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USING THE SOFTWARE control capability of the latest signal generators can lessen the impact of complexity in modern modulation techniques and communications standards.

Signal generators have had to advance beyond the tunable continuous-wave (CW) devices of the tube age to serve an industry that is densely packed with complex modulation and high-fidelity requirements (Fig. 1). Naturally, this evolution has driven signal generators to incorporate advanced software control and ever greater modulation capabilities. Aerospace/defense and telecommunications applications require signal generators that span disparate frequency regimes and have very different problem spaces. But both industries need enhanced linearity, bandwidth, and sophisticated signal creation that can only be obtained by using the latest software techniques.

There are two main frequency regimes and two main class-es of signal generators. RF signal generators, which typically operate below 6 GHz, are often designed with features geared toward the communications industry. Microwave, and now millimeter-wave, signal generators operate beyond 6 GHz. They predominantly serve the aerospace/defense, satellite, radar, and electronic-warfare (EW) markets. The application is what dic-tates which instrument features are the most appropriate.

When a high-power and very linear CW source is needed, an analog signal generator may be best. Obvious applications for an analog signal generator would be component verification or serving as a substitute for a local-oscillator (LO) input to a mix-er. In either of these cases, the harmonic and spurious content of the signal are critical limiting factors in a quality test scenario. Riadh Said, sources platform manager for Agilent Technolo-gies’ Microwaves & Communications Division, explains, “The source is the unsung hero in this situation. It is used simply for stimulus. The number-one thing the source needs to do is not interfere with the measurement. It needs to be an order of mag-nitude cleaner or better than the device under test.”

Additionally, the generator’s phase noise can impact the sensitivity of a system. For example, a signal generator with

high phase noise operating as the LO drive in a radar system could desensitize the receiver enough to block out the incom-ing signal. While analog signal generators can often operate to high frequencies, they have limited modulation capability— frequency modulation (FM), amplitude modulation (AM), and occasionally phase modulation (PM).

Signal Generators Meet The Latest Standards Head-OnUsing the software control capability of the latest signal generators can lessen the impact of complexity stemming from modern modulation techniques and communications standards.

Product TrendsJEAN-JACQUES DELISLE | Technical Contributor

1. Signal generators

have come a long

way from the tube age

(right). They can now

incorporate computer-

controlled simulations

and complex vector

signal generation.

(Courtesy of Supreme

Instruments)

MAY 2014 MICROWAVES & RF

2. Several models of analog and vector signal generators support

“ganged” operation. They can help create complex signal composi-

tions to test the latest technologies. (Courtesy of Agilent Technologies)

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When advanced modulation techniques or on-site simulated tests are necessary, vector signal generators provide improved signal-modulation capabilities over analog signal generators (Fig. 2). According to Said, “Above 6 GHz, you are usually doing radar simulation and primarily with an analog source. But in extreme high-end EW simulations, you may go to a vector sys-tem for more accurate or realistic threat simulations. That has a bearing on component design, receiver performance, and trans-mitter performance.” In contrast to analog signal generators, vector signal generators are characterized by their bandwidth capabilities as well as the error vector magnitude (EVM) of their digitally modulated functions.

Vector signal generators also benefit from greater software integration. Often, they offer modules to provide platform solu-tions for tracking/navigation, audio/video broadcasting, cellu-lar/wireless connectivity, and higher-order digital-modulation schemes. These devices can now accurately replicate the signals received and stored by an analyzer in the field so that on-site testing can be brought to the lab bench (Fig. 3). These features do come with a higher price tag, as vector signal generators often cost substantially more than analog signal generators. The cost of signal-generator units also is affected by phase noise, bandwidth, output power, frequency range, switching speed, channel flatness, modularity, and modulation performance.

Among the manufacturers of high-quality RF or micro-wave 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 sourc-ing simulations. Companies like Anritsu and BNC incorporate NI’s LabView software to enhance their instruments with auto-mation 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 stan-dards 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 4×4 multiple-input multiple-output (MIMO) tests for the latest LTE-A standards. For future standards, such as Fifth Genera-tion (5G), methods like beam steering may come into play. 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 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 sec-tors. 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 rigor-ous functional testing.

EW systems also push the boundaries of present signal gen-erators. Their extremely wide bandwidths and software suites can replay lengthy and dynamic scenarios for maximum real-ism. 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, concerns are raised about whether temperature and aging will affect accuracy. 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. The rubidium time-base degrades less in stability due to temperature variations, even below 1/20 ppm from 0º to 45º centigrade. According to SRS, the generators degrade less than 1/50 ppm per year due to aging.

3. Real signals captured by analyzers can now be replicated by the

latest signal generators and software to bring on-site testing to the

laboratory. (Courtesy of Tektronix)