Explanation of the Real-time Spectrum Analysis Function of the PXA Signal Analyzer

In the aerospace, defense and wireless communications fields, emerging challenges make system characterization and troubleshooting more difficult. Take radar and electronic warfare (EW) systems, for example. These systems are becoming more dynamic, moving faster and covering larger spaces on the battlefield. This expansion of multi-format, high-speed communication systems increases the probability of interoperability issues. As signals become more complex and

sensitive, gapless measurement technologies—real-time spectrum analysis and time capture—are gaining mainstream acceptance. The Agilent PXA signal analyzer goes a step further by integrating these new capabilities into traditional signal analyzers, eliminating the need to purchase dedicated or single-purpose instruments. (Figure 1) Agilent

's real-time spectrum analysis (RTSA) is available as an upgrade option for new and existing PXAs, making the PXA the industry's first traditional signal analyzer that supports the addition of real-time analysis capabilities after purchase. It allows users to use real-time capabilities at one-tenth the price of a new real-time spectrum analyzer. RTSA

allows users to see, capture and understand elusive signals on the lab bench or in the field. For deeper analysis, users can use the real-time PXA with Agilent 89600 VSA software to fully characterize complex modulated signals.

Figure 1. The real-time PXA is a cost-effective solution that allows you to identify spurious signals using traditional swept analysis and switch to real-time mode to observe pulsed spurious signals.

Real-time analysis definition

Although different users understand "real-time analysis" differently, it contains a consistent core concept: for spectrum or signal analyzers with digital intermediate frequency (IF) sections, real-time operation will process all signal samples for a certain measurement result or trigger operation. (Figure 2) In most cases, the measurement result is a scalar quantity (such as power or amplitude) as opposed to a traditional spectrum measurement result.

Figure 2: Real-time operation occurs when the computation is fast enough to analyze the sampled data without gaps. In this case, the CALC operation includes the Fast Fourier Transform (FFT) calculation or power spectrum calculation, as well as the average calculation, display update, etc.

Not real-time operation: Gaps between time acquisitions

Real-time operation: No gaps between time acquisitions

In addition to gapless analysis, real-time RF analyzers have four key attributes: high-speed measurements, stable measurement speeds, advanced composite displays, and frequency mask triggers (FMT).

Typically, the spectrum stream during real-time processing is used for one of two purposes: the spectrum can be combined with a composite spectrum display or compared to a limit mask to perform frequency mask testing (FMT). The real-time PXA with Option RTSA supports both capabilities.

Performing Real-Time Spectrum Analysis

with the PXA Like dedicated real-time spectrum analyzers, the real-time PXA uses ASICs and FPGAs to convert sampled signal data into signal spectra at rates approaching 300,000 spectra/second. The spectrum data is combined to build detailed displays such as density or histograms. Alternatively, the spectrum stream is tested sequentially against limits and logic criteria to generate frequency mask triggers for specific spectra and specific behavior.

To enhance the performance of real-time spectrum analysis, the Agilent design team focused on four key areas: bandwidth, dynamic range, probability of intercept (POI) and integrated analysis capabilities.

Bandwidth

As the bandwidth and frequency bandwidth of the signals being analyzed increase, it is necessary for users to use higher bandwidth. The PXA with RTSA provides up to 160 MHz analysis bandwidth for real-time measurements, which is sufficient to handle today's wideband signals and signal environments. Gapless bandwidth applies not only to real-time spectrum analysis, but also to FMT, gapless time capture, and real-time amplitude calculation for IF triggering.

Another key point is that the real-time PXA is different from some similar products, and it can always collect gapless data within the bandwidth range of up to 160 MHz. Users can be confident that the real-time mode can always capture the details of intermittent signals or rapidly changing signals.

Dynamic range

The real-time PXA provides up to -75 dB spurious-free dynamic range within the 160 MHz bandwidth, allowing users to detect small, fast, sporadic signals when large signals are present. Dynamic range is enhanced by the PXA's low noise floor, and when dealing with very small signals, it can be further enhanced by adding the "Low Noise Path" option, which improves sensitivity while dealing with high-level signals. In all cases, the PXA's low noise floor enhances the ability to distinguish small signals from noise.

Probability of Intercept (

POI) is a key benchmark for real-time spectrum analysis. The real-time PXA can detect signals as low as 5.0 ns and as low as 3.57 μs with 100% probability of intercept (full amplitude accuracy). Gapless analysis is only part of POI. Other factors that affect instrument analysis performance include analyzer and processor dynamic range (including sensitivity), sampling bandwidth, processing continuity, and FFT overlap processing (for window function shape compensation). [page]

Integrated Analysis Capabilities

In some cases, simply finding an infrequent signal is enough: just knowing that a signal exists, or finding the general spectral shape, is enough to answer some questions, confirm a problem, or suggest a solution. In other cases, finding an infrequent signal is only the first step in solving a problem in a system or signal environment.

Using VSA software in conjunction with the real-time PXA, signals acquired in real-time mode can be fully analyzed and demodulated. In addition, real-time FMT can focus the full measurement capabilities of the VSA, including demodulation and time capture (Figure 3), on hard-to-find signals. This is particularly useful when measuring modulation transients, frequency hopping, frequency settling, and unintentional transients in signal sources such as voltage-controlled oscillators (VCOs).

Figure 3. FMT can be used to capture transient events in real time. Vector signal analysis provides multiple views, showing details such as a time-gated spectrum (top) and simultaneous display of the power envelope (blue) and time waveform (green).

To get the most out of signals acquired in real-time mode,

you need to pay attention to two additional features: histogram/density display and frequency mask triggering.

Histogram or Density Display

Real-time spectrum analyzers can generate thousands of spectra per second to capture the dynamics of frequency-agile or fleeting signals. Such high-speed signals are beyond the ability of the human eye to resolve, so it is necessary to display a large amount of measurement data in a single display trace. A real-time PXA can generate nearly 300,000 spectra per second. However, most people only see 30 spectra. To better observe real-time measurement results, approximately 10,000 results should be displayed for each display update.

By compiling statistics and showing how often a specific measurement value occurs (such as a specific amplitude at a specific frequency), the real-time analyzer will build an informative display. The measurement result histogram is an enhanced spectrum measurement feature that shows how often a measurement value occurs. Some people may think of it as a crude expression of probability.

The display is color-coded or intensity-coded. As old data disappears, persistence functions can be added to focus on recent events. Trace data (the most recent display update or an average) is overlaid to produce a trace that looks similar to a traditional spectrum measurement.

This approach helps engineers see and focus on infrequent or transient events and distinguish them from other events. By changing the persistence and weighting values ​​or schemes, specific events can be highlighted. The real-time PXA provides full trace cursor capabilities for the persistence display, allowing users to interpret and analyze measurement results.

FMT Technology and Applications

When viewing a specific signal, FMT can compare a high-speed data stream to a user-defined spectrum mask. A trigger is generated when the mask range is exceeded or the signal enters the mask area. A conditional trigger can be generated when the signal enters or re-enters the mask area. This feature is particularly useful for conditional triggering when the signal leaves the mask or re-enters the mask.

In the real-time PXA, the mask consists of upper and lower limits and can be entered numerically or graphically. The analyzer automatically generates a mask using the measurement signal environment, allowing the user to modify it as needed, thereby saving measurement time. To simplify this process, the mask is displayed simultaneously with the real-time measurement trace.

FMT can be used to generate relatively frequent continuous triggers. In contrast, one of the most powerful uses of FMT is to measure behavioral events where the measured behavior is very infrequent, with intervals of up to minutes or even hours. With VSA software, pre-trigger and post-trigger delay functions can capture the start and end of an event, or any signal between two points in time.

Improve the utilization of common tools

The current economic situation has put more pressure on most companies to make the most of existing equipment, such as traditional signal analyzers. On the other hand, strict spending controls make it more difficult to adjust and replace single-purpose tools (such as dedicated real-time analyzers).

This is one of the main reasons why Agilent is committed to developing the RTSA upgrade option, which can be added to new or existing PXA signal analyzers. It is also the reason why the PXA is designed to support the addition of subsequent functions (such as RTSA) from the beginning. As a result, Agilent delivers a comprehensive instrument that combines traditional functions and real-time analysis functions to provide consistent performance regardless of which mode is used. (end)