Spectrum analysis - basics

Keysight Technologies
Spectrum Analysis Basics
Application Note 150

2 | Keysight | Spectrum Analysis Basics – Application Note 150
Keysight Technologies. Inc. dedicates this application note to Blake Peterson.
Blake’s outstanding service in technical support reached customers in all corners of
the world during and after his 45-year career with Hewlett-Packard and Keysight. For
many years, Blake trained new marketing and sales engineers in the “ABCs” of spectrum
analyzer technology, which provided the basis for understanding more advanced
technology. He is warmly regarded as a mentor and technical contributor in spectrum
analysis.
Blake’s many accomplishments include:
– Authored the original edition of the Spectrum Analysis Basics application note and
contributed to subsequent editions
– Helped launch the 8566/68 spectrum analyzers, marking the beginning of
modern spectrum analysis, and the PSA Series spectrum analyzers that set new
performance benchmarks in the industry when they were introduced
– Inspired the creation of Blake Peterson University––required training for all
engineering hires at Keysight
As a testament to his accomplishments and contributions, Blake was honored with
Microwaves & RF magazine’s first Living Legend Award in 2013.

3 | Keysight | Spectrum Analysis Basics – Application Note 150
Table of Contents
Chapter 1 – Introduction – What Is A Spectrum Analyzer? .........................................................................5
Frequency domain versus time domain .........................................................................................................5
What is a spectrum? ........................................................................................................................................ 6
Why measure spectra? ....................................................................................................................................6
Types of signal analyzers .................................................................................................................................8
Chapter 2 – Spectrum Analyzer Fundamentals ............................................................................................9
RF attenuator ...................................................................................................................................................10
Low-pass filter or preselector ........................................................................................................................10
Tuning the analyzer ........................................................................................................................................11
IF gain ...............................................................................................................................................................12
Resolving signals .............................................................................................................................................13
Residual FM .....................................................................................................................................................15
Phase noise ......................................................................................................................................................16
Sweep time ......................................................................................................................................................18
Envelope detector ..........................................................................................................................................20
Displays ............................................................................................................................................................21
Detector types .................................................................................................................................................22
Sample detection ............................................................................................................................................23
Peak (positive) detection ................................................................................................................................24
Negative peak detection ................................................................................................................................24
Normal detection ............................................................................................................................................24
Average detection ...........................................................................................................................................27
EMI detectors: average and quasi-peak detection .....................................................................................27
Averaging processes .......................................................................................................................................28
Time gating ......................................................................................................................................................31
Chapter 3 – Digital IF Overview ....................................................................................................................36
Digital filters .....................................................................................................................................................36
All-digital IF......................................................................................................................................................37
Custom digital signal processing ..................................................................................................................38
Additional video processing features ...........................................................................................................38
Frequency counting .......................................................................................................................................38
More advantages of all-digital IF ...................................................................................................................39
Chapter 4 – Amplitude and Frequency Accuracy ........................................................................................40
Relative uncertainty .......................................................................................................................................42
Absolute amplitude accuracy ........................................................................................................................42
Improving overall uncertainty ........................................................................................................................43
Specifications, typical performance and nominal values ...........................................................................43
Digital IF architecture and uncertainties ......................................................................................................43
Amplitude uncertainty examples ................................................................................................................... 44
Frequency accuracy ........................................................................................................................................44

4 | Keysight | Spectrum Analysis Basics – Application Note 150
Chapter 5 – Sensitivity and Noise ................................................................................46
Sensitivity.......................................................................................................................46
Noise floor extension ....................................................................................................48
Noise figure....................................................................................................................49
Preamplifiers .................................................................................................................. 50
Noise as a signal ...........................................................................................................53
Preamplifier for noise measurements .......................................................................... 54
Chapter 6 – Dynamic Range ........................................................................................ 55
Dynamic range versus internal distortion ...................................................................55
Attenuator test .............................................................................................................. 57
Noise .............................................................................................................................. 57
Dynamic range versus measurement uncertainty ...................................................... 58
Gain compression .......................................................................................................... 60
Display range and measurement range ...................................................................... 60
Adjacent channel power measurements ..................................................................... 61
Chapter 7 – Extending the Frequency Range .............................................................62
Internal harmonic mixing ..............................................................................................62
Preselection ................................................................................................................... 66
Amplitude calibration ....................................................................................................68
Phase noise ..................................................................................................................68
Improved dynamic range .............................................................................................. 69
Pluses and minuses of preselection .............................................................................70
External harmonic mixing ............................................................................................. 71
Signal identification ......................................................................................................73
Chapter 8 – Modern Signal Analyzers ......................................................................... 76
Application-specific measurements.............................................................................76
The need for phase information .................................................................................. 77
Digital modulation analysis ..........................................................................................79
Real-time spectrum analysis ........................................................................................80
Chapter 9 – Control and Data Transfer........................................................................ 81
Saving and printing data ..............................................................................................81
Data transfer and remote instrument control . ...........................................................81
Firmware updates .........................................................................................................82
Calibration, troubleshooting, diagnostics and repair .................................................. 82
Summary .......................................................................................................................82
Glossary of Terms ..........................................................................................................83
Table of Contents
continued

5 | Keysight | Spectrum Analysis Basics – Application Note 150
Fourier1 theory tells us any time-domain
electrical phenomenon is made up of
one or more sine waves of appropriate
frequency, amplitude, and phase.
In other words, we can transform a
time-domain signal into its frequency-
domain equivalent. Measurements in the
frequency domain tell us how much energy
is present at each particular frequency.
With proper filtering, a waveform such
as the one shown in Figure 1-1 can be
decomposed into separate sinusoidal
waves, or spectral components, which we
can then evaluate independently. Each
sine wave is characterized by its amplitude
and phase. If the signal we wish to analyze
is periodic, as in our case here, Fourier
says that the constituent sine waves are
separated in the frequency domain by 1/T,
where T is the period of the signal2.
This application note explains the
fundamentals of swept-tuned,
superheterodyne spectrum analyzers and
discusses the latest advances in spectrum
analyzer capabilities.
At the most basic level, a spectrum
analyzer can be described as a frequency-
selective, peak-responding voltmeter
calibrated to display the rms value of a
sine wave. It is important to understand
that the spectrum analyzer is not a power
meter, even though it can be used to
display power directly. As long as we know
some value of a sine wave (for example,
peak or average) and know the resistance
across which we measure this value, we can
calibrate our voltmeter to indicate power.
With the advent of digital technology,
modern spectrum analyzers have been
given many more capabilities. In this note,
we describe the basic spectrum analyzer
as well as additional capabilities made
possible using digital technology and digital
signal processing.
Frequency domain versus
time domain
Before we get into the details of
describing a spectrum analyzer, we
might first ask ourselves: “Just what
is a spectrum and why would we want
to analyze it?” Our normal frame of
reference is time. We note when certain
events occur. This includes electrical
events. We can use an oscilloscope
to view the instantaneous value of a
particular electrical event (or some
other event converted to volts through
an appropriate transducer) as a function
of time. In other words, we use the
oscilloscope to view the waveform of a
signal in the time domain.
Chapter 1. Introduction - What Is A Spectrum Analyzer?
Some measurements require that we
preserve complete information about
the signal frequency, amplitude and
phase. However, another large group
of measurements can be made without
knowing the phase relationships among
the sinusoidal components. This type of
signal analysis is called spectrum analysis.
Because spectrum analysis is simpler
to understand, yet extremely useful, we
begin by looking first at how spectrum
analyzers perform spectrum analysis
measurements, starting in Chapter 2.
Theoretically, to make the transformation
from the time domain to the frequency
domain, the signal must be evaluated over
all time, that is, over infinity. However, in
practice, we always use a finite time period
when making a measurement.
1. Jean Baptiste Joseph Fourier, 1768-1830. A French mathematician and physicist who discovered that periodic functions can be expanded into a series of
sines and cosines.
2. If the time signal occurs only once, then T is infinite, and the frequency representation is a continuum of sine waves.
Figure 1-1. Complex time-domain signal