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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.

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