast-tracking automatic-level-control loop to quickly stabilize at new power levels, allowing switching of amplitudes in as little as 150 μsec compared with more than 500 μsec for competitive generators. This fast settling reduces the time required to test devices over their full power-handling range.

Because testing of devices that operate on multiple standards requires rapid switching among numerous complex signals, the unit’s on-board arbitrary-waveform generator includes a 100M-sample memory that can hold large individual waveforms or multiple smaller ones. This deep memory combines with the instrument’s speed-optimized list mode and a sequencer operating mode to support switching from one waveform to another within a single processor-clock cycle—even with waveforms that conform to different standards and employ different modulation schemes.

A direct link from the instrument’s digital-signal-processing-circuit block to an external PC through a USB 2.0 (Universal Serial Bus) maximizes operating speed. In this high-speed desktop-control-panel operating mode, the external PC controls the instrument and acts as if it were the instrument’s onboard-processor controller. This mode supports 100-Mbps transfers of signal files from the PC directly into the instrument’s arbitrary-waveform memory and enables faster, simpler downloading of large files, such as streaming-video-test-signal files, signal files modified by channel models, and radar-profile files. With its high download speed and the manufacturer’s application software, the unit can quickly perform dynamic-frequency-selection-conformance testing on WLAN (wireless local-area-network) access points, verifying a device’s compliance with government and industry standards, which ensure that these transmissions do not interfere with radar signals, such as those for weather monitoring and air-traffic control. You can create waveform files offline and download them into the instrument’s arbitrary-waveform memory using a USB memory stick or the unit’s IEEE 488 or LAN interfaces. This transfer flexibility is convenient for those who create signal files with third-party tools, such as the MathWorks’ Matlab.

Unlike competitive signal generators, which typically maximize either throughput or signal purity, the 2920A’s design allows you to optimize whichever performance aspect is more critical in your application: High signal purity is essential when you create complex signals that conform to many wireless-communication standards. To ensure the quality of these signals, the 2920A-UPN (ultralow-phase-noise) option provides noise levels of ≤−135 dBc/Hz at a 300-kHz offset from a 2-GHz carrier. This performance allows the generation of wideband-code-division multiple-access signals with EVMs (error-vector magnitudes) of less than 0.85% of the signal amplitude. This option also permits the 2920A to generate 5.8-GHz, 40-MHz-bandwidth IEEE 802.11n WLAN signals with an EVM of less than −43 dB and WiMax signals with a residual relative constellation error of ≤−43 dB. In addition, when you use the UPN option, the 2920A provides tuning as fast as 750 μsec.

The 2920A’s wide signal range suits it to testing a broad spectrum of receivers and components. The 2920A-LAR (low-amplitude-range) option extends that range to allow the generator to output signals of −130 to +13 dBm. The unit’s accuracy, repeatability, and stability allow you to set narrow limit bands in your test protocols. Absolute amplitude accuracy to 3 GHz is ±0.6 dB maximum, or ±0.3 dB typical, from −110 to +13 dBm. Relative amplitude accuracy, or linearity, is ±0.05 dB, and amplitude repeatability is ±0.05dB. The 2920A is the latest addition to the manufacturer’s MIMO (multiple-input/multiple-output) testing systems. With the manufacturer’s SignalMeister RF-communications-test-tool-kit software and Model 2820A vector-signal analyzer, it can easily generate and analyze MIMO signals. With signal personalities that the software provides, the 2920A can not only generate pure signals but can also replicate impaired signals for in-depth receiver characterization.