Measure the Impossible: A New Approach to Spectrum Analysis with the FSWX

Designed to overcome the limitations of current measurement methods, the new FSWX signal and spectrum analyzer from Rohde & Schwarz features a groundbreaking architecture that integrates multiple input ports with advanced cross-correlation technology and will enable entirely new measurement scenarios in RF system testing.

The field of signal and spectrum analysis has long been at the heart of innovation in RF system design, enabling engineers to characterize devices and systems critical to applications in radar, satellite, wireless communication, and more. However, as technologies evolve to demand wider modulation bandwidths, higher data rates, and more complex signal structures, traditional approaches to signal analysis are beginning to show their limitations.     

For decades, engineers have relied on signal and spectrum analyzers to measure and characterize RF performance. While these instruments have served the industry well, their inherent design constraints present challenges as demands on performance and measurement precision increase. In particular, dynamic range for modulation analysis, noise performance, and the ability to analyze signals across multiple channels and frequencies have emerged as critical bottlenecks. To address these challenges, we at Rohde & Schwarz have developed a fundamentally new approach to spectrum analysis, embodied in the FSWX signal and spectrum analyzer.

Fig 1: The FSWX from Rohde & Schwarz helps to discover signals once out of reach.

The Challenge of Modern RF Systems   

In today’s RF landscape, engineers face increasingly complex measurement scenarios. For example, wideband signal analysis often requires extremely low noise floors to detect small spurs or weak signals, even in the presence of much stronger carriers. Traditional signal and spectrum analyzers are inherently limited by their own noise floor, which obscures these weak signals.

Dynamic range is another critical factor, particularly for applications that assess the modulation quality like error vector magnitude (EVM) measurements. EVM depends on distortion and a high signal-to-noise ratio. Unfortunately, the noise introduced by the measurement instrument itself can often mask the true performance of the device under test, limiting the accuracy of these measurements.

Moreover, as RF systems become more sophisticated, there is a growing need to analyze signals across multiple input ports simultaneously. Phased array antennas, for instance, require precise phase and amplitude measurements across multiple paths to ensure optimal performance. For component characterization, the input signal can be used as a reference and directly related to the output signal to measure group delay, distortion or EVM. Similarly, engineers working on multi-standard communication systems or interference analysis need tools capable of handling signals from different sources or frequencies simultaneously. Traditional single-port spectrum analyzers are simply not equipped to handle these scenarios efficiently.

Finally, the issue of preselection becomes critical at higher frequencies. Many analyzers rely on yttrium iron garnet (YIG) filters, which, while effective for narrowband measurements, can introduce challenges in terms of repeatability, accuracy, and frequency response. In addition, the maximum available bandwidth is mostly limited to around 50 MHz. When these filters are bypassed for wideband signal analysis, unwanted signals and mixing products can leak into the measurement band, further complicating the analysis. Furthermore, for spectrum analysis when sweeping over a wide frequency range, YIG filters significantly slow down the measurement time.

The above-mentioned limitations have forced engineers to adopt various workarounds, such as noise cancellation techniques like IQ averaging, using multiple instruments in parallel, or applying external preselection. While these methods can be effective, they often introduce additional complexity, reduce efficiency, and increase testing costs.

A New Approach to Spectrum Analysis  

Solving these challenges requires more than incremental improvements — it requires a rethinking of the way signal and spectrum analyzers are designed. With the new FSWX signal and spectrum analyzer (Fig 1), Rohde & Schwarz set out to create an instrument that breaks free from traditional constraints, offering engineers a tool capable of tackling the most demanding measurement tasks with precision and simplicity.

The FSWX introduces a completely new architecture that features multiple input ports and built-in cross-correlation technology. By addressing the root causes of limitations in traditional analyzers, the FSWX opens the door to measurement scenarios that were previously considered impossible.

Central to this new architecture is the ability to eliminate the noise introduced by the instrument itself. Built-in cross-correlation technology allows the FSWX to suppress its own noise floor, enabling engineers to detect signals that were previously masked (Fig 2). This capability is particularly transformative for spectrum analysis, where capturing small spurs or weak signals near thermal noise levels is critical. The result is a dramatic improvement in dynamic range, allowing for more accurate and reliable measurements.

Fig 2: The cross-correlation technique of the FSWX can lower the test noise floor with only a slight increase in measurement time: The blue trace with applied cross-correlation reveals spurs hidden in the yellow trace without cross-correlation.

Another key innovation is the multi-port input architecture of the FSWX. By incorporating multiple input ports and internal signal paths, the FSWX allows engineers to perform simultaneous measurements across different channels or frequencies. This capability simplifies complex setups, such as those required for phased array antenna testing, where precise phase and amplitude alignment across multiple signal paths is essential. In addition, engineers can now analyze mutual interference of different signals at different frequencies, assess timing differences across multiple ports or characterize components by capturing a wideband input signal on one port and the output signal at the other port — all within a single instrument.   

Preselection, often a limiting factor in traditional analyzers, has also been reimagined in the FSWX. The instrument incorporates advanced preselection capabilities with switched filter banks to suppress unwanted signals and mixing products, ensuring clean and accurate measurements even at higher frequencies. This is particularly beneficial for applications such as wideband signal analysis or amplifier characterization, where signal integrity is paramount.

Transforming Measurement Scenarios     

The practical implications of these innovations are far-reaching. Consider the challenge of EVM measurements in scenarios where the signal-to-noise ratio is inherently limited. With traditional tools, engineers often struggle to achieve meaningful results, especially when the signal applied to the instrument is already low in power or heavily attenuated by the test setup. The cross-correlation technology extends the low-power range of the EVM bathtub curve, enabling accurate measurements even in these challenging conditions. Moreover, it improves EVM performance in the mid-to-high power range, where phase noise from the analyzer itself would typically dominate.

Phased array systems, another demanding application, also benefit significantly from the FSWX’s capabilities. These systems, used in radar, autonomous vehicles, and advanced communication systems, rely on precise phase and amplitude alignment across multiple channels. The new analyzer’s multi-port architecture makes it possible to measure signals across identical or even different frequencies simultaneously, providing insight into amplitude, phase, and timing variations across antenna ports (Fig 3). This capability not only simplifies testing but also enhances the accuracy of the resulting measurements.

Fig 3: Phased array antenna test under modulated conditions requires comparing one element to the nth element.

Engineers working on amplifier characterization can take advantage of the simultaneous input and output signal analysis offered by the FSWX. By capturing both the input and output signals of an amplifier in real-time, the FSWX enables direct measurement of spectral regrowth caused by nonlinearities, as well as AM-AM conversion near the compression point (Fig 4). This holistic view of amplifier performance simplifies the characterization process and provides more comprehensive insights.

Fig 4: Amplifier characterization with simultaneous input/output signal measurement and spectral regrowth, featuring view of both frequency and time domain data.

Even in the realm of electronic warfare, where capturing and analyzing modulated pulsed radar signals is critical, the FSWX offers unique advantages. Engineers can capture these signals at both the input and output of a device simultaneously, characterizing amplitude and phase variations with a single instrument. The FSWX’s advanced triggering options, including independent frequency settings for each receive path, further enhance its versatility in these applications.

Measuring the Impossible

With the FSWX, Rohde & Schwarz presents a paradigm shift in signal and spectrum analysis, offering engineers a tool designed not just to meet today’s challenges but to anticipate future needs. By combining multi-port architecture, cross-correlation technology, and advanced preselection, the FSWX enables measurements that were previously out of reach. Whether it’s detecting the faintest signals in the presence of strong carriers, analyzing complex multi-channel systems, or simplifying intricate test setups, the FSWX empowers engineers to push the boundaries of what’s possible.