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Spectrum Analyzer
Measurements and Noise
Application Note 1303
Measuring Noise and
Noise-like Digital
Communications Signals
with a Spectrum Analyzer
2
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 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) ...................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
The standard deviation of measurement noise...............................21
Examples...........................................................................................22
Part IV: Compensation for instrumentation noise ...................23
CW signals and log versus power detection ....................................23
Power-detection measurements and noise subtraction ..................24
Log scale ideal for CW measurements.............................................25
Bibliography .......................................................................................27
Glossary of terms ...............................................................................28
3
Part I: Noise measurements
Introduction
Noise. It is the classical limitation of electronics. In measurements,
noise and distortions limit the dynamic range of test results.
In this four-part paper, the characteristics of noise and its direct
measurement are discussed in Part I. Part II contains a discussion
of the measurement of noise-like signals exemplified by digital CDMA
and TDMA signals. Part III discusses using averaging techniques to
reduce noise. Part IV is about compensating for the noise in instru-
mentation while measuring CW (sinusoidal) and noise-like signals.
Simple noise--Baseband, Real, Gaussian
Noise occurs due to the random motion of electrons. The number
of electrons involved is large, and their motions are independent.
Therefore, the variation in the rate of current flow takes on a bell-
shaped curve known as the Gaussian Probability Density Function
(PDF) in accordance with the central limit theorem from statistics.
The Gaussian PDF is shown in Figure 1.
Figure 1. The Gaussian PDF is
maximum at zero current and i
falls off away from zero, as shown
3
i
(rotated 90 degrees) on the left. 3
A typical noise waveform is shown
on the right. 2
2
1 1
0 0
PDF (i)
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