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                                         Agilent
                                         Spectrum and Signal Analyzer
                                         Measurements and Noise

                                         Application Note




Measuring Noise and Noise-like Digital
Communications Signals with Spectrum
and Signal Analyzers
Table of Contents

Part I: Noise Measurements ............................................................................................................................................................. 3
Introduction....................................................................................................................................................................................................... 3
Simple noise--Baseband, Real, Gaussian ............................................................................................................................................... 3
Bandpassed noise--I and Q ........................................................................................................................................................................ 3
Measuring the power of noise with an envelope detector .................................................................................................................. 6
Logarithmic processing.................................................................................................................................................................................. 7
Measuring the power of noise with a log-envelope scale ................................................................................................................... 8
Equivalent noise bandwidth ......................................................................................................................................................................... 8
The noise marker ............................................................................................................................................................................................. 9
Spectrum analyzers and envelope detectors ......................................................................................................................................... 10
Cautions when measuring noise with spectrum and signal analyzers........................................................................................... 12

Part II: Measurements of Noise-like Signals .............................................................................................................................. 14
The noise-like nature of digital signals ................................................................................................................................................... 14
Channel-power measurements.................................................................................................................................................................. 14
Adjacent-Channel Power (ACP)................................................................................................................................................................. 16
Carrier power .................................................................................................................................................................................................. 16
Peak-detected noise and TDMA ACP measurements......................................................................................................................... 17
Peak-detected noise in continuous and sampled measurements .............................................................................................. 18

Part III: Averaging and the Noisiness of Noise Measurements.............................................................................................. 19
Variance and averaging................................................................................................................................................................................ 19
Averaging a number of computed results............................................................................................................................................... 20
Swept versus FFT analysis ......................................................................................................................................................................... 20
Zero span ...................................................................................................................................................................................................... 20
Averaging with an average detector ........................................................................................................................................................ 20
Measuring the power of noise with a power envelope scale ........................................................................................................... 20
The standard deviation of measurement noise ..................................................................................................................................... 21
Examples .........................................................................................................................................................................................................22
The standard deviation of CW measurements...................................................................................................................................... 23

Part IV: Compensation for Instrumentation Noise ..................................................................................................................... 24
CW signals and log versus power detection ......................................................................................................................................... 24
Power-detection measurements and noise subtraction ..................................................................................................................... 25
Noise Floor Extension (NFE) for noise compensation................................................................................................................... 25
Log scale ideal for CW measurements ................................................................................................................................................... 26

Bibliography ....................................................................................................................................................................................... 28

Glossary of Terms.............................................................................................................................................................................. 29




                                                                                                             2
Part I: Noise Measurements

Introduction                                                                The Gaussian PDF explains some of the characteristics of
Noise. It is the classical limitation of electronics. In                    a noise signal seen on a baseband instrument such as an
measurements, noise and distortion limit the dynamic                        oscilloscope. The baseband signal is a real signal; it has no
range of test results.                                                      imaginary components.

In this four-part paper, the characteristics of noise and its               Bandpassed noise--I and Q
direct measurement are discussed in Part I. Part II contains                In RF design work and when using spectrum analyzers, we
a discussion of the measurement of noise-like signals                       usually deal with signals within a passband, such as a com-
exemplified by digital CDMA and TDMA signals. Part III                      munications channel or the resolution bandwidth (RBW,
discusses using averaging techniques to reduce noise. Part                  the bandwidth of the final IF) of a spectrum analyzer. Noise
IV is about compensating for the noise in instrumentation                   in this bandwidth still has a Gaussian PDF, but few RF
while measuring CW (sinusoidal) and noise-like signals.                     instruments display PDF-related metrics.

Simple noise--Baseband, Real, Gaussian                                      Instead, we deal with a signal's magnitude and phase
Noise occurs due to the random motion of electrons. The                     (polar coordinates) or I/Q components. The latter are the
number of electrons involved is large, and their motions are                in-phase (I) and quadrature (Q) parts of a signal, or the real
independent. Therefore, the variation in the rate of current                and imaginary components of a rectangular-coordinate
flow takes on a bell-shaped curve known as the Gaussian                     representation of a signal. Basic (scalar) spectrum analyzers
Probability Density Function (PDF) in accordance with the                   measure only the magnitude of a signal. We are interested
central limit theorem from statistics. The Gaussian PDF is                  in the characteristics of the magnitude of a noise signal.
shown in Figure 1.




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