Product Trends: Signal Generators Meet The Latest Standards Head-On

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. 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 classes 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. As with the majority of RF/microwave systems, the application is what dictates which instrument features are the most appropriate for the job.

When a high-power and very linear CW source is needed, an analog signal generator may be the best choice. Obvious applications for an analog signal generator would be component verification or serving as a substitute for a local-oscillator (LO) input to a mixer. 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 Technologies’ 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 magnitude cleaner or better than the device under test.”

Additionally, the generator’s phase noise can directly 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 incoming signal. Although 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).

When advanced modulation techniques or on-site simulated tests are necessary, vector signal generators provide improved signal-modulation capabilities over analog signal generators. 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 system for more accurate or realistic threat simulations. That has a bearing on component design, receiver performance, and transmitter 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 solutions for tracking/navigation, audio/video broadcasting, cellular/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. 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.

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.