Jeff Thomas
Tom Holmes
Terri Hightower
Learn RF Spectrum
Analysis Basics
Agenda
• Overview: Spectrum analysis and its
measurements
• Theory of Operation: Spectrum analyzer
hardware
• Frequency Specifications
• Questions and Answers break
• Amplitude Specifications
• Summary
• Questions and Answers break
Page 2
Learning Objectives
• Name the major measurement strengths of a
swept-tuned spectrum analyzer
• Explain the importance of frequency resolution,
sensitivity, and dynamic range in making analyzer
measurements
• Outline the procedure making accurate distortion
measurements
Page 3
Overview: What is Spectrum Analysis?
8563A
Page 4
SPECTRUM ANALYZER
9 kHz - 26.5 GHz
Types of Tests Made
.
Modulation
Modulation
Noise
Noise
Distortion
Distortion
Page 5
Frequency Versus Time Domain
Amplitude
(power)
f
cy
n
e
requ
ti m
e
Time domain
Measurements
Page 6
Frequency Domain
Measurements
Fourier Spectrum Analyzer
Fourier analyzer transforms a signal over time
into a frequency spectrum
Amplitude
Display
f1
Page 7
f2
Frequency
Swept-Tuned Spectrum Analyzer
Filter “sweeps” over a frequency range
Amplitude
Display
f1
Page 8
f2
Frequency f
Agenda
• Overview: Spectrum analysis and its
measurements
• Theory of Operation: Spectrum analyzer
hardware
• Frequency Specifications
• Questions and Answers break
• Amplitude Specifications
• Summary
• Questions and Answers break
Page 9
Spectrum Analyzer Block Diagram
RF Input
Attenuator
Mixer
IF
IF
Gain Filter
Detector
Input
Filter
Local
Oscillator
Frequency
Reference
Page 10
Log
Amp
Video
Filter
Sweep
Generator
Display
The Mixer: Key to a Wide Frequency
MIXER
Range
Input
RF
fLO+fin
LO
0 fin
Page 11
fLO-fin
IF
fLO
0
0
fLO
RF = Radio frequency
LO = local oscillator
IF = intermediate
frequency
Intermediate Frequency (IF) Filter
IF FILTER
Input
• IF Bandwidth: also known as
resolution bandwidth and
RBW
• Provides shape of frequency
domain signal
Page 12
Actual
Displayed
f
f
Detector
Input
DETECTOR
Amplitude
Values Displayed
Positive detection: largest
Negative detection: smallest
Sample detection: last
Page 13
Video Filter
Input
VIDEO
FILTER
Without video filtering
Page 14
With video filtering
Local Oscillator and Sweep Generator
LO
SWEEP
GEN
frequency
LCD DISPLAY
• Provides swept display
Page 15
Input Attenuator and IF Gain
Circuits
RF INPUT
ATTENUATOR
IF GAIN
• Protects input circuits
• Calibrates signal amplitude
• Keeps signal display position constant
Page 16
Agenda
• Overview: Spectrum analysis and its
measurements
• Theory of Operation: Spectrum analyzer
hardware
• Frequency Specifications
• Questions and Answers break
• Amplitude Specifications
• Summary
• Questions and Answers break
Page 17
What Spectrum Analyzer
Specifications are Important?
•
•
•
•
•
•
Page 18
Frequency Range
Frequency and Amplitude Accuracy
Frequency Resolution
Sensitivity
Distortion
Dynamic Range
Frequency Range
Low frequencies for
baseband and IF
High frequencies for
harmonics and beyond
Page 19
Getting the Frequency Range You Need
fif
1
0 fin Range
IF signal, fif
1 fin
Mixer
3
2
0
LO
2
The input signal is
displayed when
fLO - fin = fIF
Page 20
LO Range
3
fLO-fin
fLO
0 frequency
fLO+fin
Getting the Frequency Range You Need
LO Feedthrough
3
fLO
fLO-fin fLO+fin
fif
1
fin
IF
filter
0 fin Range
LO
2
0
Page 21
Sweep
generator
LO Range
4 0 Display
Getting the Frequency Range You Need
Input signal
displayed
fLO - fin = fIF
3
fLO-fin fLO fLO+fin
fif
1
fin
0 fin Range
fin
LO
2
0
Page 22
LO Range
4 0
Getting the Frequency Range You Need
• Lower frequency limited by LO feedthrough
• Upper frequency limited by LO range and IF
frequency
• Microwave frequency measurement uses
harmonic mixing
Page 23
Frequency and Amplitude Accuracy
Absolute
Amplitude
in dBm
Relative
Amplitude
in dB
Frequency
Relative
Frequency
Page 24
Frequency and Amplitude Accuracy
•Frequency accuracy:
−Internal/external frequency reference
−Use of internal counter
•Amplitude accuracy:
−Not as good as a power meter
−Dependent upon measurement procedure
−Excellent relative measurements
Page 25
Signal Resolution
What Determines Resolution?
Resolution Bandwidth
RBW Type and Selectivity
Page 26
Residual FM
Noise Sidebands
IF Filter Bandwidth
Mixer
Input
Spectrum
LO
3 dB
3 dB BW
IF Filter/
Resolution Bandwidth Filter (RBW)
Sweep
RBW
Display
Page 27
Detector
Resolving Two Equal-level Signals
10 kHz RBW
3 dB
10 kHz
Page 28
Resolving Two Unequal-level Signals
•3 dB bandwidth
•Selectivity (filter shape)
3 dB
3 dB BW
60 dB
60 dB BW
Selectivity =
Page 29
3 dB BW
60 dB BW
Resolving Two Unequal-level Signals
• For a RBW of 1 kHz and a selectivity of 15:1,
the 60 dB bandwidth is 15 x 1 kHz = 15 kHz...
• …so the filter skirt is 7.5 kHz away from the filter’s
center frequency
60 dB
10 kHz
7.5 kHz
Page 30
Distortion
Residual FM
Residual FM
"Smears" the Signal
Page 31
Noise Sidebands (Phase Noise)
Phase Noise
Noise Sidebands can prevent resolution of
unequal signals
Page 32
Sweep Rate
Swept too fast
Penalty For Sweeping Too Fast
Is An Uncalibrated Display
Page 33
Analog versus Digital Resolution
Bandwidths
Analog
Filter
RES BW 100 Hz
Page 34
Digital
Filter
SPAN 3 kHz
Typical
Selectivity
Analog 15:1
Digital
5:1
Rules to Analyze By:
Use the Analyzer’s Automatic
Settings Whenever Possible
• When using the analyzer in its preset
mode, most measurements will be easy,
fast, and accurate
• Automatic selection of resolution
bandwidth, video bandwidth, sweep time
and input attenuation
• When manually changing the analyzer
parameters, check for “uncal” messages
Page 35
Are There Any Questions?
Agenda
• Overview: Spectrum analysis and its
measurements
• Theory of Operation: Spectrum analyzer
hardware
• Frequency Specifications
• Questions and Answers break
• Amplitude Specifications
• Summary
• Questions and Answers break
Page 37
Sensitivity and Displayed Average
Noise Level
RF
Input
Detector
Mixer
RES BW
Filter
LO
Sweep
A spectrum analyzer generates and
amplifies noise just like any active circuit.
Page 38
RF Input Attenuator Effects
Displayed noise is a function of RF input
attenuation
signal level
10 dB
Attenuation = 20 dB
Attenuation = 10 dB
Signal-to-noise ratio decreases as
RF input attenuation is increased
Page 39
IF Filter (Resolution Bandwidth)
Effects
Displayed noise is a function of IF filter bandwidth
Decreased BW = Decreased Noise
10 dB
10 dB
Best sensitivity = narrowest RBW
Page 40
100 kHz RBW
10 kHz RBW
1 kHz RBW
Video Bandwidth Effects
Video BW smoothes noise for easier
identification and measurement of lowlevel signals
Page 41
Sensitivity - the smallest signal that
can be measured
Signal
equals
noise
Page 42
~2.2 dB
Rules to Analyze By:
Getting the Best Sensitivity Requires
Three Settings
•
•
•
Page 43
Narrowest resolution bandwidth
Minimum RF attenuation
Sufficient video filter to smooth noise
(VBW < 0.01 Resolution BW)
Where is Distortion Generated?
Mixers Generate Distortion
Signal to be
measured
Frequency
translated signals
Mixer-generated
distortion
Page 44
Resulting
signal
Most Influential Distortion is the
Second and Third Order
< -50 dBc
Two-Toned Intermod
Page 45
< -40 dBc
< -50 dBc
Harmonic Distortion
Distortion Increases as a Function of
the Fundamental’s Power
2nd
3rd
Fundamental Harmonic Harmonic
∆ =1 dB
Power
in dB
∆=2 dB
∆=3 dB
3f
f
2f
For every dB fundamental level change,
the 2nd changes 2 dB and the 3rd changes 3 dB.
Page 46
How Distortion Amplitudes Change
∆ =1 dB
1 dB/dBfund
2 dB/dBfund
∆=2 dB
f
∆=3 dB
2f
3f
Since distortion changes relative to the
fundamental, a graphical solution is practical.
Page 47
Plotting Distortion as a Function of
Third Order
Mixer Level
Intercept - TOI
Distortion, dBc
0
.
-20
Second
Order
-40
-60
Third
Order
-80
-100
-60
-30
0 +5
+30
Power at the mixer =
Input level minus the attenuator setting, dBm
Page 48
Rules to Analyze by:
A Simple Distortion Test
Is the distortion from the signal or from the analyzer?
1
Change Input
Attenuation by 10 dB
RF INPUT
ATTENUATOR
No change in amplitude distortion is part of input
signal (external)
Page 49
2
Watch Signal on Screen:
IF GAIN
Change in amplitude - at least some
of the distortion is being generated
inside the analyzer (internal)
Dynamic Range Optimum Amplitude Difference
Between Large and Small Signals
Dynamic
Range
Page 50
Displayed Noise Limits
Dynamic Range
Displayed average
noise level can be
plotted like distortion.
Distortion, dBc
0
-20
.
-40
-60
-80
-100
1 kHz
RBW
-30
0
+30
Power at the mixer =
Input level minus the attenuator setting, dBm
Page 51
-60
Noise at 10 kHz
RBW
Distortion and Signalto-Noise, dBc
Dynamic Range as a Function of
Distortion and Noise Level
Page 52
0
-20
.
Maximum 2nd
Order Dynamic
Range
-40
Maximum 3rd
Order Dynamic
Range
-60
-80
-100
-60
-30
0
+30
Power at the mixer =
Input level minus the attenuator setting, dBm
.
Close-in Dynamic Range Limited by
Noise Sidebands
Dynamic Range
Limited By Noise
Sidebands, dBc/Hz
Sideb
and N
oise
100 kHz
to 1 MHz
Page 53
Dynamic Range
Limited By
Distortion and
Displayed Noise
Displayed
Noise Level
Rules to Analyze by:
Determining Dynamic Range
Your spectrum analyzer’s dynamic range
is dependent upon:
• Internal second and/or third order
distortion
• Displayed noise level
• Noise sidebands when close to
large signals
Page 54
Dynamic Range is
Defined by Your Application
+30 dBm Maximum Power Level
+10 dBm Mixer Compression
Measurement
Range 145 dB
-35 dBm Third-order Distortion
-45 dBm Second-order Distortion
Signal/Noise
Range
105 dB Third Order
0 dBc Noise Sidebands
Distortion
~80 dB
Second Order
Noise
Distortion
Sidebands
~70 dB
60 dBc/1 kHz
-115 dBm Displayed Noise (1 kHz RBW, 0 dB attenuation)
Page 55
Summary
• The RF spectrum analyzer is a heterodyne receiver
• Offers a narrow resolution capability over a wide
frequency range
• Measures small signals in presence of large signals
• Remember to:
—Adjust the measurement procedure for specific
application
—Test for internal distortion
—Take sideband noise into account
Page 56
Agilent Spectrum Analyzer Product Families - Swept Tuned
PSA Series
Highest performance SA!
O 3 Hz to 50 GHz
O Pre-selection to 50 GHz
O Worlds best accuracy
(0.24dB)
O 160 RBW settings
O Phase noise optimization
O FFT or swept at any RBW
O Complete set of detectors
O Fastest spur search
O Vector signal analysis.
O
ESA-E Series
Mid-Performance
O 30 Hz to 26.5 / 325 GHz
O Rugged/Portable
O Fast & Accurate
O Unparalled range of
performance and
application options.
O Remote WEB interface
O
ESA-L Series
856X- EC Series
Super Mid-Performance
O 30 Hz to 50 / 325 GHz
O Rugged/Portable
O Pre-selection to 50 GHz
O Color LCD Display
O Low Phase Noise
O Digital 1 Hz RBW
O
Page 57
Low cost
O 9 kHz up to 26.5 GHz
O General Purpose
O Rugged/Portable
O Fully synthesized
O
Agilent Vector Signal Analyzer Product Families
E4406A
89400 Series
Multi-Format wireless
capabilities
O 7 MHz - 4 GHz
O Fast & Accurate
O Simple User Interface
O Base-band IQ
inputs
Flexible Signal Analysis
O DC to 2.65 GHz
O 10 MHz Signal Bandwidth
O Block Digital demodulation
O Integral Signal Source
O Spectrum & Time waveform
Analysis
O Complex time varying signals
O Color LCD Display
O
O
89600 Series
Multi-Format & Flexible vector signal analysis
O DC – 6.0 GHz
O Bandwidth: 36 MHz RF, 40 MHz Baseband
O RF and modulation quality of digital
communications signals including WLAN.
O Spectrum & Time (FFT) Analysis
O OFDM Analysis (802.11a)
O Links to design software (ADS)
O PC Based for the Ultimate in Connectivity
O Analysis software links to PSA, ESA, E4406A signal
analyzers.
O
Page 58
89600 Ultra-wide bandwidth
500+ MHz Signal Bandwidth!
O 89600 Analysis Capability
O Low Cost Oscilloscope Front-end for
“RF Scope” measurements
O
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