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Among the manufacturers of high-quality RF or microwave signal generators are Agilent, Anritsu, Rohde & Schwarz, National Instruments, Tektronix, Aeroflex, BNC, SRS, AnaPico, and Averna. When considering the modular PXIe platform for analog and vector signal generators, National Instruments, Agilent, and Aeroflex offer units that can be combined in a chassis with several signal generators or other test components for a compact and space-efficient test bench. To enable highly configurable modulation and test scenarios, Rohde & Schwarz, Agilent, National Instruments, Tektronix, and Averna offer software suites that are compatible with their signal generators. Some of these software suites even support global navigation satellite system (GNSS) simulations and other advanced sourcing simulations. Companies such as Anritsu and BNC incorporate NI’s LabView software to enhance their instruments with automation and control capabilities.

 

To keep up with cellular and wireless communications, many signal-generator software suites include modules for specific common standards, such as LTE, LTE-A, Wideband-CDMA, IEEE 802.11x, and many others. These packages automate standards testing in order to decrease the overhead when testing for new or complex standards. To help ensure proper testing, each test structure is labeled in a manner that is precisely aligned with the standard conformance specifications. The software will even automatically configure the power, frequency, and modulation of the signal generator and any coupled analyzers to aid in properly performing the characterization.

The most advanced RF vector signal generators are even capable of synchronizing with other generators to perform 4x4 multiple-input multiple-output (MIMO) tests for the latest LTE-A standards. For future standards, such as fifth generation (5G), methods like beam steering may come into play. Here, time and coherence synchronization among an array of generators is critical. For signal generators in a ganged configuration, achieving tight phase and amplitude control over many receive and transmit elements in an antenna array demands the implementation of challenging specifications. Currently, companies such as Agilent offer signal generators that can be ganged in configurations to 16 units.

The bandwidth demands for the communications industry are seen at higher frequencies in the aerospace/defense sectors. Modern EW and radar systems also have posed complex problems, which signal generator software is primed to tackle. For example, new pulse-compression techniques improve the range and resolution of radar systems. But they also increase test complexity. Such dynamic operations require much more rigorous functional testing.

With their complex tracking and identification algorithms, EW systems also push the boundaries of present signal generators. Their extremely wide bandwidths and software suites can replay lengthy and dynamic scenarios for maximum realism. As a result, sophisticated tests can be performed to stress-test these systems. According to Said, “You can couple two instruments together and create up to a 2-GHz bandwidth. If you are working in the Ka-band and centered at 18 GHz with that system, you can simulate multiple radar signals at the same time with that one source.”

With any precision test instrument, substantial concerns are raised about whether temperature and aging will affect the accuracy of the instrument. The cost of calibration and downtime for these instruments also poses a challenge. An example of an attempt to limit these concerns can be seen at SRS, which includes an option in its signal generators for a rubidium time-base. According to the company, the rubidium time-base degrades less in stability due to temperature variations, even below 1/20 ppm from 0° to 45° Centigrade. In addition, SRF says that the generators degrade less than 1/50 ppm per year due to aging. 

 

Locking In Channel Flatness For Linearity’s Sake

For all signal sources, the demands for linearity and low harmonic/spurious content are necessarily low for the latest applications. Some techniques to maintain these characteristics across the frequency and power ranges include active channel-flatness correction. Using Agilent’s method as an example, the signal source’s instantaneous channel response is characterized for both frequency and power immediately after manufacturing. The calibration coefficients derived from this testing are retained in the device’s internal memory. When the device is being tuned to a specific frequency and power level, the corrections for linear errors in the channel response are provided from memory. Intelligent correction algorithms are implemented with an application-specific integrated circuit to enhance the output performance of the source.

This method offers the greatest benefit in device characterization and high-end fidelity testing. An example would be the characterization of an amplifier. Any nonlinearities or linear errors in the output of the signal source will directly impact the amplifier’s output parameters. This could lead to inaccurate amplifier performance if the signal source is the limiting factor at the test bench.

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