Real-Time Analysis Techniques for Making Wireless Measurements

Content

• Overview
  
• Using real-time measurements in wireless applications
• Visualizing agile signals and complex signal environments
• Triggering to find specific signals or signal behavior
• Aligning measurements with bursts Enabling flexible vector analysis and demodulation Reusing captured signals
• More information and examples

Overview

The Keysight Technologies, Inc. PXA and MXA X-Series signal analyzers have real-time functionality, allowing them to continuously process all the samples in an RF bandwidth from 40 to 160 MHz. As a result, analysis and triggering can be done without gaps or missing signal behavior. Real-time analysis is available as an option or retrofit for the MXA and PXA.

This application note will describe the two major functions of a real-time spectrum analyzer and how this functionality relates to the gap-free time capture with the PXA and MXA when used with Keysight 89600 VSA software. The capabilities can be used together in a complementary fashion to accelerate wireless system design and troubleshooting.

The first type of real-time analysis focuses on one or more advanced displays, such as density or spectrogram, to represent agile or elusive signals as well as the behavior of complex signal environments such as wireless communications bands.

The second type of analysis uses real-time spectrum processing to evaluate individual spectra as a way to trigger measurements of specific signals or signal behavior. Both capabilities can be useful in today’s dynamic, complex and demanding wireless environment.

Because they can find elusive signals and trigger multiple elements of signal behavior, these real-time capabilities are powerful tools for discovering problems or anomalies even when they are not suspected. These tools, and the associated analysis techniques, will allow wireless engineers to have more confidence in their designs and to achieve this confidence more quickly.

In many cases, though, discovery is often just the first step in perfecting wireless designs. Frequently, additional engineering is required to isolate signals or problems, quantify them, and determine cause and effect. Once accomplished, designs can be quickly optimized for the desired combination of cost, performance, power consumption, or other factors, and placed into efficient production.

Using real-time measurements in wireless applications

The most common real-time (or gap-free) analysis types used in wireless applications are continuous power-spectrum measurements and time capture with flexible postprocessing during capture playback. The continuous spectrum measurements are associated specifically with real-time spectrum analyzers (RTSAs), while capture/playback operations are generally performed by vector signal analyzers (VSAs). The continuous power-spectrum measurements of RTSAs are generally used in two ways:

• Generating spectrum displays such as density or spectrogram
  
• Testing spectral results against a mask to generate a frequency-mask trigger

An important additional, though less prominent, real-time scalar measurement is instantaneous channel amplitude or magnitude in which the “channel” is defined by the analyzer’s digitizing span. These magnitude measurements are provided in two ways. The Keysight RTSA provides a measurement type known as power versus time (PVT), and Keysight VSAs provide a magnitude-based gap-free trigger function along with a magnitude trace for measurement results.

As mentioned previously, the Keysight PXA and MXA X-Series RF analyzers are capable of RTSA, vector signal analysis, and traditional swept spectrum analysis using the same hardware platform.

All these measurements can be useful in wireless applications and will be described in the sections that follow. A summary of the types of tools, along with their benefits and limitations for real-time analysis in wireless applications, is shown in Table 1.

Visualizing agile signals and complex signal environments

As signals become more agile and signal environments become more complex, it’s increasingly useful to represent a large amount of measurement data on a single screen. In modern RF applications, perhaps the most familiar real-time measurements are described by the terms density, histogram or cumulative history, or spectrograms.

These new displays take advantage of the high-speed digital signal processing in RTSAs that generate thousands of spectra every second—many more than can be discerned individually by the human eye. In such cases, the most informative displays are created by compiling statistics and displaying how often a particular measurement value occurs (e.g., a specific amplitude at a specific frequency). An example result is shown in Figure 1.

The histogram of measurement results shown in Figure 1 is a spectrum measurement enhanced to show the frequency of occurrence. More than just a visual tool, the frequency of occurrence is quantified (typically in percent) and can be read at any frequency/amplitude point by a marker. The displays are coded using color or trace intensity, and a persistence function can be added to focus attention on more recent events as older data fades away. This allows engineers to see and focus on infrequent events or transients and separate them from other behavior. By changing persistence and color-weighting values or schemes, specific behaviors can be highlighted, providing an excellent way to quickly and comprehensively assess the spectral occupancy of a frequency band.