Techniques for Characterizing
Spurious Signals
October 21, 2014
Riadh Said
Product Manager
Microwave and Communications Division
Keysight Technologies
Our Goals today
Review the sweep time equation to tradeoff dynamic range for sweep speed.
Review the basics of applying good spur
searching strategy.
Introduce new spectrum analysis
technologies to accelerate spur searching
for both R&D and manufacturing.
Page 2
Agenda:
1. What is a spur and why we care about them.
2. Introduce the sweep time equation for spectrum analysis.
3. Establishing your spur searching strategy.
4. Translate sweep time equation to real-life example
5. Techniques and technologies to manage your spur search time more effectively:
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New wideband, high dynamic range ADC & display signal processing
•
New fast sweep capabilities vs. traditional sweep
•
New stepped FFT approach with spur subtraction
•
Noise subtraction & new pre-amps for best sensitivity
Page 3
Assumptions & Definitions
– Spectrum analyzer basics such RBW, TOI, and SHI
– “Noise” = Displayed average Noise level (DANL), sensitivity, noise
figure
– Distortion = Harmonics 2nd and 3rd order and their products
Page 4
What is a spur?
Definition: spu·ri·ous, \ˈspyu̇r-ē-əs\  Adjective
1. not genuine, sincere, or authentic
2. of illegitimate birth
3. outwardly similar or corresponding to something without having its genuine qualities
4. of falsified or erroneously attributed origin
5. of a deceitful nature or quality
Your Design
What you wanted.
What you got.
Spurs & noise
Page 5
Where do spurs come from?
…Many many places
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Mixers  2nd and 3rd order harmonics + their mixing products with LO and IF inputs…
Multipliers  ½ rate , ¾ rate harmonics or “sub-harmonics”
Dividers  Odd Harmonics
Oscillators, LO, VCO’s, clocks  Leakage
PLL’s  Frac-N-Loops spurs
Amplifiers  2nd and 3rd order Harmonics
DAC’s  repetitive quantization errors…
Poor filtering & isolation/coupling
Incidental resonance from parasitic capacitance or inductance in circuit
Vibration  microphonic noise/spurs
Power supply: line noise or switching harmonics, 50/60 Hz and their harmonics
Glitch's or discontinuities in digital IF or baseband FPGA’s, ASIC’s etc…
The mixing products of any of the above.
When a non-linear device is presented with two or more input frequencies the output will generator both the input frequencies
and the intermodulation distortion products of those input at the same time.
Page 6
Why do we care?
…Link Budgets, Sensitivity, Range, Quality of Service
Radar/EW example: False target /threat detection or inference
Transmitter
Signal, f0
Spurious Signal
Return Signal, f1 (-85 dBm)
Satellite example: In channel Rx interference degrades sensitivity & range
Spurious Signal – Jam yourself
Very small received signal (-130 dBm)
Cellular example: Out of channel Interference pollutes neighbors receiver and
degrades range & data rates. Regulatory requirements. (i.e. FCC)
Spurious Signal – Jam your neighbor
Small received signal (-50 dBm)
Page 7
Spectrum Analyzer Dynamic range
Balancing Distortion, Noise & Test time
Block diagram of a classic superheterodyne spectrum analyzer
Spectrum Analyzer Dynamic range is limited by three factors:
1.
2.
3.
Distortion performance of the input mixer  2nd & 3rd order products
Broadband noise floor of the system.  Sensitivity/Displayed Average Noise level (DANL)
Phase noise of the local oscillator  Narrow band measurements.
The above three factors must be optimized in combination with your DUT and measurement
uncertainties requirements.
Page 8
Spectrum Analyzer Dynamic range
Distortion and Noise
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Increasing input attenuation reduces harmonic
distortion from spectrum analyzer. However this
also adds more IF gain which degrades noise
floor.
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To compensate for this you can reduce the RBW
and reduce the noise floor.
For more information see:
App Note 150: Spectrum analyzer basics
Page 9
Spectrum Analyzer Dynamic range
Uncertainties Budget for Noise + Distortion
Spectrum Analyzer Dynamic range must be optimized in combination with
your DUT requirements and measurement uncertainties you can tolerate.
Example of uncertainty budget:
Distortion Error budget: +/- 1 dB error = -18 dBc margin relative to DUT input
Noise Error budget: +/- 0.3 dB error = 5 dB margin relative to noise floor
Maximum total error: (+/- 1 dB) + (+/- 0.3 dB) = +/- 1.3 dB (excludes instrument uncertainty)
Distortion Error
Uncertainty versus difference in
amplitude between two sinusoids
at the same frequency
Noise Error
Error in displayed signal
amplitude due to noise
For more information see: App Note 150: Spectrum
analyzer basics and http://mwrf.com/author/bob-nelson
Page 10
The sweep time equation for spectrum analysis
Balancing Dynamic Range & Test Time
ST =
RBW = Resolution Bandwidth Filter
ST = Sweep Time
k = the constant of proportionality
k (Span)
RBW 2
The rise time of a filter (RBW) is inversely
proportional to its bandwidth, and if we
include a constant of proportionality, k,
then: Rise time = k/RBW
100x delta in sweep time
with a 10x delta in RBW!
RBW has a squared
relationship with time.
42 ms (300kHz RBW)
Noise floor change* =
10 log (BW2/BW1)
Where
BW1 = starting resolution bandwidth
BW2 = ending resolution bandwidth
-10 dB
4.2 sec (30 kHz RBW)
* Peak-detectors do not accurately represent the noise floor.
Note: The k value varies based on a number of conditions including filter shape for
RBW, VBW and detector types. Generally a value of 2 or 3 for Gaussian filters.
Page 11
Probability of Intercept for Swept Analysis
Odds of detecting intermittent spurious signals
Perfect POI = 1
Range: zero to one
POI =
(R+T)
(R+R’)
T = duration of the signal of interest
R = listening time at frequency
R’ = time not listening
R+R’ = revisit time
Note: Assumes signal can be discerned
R (approx) =
(RBW+SBW)*ST
Span
RBW = Resolution BW
SBW = spectral width of the signal
ST = spectrum analyzer sweep time
Span = spectrum analyzer span
The Term (R+R’) is the sum of the sweep time and the dead time
between sweeps. And it is also called the revisit time.
Listening time of swept-LO spectrum analyzers can easily be
approximated as the amount of time that some portion of the
resolution–bandwidth filter overlaps some part of the signal energy.
Page 12
Establishing your spur searching strategy
maximize test efficiency
Know where & what to look for:
Start with DUT block diagram, modes of operation & know design issues.
• Establish required target levels (Balance schedule & search time)
• Establish the type of spurious signals (static, moving, harmonic, random, modulated)
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Balance the needs in production vs. R&D:
• R&D and production teams should partner to focus on problem areas.
• Production target test levels ideally more relaxed if designed with margin.
• Focus on known product/component variation problems.
Determine uncertainty budget:
• Just enough margin to yield required results and minimize total test time.
• Remove tests that deliver 100% yield with high margins. Go to sampling.
• Apply appropriate measuring tools for max speed. (Swept, FFT, Real-time)
Band Filters
1
2
3
Unexpected ASIC errors
Baseband
/IF/ ASIC
LPF
LO
X2
Don’t waste time
Focus on known
looking outside
problems first.
filtered bands?
In Band
Out of Band
Don’t waste time
looking outside
filtered bands?
N
Page 13
Real-life example
The sweep time equation in action
 Target non-harmonic spurs: >-100 dBm
 Max signal input size = - 10 dBm
 Target attenuation = 16 dB
 Target measurement error budget ~ 2 to 3 dB
 RBW = 3 kHz, (peak detector, with pre-amp)
 Span = 10 MHz to 18 GHz
 Sweep time = 2.4k sec or 40 minutes
 300 different test modes of DUT
 Total test time = 200 hours!
 Repeat every time a new design change is made…
Page 14
Manage your spur search time more effectively
New Techniques and technologies
Large spurs > -75 dBc
Medium spurs > -100 dBc
Small Spurs < - 100 dBc
Page 15
Large Spurs >-75 dBc
New wideband high dynamic range digitization
& Processing
510 MHz BW
SDRAM
500 MHz
ADC
FPGA
FPGA
ASIC
CLK
Page 16
The Follow features available on Keysight X-Series
Analyzer & new N9040B UXA Signal Analyzer
Page
Page 17
Agilent Confidential
July 2014
510 MHz, High Performance IF for spur searching
New N9040B UXA Spectrum Analyzer
– See your spurs clearly with SFDR
of > -75dBc across 510MHz BW
– Monitor and capture highly elusive
spurs across the full analysis
bandwidth with real time signal
analysis
– Maximise dynamic range and
accuracy with excellent IF
frequency response of <0.7dB
For more information see application note:
Using Wider, Deeper Views of Elusive Signals to Characterize Complex
Systems and Environments 5992-0102EN
Page
Page 18
How to Capture the Intermittent Spurious Signal
– Questions about the signal of
interest
• What Frequency?
• How Often?
• How much Power?
• What is the Bandwidth?
• What Modulation?
• Where is the Noise Floor?
• What is the Phase Noise?
• Is there more than one signal?
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The Swept Analysis Mode
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A swept LO w/ an assigned RBW.
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Covers much wider span.
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Good for events that are stable in
the freq domain.
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Swept LO
Freq
Lost Information
Magnitude ONLY, no phase
information (scalar info).
Lost Information
Captures only events that occur at
right time and right frequency
point.
Lost Information
Data (info) loss when LO is “not
there”.
Time
Page 20
IQ Analyzer (Basic) Mode – Complex Spectrum and Waveform
Measurements
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A parked LO w/ a given IF BW
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Collects IQ data over an interval
of time.
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Performs FFT for time- freqdomain conversion
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Captures both magnitude and
phase information (vector info).
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Data is collected in bursts with
data loss between acquisitions.
Parked LO
Freq
Meas Time
or
FFT
Window
Length
Lost Information
Meas Time
or
FFT
Window
Length
Analysis BW
Time
Page 21
Real–Time Spectrum Analysis
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A parked LO w/ a given IF BW
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Collects IQ data over an interval
of time.
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Data is corrected and FFT’d in
parallel
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Parked LO
Freq
Vector information is lost
Acquisition or
slice time
Advanced displays for large
amounts of FFT’s
Acquisition or
slice time
Real-time BW
Time
Page 22
Real-Time Spectrum Analysis Hardware
ADC
(400 MSA/s, 14-bit)
Real-time corrections and decimation
Time Domain
Processor
Overlap
Memory
FFT Engine
(292,968 FFT’s/s)
Power vs Time
trace memory
Spectrum
trace memory
Density trace
memory
Frequency
Mask Trigger
Display processor
Page 23
Using Real-Time Spectrum Analysis
Benefits
– Gap free capture
– Supports wide bandwidths
– Real-Time capture of signals that are
present for only 2 ns with large S/N
ratio’s.
– For full amplitude accuracy UXA POI
expressed in time = 3.57 us
– Best at measuring the
shortest duration signals that
are infrequent or occur only
one time within 510 MHz
Window size = FFT windowing in points
Time record length = FFT bin size in points
P = Overlap FFT processing points
fs= sample rate
POI express in time duration T is the minimum length of the
signal of interest if it is to be detected with 100-percent
probability and measured with the same amplitude accuracy as
that of a CW signal.
For more information see application note:
Understanding and Applying Probability of Intercept in Real-time Spectrum
Analysis 5991-4317EN
Using Wider, Deeper Views of Elusive Signals to Characterize Complex
Systems and Environments 5992-0102EN
Page 24
Real-Time Spectrum Analysis – Density Display
Color coded for fast visualization & triggering
Page 25
Wider, cleaner analysis BW
>-75 dBc
Quickly Analyze large spans for spurs with confidence
Maximize the dynamic range for
optimum headroom.
510 MHz
Page
Minimize Measurement Uncertainty
IF Frequency Response UXA N9040B
<0.7
Minimize measurement
uncertainties across wide
instantaneous bandwidths
Page
Page 27
Stepped Density Method
Using Real-time Dwell
Capture of repetitive non CW signals over large bandwidths
Freq
Time
For more information see application note:
Using Wider, Deeper Views of Elusive Signals to Characterize Complex
Systems and Environments 5992-0102EN
Page
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Stepped Density Method
Using Real-time Dwell over full span
Full span of spectrum analyzer
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Medium to low Spurs (<-75 dBc)
New fast sweep vs. traditional sweep
SDRAM
500 MHz
ADC
FPGA
FPGA
ASIC
CLK
Page 30
The Follow features available on Keysight X-Series
Analyzer & new N9040B UXA Signal Analyzer
Page 31
Agilent Confidential
July 2014
Traditional Sweep – non-continuous signals
Limited by analog RBW rise time vs. accuracy needs
Standard Analog Sweep
Freq
Sweep
RBW
k (Span)
ST =
RBW 2
Signals
(R+T)
POI =
R =
(R+R’)
(RBW+SBW)*ST
Span
Time
Page 32
Fast Sweep – non-continuous signals
Compensated RBW rise time with improved accuracy
UXA Fast Sweep
Freq
Modified
k value
ST =
k (Span)
RBW 2
(R+T)
POI =
R =
(R+R’)
(RBW+SBW)*ST
Span
Time
For more information see :
5991-3739EN-Using Fast-Sweep Techniques to Accelerate Spur Searches
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Up to 50x faster vs. Traditional Sweep
Traditional Sweep
Fast Sweep
Full 26.5 GHz span
Full 26.5 GHz span
For more information see :
5991-3739EN-Using Fast-Sweep Techniques to Accelerate Spur Searches
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The Effects of Over-sweeping
RBW filter limits the rise time of signal
Oversweeping produces errors in frequency, amplitude and bandwidth
• Low amplitude: The displayed amplitude of the spurious
and other signals is lower than the true value, and by an
amount larger than the analyzer specifications would
indicate.
• Bandwidth spreading: The effective RBW of the
measurement is significantly wider than the selected value.
• Frequency shift: The apparent center frequency of
spurious and other signals is higher than the true value, and
by an amount larger than the analyzer specifica- tions would
indicate.
Leveraging real-time DSP during fast sweep, the phase response of the RBW filter is
adjusted based on the sweep rate to compensate for oversweeping effects. This maintains
the correct amplitude and bandwidth of the detected signal, even at very high sweep rates.
For more information see :
5991-3739EN-Using Fast-Sweep Techniques to Accelerate Spur Searches
Page 35
FFT S WEEP SPEED IMPROVEMENT
Sweep Speed Improvement with IF CHIRP Processing
> 10 x Faster!
Without Chirp
Area where Chirp IF
processing improves
speed
With Chirp
FFT Swept
Page
Page 36
Fast Sweep Repeatability
Holding sweep time constant while
using a narrower RBW to measure
CW signals reduces measurement
variance because the narrower
filter blocks more of the broadband
noise.
Comparing fast sweep to traditional sweep, the lower values and shallower slope of the blue data points
(fast sweep) show that repeatability is improved and varies less with sweep time.
For more information see :
5991-3739EN-Using Fast-Sweep Techniques to Accelerate Spur Searches
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Fast Sweep Benefits Summary
Fast-sweep technology provides at least four important benefits:
1. Dramatically reduced sweep times for CW spur searches over wide spans and
narrow RBWs. (> 10x faster)
2. Improved measurement throughput while maintaining accuracy, frequency
selectivity and consistent bandwidth
3. Improved measurement repeatability at faster sweep rates
4. Simplified selection of RBW to get a desired combination of dynamic range and
repeatability, because repeatability depends almost entirely on dynamic range
rather than both dynamic range and sweep time.
Note: Installed base Keysight (Agilent) X-Series spectrum analyzers can be upgraded with fast sweep.
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Medium to Low Spurs (< -75 dBc)
Stepped FFT
FFT- Based
Page 39
The following features available on the M9393A
PXIe Performance Vector Signal Analyzer
Page 40
Stepped spectrum analysis
High-speed stepped FFTs to analyze wide spans
 Benefits:
 Fast “sweeps” and excellent dynamic
range with narrow RBW
Single FFT up to max
analysis BW (160 MHz)
 Search for spurs, measure harmonics
or analyze multiple signals at once.
 Trade-off
 Solid state front end limits max
sensitivity, but can be balance with
narrower RBW’s and speed.
 Method:
 Multiple FFTs are concatenated to
create a span >the IF bandwidth
 High speed LO and digitizer list mode
enables fast stepping across span
 Software stitches together FFTs and
displays single trace result up to 27GHz
Span up to frequency range of analyzer (27 GHz)
 Fastest stepped spectrum analysis requires wide
analysis bandwidth and fast frequency tuning
options, and powerful computer.
Page 41
Digital Image Rejection
Fast, accurate measurements without hardware pre-selector
High & Low Side Mixing
Minimum
detect
Smart Image rejection processing
 Method:
1. Adjust LO making 2 acquisitions: high-side
mix then low-side mix
2. Use minimum detection algorithm to
determine real signals
3. Use additional techniques including maxhold, IF dithering and using narrow RBW to
accurately measure even challenging signals
For more information see application notes:
5991-4039EN - Achieving Excellent Spectrum Analysis Results Using
Innovative Noise, Image and Spur-Suppression Techniques
Page 42
Digital image rejection
Fast, accurate measurements without hardware pre-selector
 Benefits:
 Fast tuning speed
 Excellent amplitude accuracy
 Compact physical size
2 GHz LFM
signal
AWG 12
GHz clock
AWG alias
product
Challenging test scenario: Measuring 2 GHz wide linear FM chirp
from arbitrary waveform generator centered at 2 GHz
For more information see application notes:
5991-4039EN - Achieving Excellent Spectrum Analysis Results Using
Innovative Noise, Image and Spur-Suppression Techniques
Image-protect
Page 43
Power Spectrum mode for high-speed stepped FFT analysis
Results
– Measure spurs & harmonics across 27 GHz @ 10 kHz RBW in < 1
second
– Achieve > 300 GHz/sec sweep speeds
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Stitched FFT with Digital Image Rejection Summary
Optimized for speed & compact form factor
– Fastest tuning speed enabled by:
Frequency tuning speed (nominal)
< 3.6 GHz
175 us
3.6 to 8.4 GHz
135 us
8.4 to 13.6 GHz
135 us
13.6 to 17.1 GHz
155 us
17.1 to 27 GHz
145 us
105 dB
dynamic range
1 GHz span
Meas. time:
1 second
• Very fast LO & all solid-state design
• List mode executes predefined set
of acquisitions from digitizer FPGA
• Takes advantage of latest processor
technology – M9037A or high-power
PC
– Outstanding speed to dynamic range to
see low-level signals quickly
– Leading to shorter test times & higher
throughput in design validation &
production
Page 45
Low Spurs (< -100 dBc)
How to improve the noise floor
Pre-Amps
Noise subtraction
Page 46
Noise Floor Extensions/Corrections
Subtracting the noise floor of the spectrum analyzer
Benefit:
 Accurately measure signals
close to the noise floor
Trade off:
 Increases variability typically
requires more averaging when
near the noise floor and
therefore more time.
Method:
1. Measures internally generated noise (N)
2. Measures input signal and noise (S + N)
Feature available on both X-Series & M9393A
3. Subtract the two measurements for
corrected result: (S + N) – N = S
4. Use averaging/VBW filter to see effects of
noise correction
Software driven
For more information see application notes:
5990-5340EN - Using Noise Floor Extension in the PXA Signal Analyzer
5991-4039EN - Achieving Excellent Spectrum Analysis Results Using
Innovative Noise, Image and Spur-Suppression Techniques
Real-time ASIC
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USB Pre-Amps
Pre-calibrated plug & play operation
Page 48
U7227x USB Preamplifiers
– Pre-calibrated and ready to use with X-Series
– Noise Fig ~ 5dB
U7227x USB Preamps
– Gain >17 dB (makes NF of SA
negligible).
– USB provides power to Preamp,
and reads gain, noise fig, and Sparameter data from flash.
– UXA SA app can use preamp as
“remote front end”; correct absolute
amplitude vs frequency displayed.
Model
Frequency Range
X-series Analyzers + USB Preamplifiers provide:
U7227A
10 MHz to 4 GHz
U7227C
100 MHz to 26.5 GHz
U7227F
2 GHz to 50 GHz
 2X improved noise figure beyond 10 GHz up to 50 GHz
 Improved measurement uncertainty up to 1/3
 Lower DANL/noise floor improving to -171 dBm/Hz
Keysight USB Pre-amplifiers: 5991-4246EN:
Page 49
Summary
1. Start with a smart spur search strategy
2. Balance it with the Sweep time considerations
3. Leverage modern digital IF processing:
• Real-Time
•
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Stepped Density Capture of repetitive non CW signals over large bandwidths
Real-time Density Best at measuring the shortest duration signals that are infrequent or
occur only one time within 510 MHz
• Pre-Selected Fast sweep  Best at measuring both small and large signals
with or without modulation at high speed.
• Stitched FFT with digital image & spur rejection  Ideal for continuous
CW like spurs. Ideal for the fastest speeds when absolute noise floor can be
traded off.
• Noise Floor corrections – When you can trade-off speed for dynamic range.
4. USB Pre-Amps – Simplified calibrated setup to extend noise floor, improve
uncertainties, or increase sweep speed with wider RBW’s.
Page 50
Where to learn more/References
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Spectrum Analyzer Basics, App Note 150, 5952-0292:
http://literature.cdn.keysight.com/litweb/pdf/5952-0292.pdf
Using Wider, Deeper Views of Elusive Signals to Characterize Complex Systems and Environments:
http://literature.cdn.keysight.com/litweb/pdf/5992-0102EN.pdf
Using Fast-Sweep Techniques to Accelerate Spur Searches, 5991-3739EN:
http://literature.cdn.keysight.com/litweb/pdf/5991-3739EN.pdf
Achieving Excellent Spectrum Analysis Results Using Innovative Noise, Image and Spur-Suppression Techniques
http://cp.literature.agilent.com/litweb/pdf/5991-4039EN.pdf
Understanding and Applying Probability of Intercept in Real-time Spectrum Analysis :
http://cp.literature.agilent.com/litweb/pdf/5991-4317EN.pdf
Measuring Agile Signals and Dynamic Signal Environments: 5991-2119EN:
http://cp.literature.agilent.com/litweb/pdf/5991-2119EN.pdf
Using Noise Floor Extension in the PXA Signal Analyzer:
http://cp.literature.agilent.com/litweb/pdf/5990-5340EN.pdf
N9040B UXA X-Series Signal Analyzer: www.keysight.com/find/UXA
M9393A PXIe Performance Vector Signal Analyzer: www.keysight.com/find/M9393A
Keysight USB Pre-amplifiers: 5991-4246EN: http://literature.cdn.keysight.com/litweb/pdf/5991-4246EN.pdf
Spur Calculator from Marki Microwave: http://www.markimicrowave.com/WepApps/Spur_Calculator.aspx
Technical Expert Bob Nelson: http://mwrf.com/author/bob-nelson
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Thank You!
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