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Keysight Technologies

Understanding and Applying Probability of

Intercept in Real-time Spectrum Analysis

Application Note

Introduction

As today's wireless signals become more complex, the process of analyzing and understanding those

signals becomes more difficult. Examples include signals that have some combination of highly agile carrier

frequencies, digital modulation, time-division multiplexing and burst shapes. In addition, more devices and

systems are using the same frequency bands. As a result, it's becoming increasingly difficult to identify and

characterize interference and signal errors, especially when they come from transient signals.

When chasing an elusive signal, several attributes are important: when it occurs, how long it lasts, where

it occurs within the spectrum, and how large or small it is. Determining these attributes requires a signal

analyzer capable of performing real-time spectrum analysis (RTSA), real-time data capture, or both. Real-

time spectrum analysis is crucial for detecting, observing and identifying transient signals. Real-time data

capture enables detailed post-processing analysis, including demodulation.

The first step is detecting the transient signal, and the main question in RTSA is, "Which signals can I

see?" The key specification is probability of intercept (POI), which is actually a statistical property (see

the sidebar on page 2). In the specifications for a signal analyzer, POI is often expressed as the minimum

duration of a signal that can be observed with 100 percent probability--and accurately measured--if that

signal is a specific amount above the instrument's noise floor. As an example, a Keysight Technologies, Inc.

PXA X-Series signal analyzer equipped with 160-MHz analysis bandwidth and real-time spectrum analyzer

capability (both are optional) can detect a signal as short as 5