practical rf measurement techniques & probing tools for ... · pdf fileılee is the...

Hosted by: Rohde & Schwarz and Interference TechnologyPresented by: Lee Hill, SILENT Solutions LLC

Dates & LocationsThursday, March 10 – Austin, TXTuesday, March 15 – Irvine, CAThursday, March 17 – Milpitas, CATuesday, March 22 – Livonia, MIThursday, March 24- Chelmsford, MA

Practical RF MeasurementTechniques & Probing Tools forEMC Troubleshooting

Program Presenter: Mr. Lee Hill

ı Lee is the Founding Partner of SILENT Solutions LLC, an EMC and RF design firm he established in 1992. Lee is a member of adjunct faculty at Worcester Polytechnic Institute (WPI) where he teaches graduate-level classes in EMC. He is also an EMC instructor at University of Oxford (England), and for the IEEE EMC Society’s annual Global University and EMC Fundamentals program.He earned his MSEE in electromagnetics from the Missouri University of Science and Technology EMC Laboratory under Dr.’s Tom Van Doren,Todd Hubing, and James Drewniak.

ı Contact Lee or his business partner Randal Vaughn for more information about SILENT Solutions EMC Design Reviews, Troubleshooting and Intensive Courses

www.silent-solutions.com Tel: +1 [email protected]

2© 2016 ROHDE&SCHWARZ

http://www.silent-solutions.com/

mailto:[email protected]

Outline

ı Energy Coupling Mechanismsı Why CURRENT is important in typical regulatory noise problemsı Key PASSIVE Probe Characteristicsı Voltage Probesı Clamp-on Current Probesı Magnetic Field Probesı Electric Field Probesı Self contained Immunity Generator “active probes”ı Our Favorite Tools

ı Special Thanks to Dr. Tom Van Doren, Professor Emeritus, Electrical and Computer Engineering Department, Missouri University of Science & Technology, for use of his original source material contained in this section

PART 1: Probing Tools


WHAT Are We Trying To Do With A Probe?

LISTEN / FIND / MEASUREa noise quantity FROM an existing source

INDUCE / REPRODUCEa noise quantity INTO an existing victim


If We Have An Electrical Noise Problem,We Must Be Able to Find:

NoiseSource “Victim”Path


Most “Radiated” Noise Problems Can Be Understood by Locating the Path of the Responsible Noise Current

Radiated EMISSIONSRadio Wave

E,H

Radiated Immunity

Currentdi/dt Antenna

Radio WaveE,H

Currentdi/dt Antenna

“transmitting”

“receiving”

Radiated Emission

Radiated IMMUNITY


Presenter

Presentation Notes

DEMO

Most “Conducted” Noise Problems Can Be Understood by Locating the Path of the Responsible Noise Current

Conducted EMISSIONS

Measured Noise Voltage

Conducted Immunity

EUTNoise

Current

Noise Generator Current

Current driven into EUT

Conducted Emission

Conducted IMMUNITY

V+-


Presenter

Presentation Notes

DEMO

WHY Do We Use PROBES?

ı Pass a “stupid” EMI test; fix a problem that causes EMC regulatory test failure

ı Understand or fix a “functional” noise problem

ı Characterize an environment

ı Characterize a device


What Makes a Good EMC Probe?

ı BandwidthF1 < Frequency < F2

ı Sensitivity (Listening), Efficiency (Injecting)Small signal preamplifier (listening) or big signal amplifier (injecting) necessary?

ı Impedance Loading (of circuit under test)Does the probe disturb the (SSS, I,V) quantity being measured?

ı Field PerturbationDoes the probe disturb the field quantity being measured?

ı Impedance BalanceWhen trying to measure Idm or Vdm, does Icm increase?

ı Repeatable Positioningı Ruggedness

Usually the difference between purchased and homemadeı Costı Calibration


What Are We Going to Measure with Our Probe?Coupling

MechanismQuantity

MeasuredMeasuringProcess

ConductedVoltage (V) Resistive Divider

+ Amplifier

Current (I)Resistive Shunt (I=V/R )

Magnetic Coupling (Xfrmr)

Near FieldMagnetic Field (H) Magnetic Coupling

V=Mdi/dt ∝ dH/dt

Electric Field (E) Displacement CurrentI = CdV/dt ∝ dE/dt

Far Field EM Wave (E,H) AntennaVout= AF x |E|


Passive Voltage Probes (Listening to locate sources)

Radiated Emissions:

ı Evaluating Radiated Emissions at Aperture of Conductive Chassis byMeasuring Voltage Differences Across Seams, Apertures, Connector Pins

ı Finding Circuit Nodes with “Bonus Noise” (Unexpected RF sources)


Problems with Passive Voltage Probes (Listening to locate sources)

Sources of Measurement Error:

ı To Reduce Error, minimize the “CRITICAL AREA”ı Maybe Can Reduce Error by Reducing Icm with ferrite beads around coax cable

A

B

VAB

“CRITICAL AREA”

dH/dt

Vmeasured+-

Icm

-

+

|Vmeasured| = VAB ± ω1MICM ± ω2∫∫H·ds


Problems with PEOPLE Trying to Measure“Noisy Ground” or “Ground Plane Inductance” or “Voltage”Sources of Measurement Error:

v+ -

V depends on location of wiresand amount of flux wrapping the plane

Do we want a wide plane or narrow plane?

“Critical Area”is obvious


Passive, Unbalanced X10 Voltage Probe Design

ı Inexpensive. Only 500 Ω Zin . Flat Freq. Response >> 1 GHz, ESD insensitive

SolderBrass Tube

Epoxy5 mmMax

450 ΩResistor

SemirigidCoaxCable

SMA Connector

To50 ΩLoad


Passive Voltage Probes (Listening to locate noise sources)Radiated Emissions:


APERTURE CONNECTOR PIN

ı Measuring voltage difference across many different seams or pins might identify the location of a dominant energy leakage

conductiveenclosure

50 Ω450 Ω

X10 Coax Probe

50 Ω


Passive Voltage Probes (Listening to locate noise sources)Radiated Emissions or E field (dv/dt) Source:


ı Finding Circuit Nodes with “Bonus Noise” (Unexpected RF sources)

Vmeasured ≈ Vnoise

when

50 Ω >>1/ωC

50ΩC

Equivalent Circuit

Vnoise

PCB

CoaxCable

CIC

DC Block50 Ω


Presenter

Presentation Notes

Demo

Active Voltage Probes (Listening to SIGNAL during NOISE Event)

PCB

Fiber

IC50 Ω Scope

ı Fiber optic analog or digital signal detector, hardened against RF, monitors critical EUT signal via fiber to active receiver.

ı For use during conducted immunity or radiated immunity or ESD tests.Measure health of intended signals with isolated sensor to avoidRF coupling via measurement wires.

Sensor ActiveReceiver

Induce Noise


Presenter

Presentation Notes

Demo

Using an Active Voltage Probe (Generator) to Induce Noise to Locate Victim

Electrostatic Discharge

ı Identifying the precise location of victim circuits that cause system level ESD failure

ı Finding Circuit Nodes with “Bonus Sensitivity” (Unexpected RF victims)


Active Noise Probes (Generators) To Localize Victim Circuits Sensitive to ESDElectrostatic Discharge (ESD)Manual Injection Methods can be Tricky / Ineffective

ı Identifying the precise location of victim circuits that cause system level ESD failure

IC

Which IC / Trace Causes System ESD Failure?

PCB

ICIC

IC

IC


Active Noise Probes (Generators) To Localize Victim Circuits Sensitive to ESDElectrostatic Discharge (ESD)

ı Identifying the precise location of victim circuits that cause system level ESD failure by using handheld generator with (contact + capacitance) connection

PCB

ICIC

IC

IC

Which IC / Trace Causes System ESD Failure?

PCB

ICIC

IC

IC1.2 kV, 1.8 nS min edgetime,single or 5 kHz

|E|


Passive Current Probes (Listening for CE or REInjecting for CI or “Simulated” RI)Clamp-on Current Probe

Shielded – We only want to detect dH/dt and thus the current (listening)We only want to deliver i(t) (injecting)

Loads

Inet = Icm

Net current Enclosed = Probe withElectric Fieldshield makesthis term = 0

=•∫ dlHconduction∫ •dSJ

displacement

∫ •+ dSDdtd

lListen Inject


Passive Current Probes (Listening for RE)

Clamp-on Current Probe

Under far field conditions, at a distance of R meters from a λ/2 dipole

𝐸𝐸 = 60 𝐼𝐼𝑅𝑅

where I = current (A) at center of dipole

I < 𝑅𝑅 |𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸|𝑅𝑅

= 3𝐸𝐸 (100 𝜇𝜇𝜇𝜇/𝐸𝐸)60 Ω

= 5 µA (FCC Class B) @ 3m)

LoadsInet = Icm


Passive Current Probes (Listening for CE or REInjecting for CI or RI)Clamp-on Current Probe - Requires Electric Field Shielding

Magnetic Core

Electric Field Shield

Gap to block eddy currents

The wire under test can capacitivelycouple to the probe winding through the slot. This is usually minor atF< ~ 400 MHz.

Some folks use a foam “donut” to prevent cables from getting veryclose to the gap, causing loadingand measurement repeatabilityproblems.


Passive Current Probes (Listening for CE or RE,Injecting for CI or “Simulated” RI)

| I | = | V | / | ZT | IdB = VdB – ZT dB , VdB = IdB + ZT dB

For listening to noise current, we want a “sensitive” probe that delivers max V for a given Inoise so we want ZT to be ___.

For inducing a noise current, for a given available RF output, we want a probe that delivers max I for a given power so ZT = _____

Clamp-on Current Probe

IL (R=50 Ω)

+V-

Magnetic CoreHow do you likea 0 dB Probe?


Passive Current Probes

Clamp-on Current Probe – Simplified Equivalent Circuit of Output

L +V-

R = 50ΩSpec An.

+

- ( ) T22ZI

LR

MRIV =ω+

ω=

|ZT|

Freq.

For ωL << R, |ZT| = ωM

For ωL >> R, |ZT| = MR/L


Passive Magnetic H Field Probes (Listening for CE or RE, or Injecting H)

I

We can use a simple loop probe to:1) Detect the relative amplitude AND direction of time-changing current(nice if it’s calibrated)

2) Detect the relative amplitude of a time changing magnetic fielde.g., if we’re failing a magnetic field emissions test or lookingfor “leaky” inductive components.

dHVdt

∝

=•∫ dlHconduction∫ •dSJ

+ V

- +-

+ -

V +-H

Coax Cable


Passive Magnetic Field Probes

27

Where’s the Magnetic Flux?

R+

-

1 cm loop

> 1 cm aperture

The probes below can help find larger problems, such as noise exiting an enclosure slot, a big IC, or big connector. Not calibrated. Robust. F< 2 GHz

Why Not Use This?

© 2016 ROHDE&SCHWARZ

Passive and Active Magnetic Field Probes

28

For locating sources of radiated emissions, wireless self-interference

Passive: These can localize noise to aconnector pin or region of a small IC.Calibrated. 1 GHz ≤ F ≤ 10 GHzResolution ~ 1 to 2 mm

Active: These can localize noise to a via or BGA bump.Calibrated. 100 MHz ≤ F ≤ 6 GHzResolution ~ 200 um

H


Use Passive H field Probes for Immunity Debug

29

Some Vendors Characterize Max Forward Power and Input Match

H

H

or

PCB

ICIC

IC

IC


Passive E field Probes

30

How do these work? Where is the loop of currentthat drives the spectrum analyzer input?

50 ΩE


Presenter

Presentation Notes

Demo

Passive E field Probes

31

How do these work? Where is the loop of currentthat drives the spectrum analyzer input?

ZOZO

CP

Circuit emittingE-field

CI

V+

-E

VO-+

ZX

V α E;|I| = |ωCV| α E

ZCp Zo

I

|V O | = |OP

O

ZCj1VC Z

ω+ω

| α E

50 ΩE

ZX


Presenter

Presentation Notes

Demo

Use ACTIVE H and E field Probes With Internal Noise Generator for EFT, ESD Debug

32

Non-Contact. Handheld E or H. 1 pulse or 5 KHz. Edgetime < 5 nS

PCB

ICIC

IC

IC

H

H

or

E


Pitfalls of Homemade Probes

ı H field loop probes and current probes must be shielded, otherwise; Confusing measurement results

May interact with human’s capacitance May interact with E, dv/dt of circuit under test

Poor spatial resolution – It’s tough to makereally tiny, shielded, homemade H-field probes.

ı Homemade probes are often uncalibrated Difficult to distinguish between “interesting” and “important” when multiple

sources exist. Large bandwidth of commercial probes is often tough to duplicate.


Favorite Passive Current Probes

Fischer F-61Listening

1 MHz ≤ F ≤ 1 GHz

Fischer F-16Listening

10 Hz ≤ F ≤ 70 MHz

Fischer F-16Injection

10 kHz ≤ F ≤ 400 MHz


Favorite Passive Near Field E + H Probes

Van Doren CompanyMFH-23MLF-11

40 MHz ≤ F ≤ 400 MHz3 kHz ≤ F ≤ 300 kHz Langer EMV-Technik

RF-R400-1RF-E 02RF-E-10

RF-U 2.5-2RF-B 3-2

F ≤ 6 GHzFinest Spatial Res.

Rohde & SchwarzHZ-15 Probe Set

ETS 1 and 3 cm loop probesF ≤ 1.5 or 2.3 GHz


Favorite “Active” E + H Probes

“Pulsers”: P1 set, P23Noise Generators

All Made By Langer EMV-Technik

MFA Active H Field Probes For Listening


Where to Buy the Tools

Places to buy the good stuff that SILENT uses:

Real-Time EMC Spectrum Analyzers and Test ReceiversRohde & Schwarz www.rohde-schwarz.com

Handheld E & H Probes, Noise Generators Langer EMV-Technik www.langer-emv.comVan Doren Company www.emc-education.com/

Clamp-on Current ProbesFischer Custom Communications www.fischercc.com


http://www.emc-education.com/

http://www.rohde-schwarz.com/

http://www.langer-emv.com/

http://www.emc-education.com/

http://www.fischercc.com/

PART 2: RETURN LOSS – How Good Is That RF Cable, Adapter….


Let’s Do Some RF Characterizations ofPassive and Active Two Port Devicesı Insertion Loss and Return Loss RF test cables RF adapters

ı Input impedance Match & Resonant Frequency Preamplifiers Antennas Filters

ı Broadband RF "Snapshots" Antennas LISNS

ı DO IT _ALMOST_ FOR “FREE” IF you already have access to aspectrum analyzer / test receiver with internal tracking generator.


Outline

ı What are the different types of equipment we can use?

ı Advantages & disadvantages of each

ı Demonstrations of measurements

ı A few recommendations of equipment to get started


Different Types Of Equipment We Can Use

ı PROS: "Gold Standard". If you have one, GREAT!

ı CONS: If you've never used one, lots of things to learn; calibration, measurement

techniques, S parameters.

If already have one at work, it is often the guarded, locked up machine that belongs to product design...don't touch! And you have to SHARE

41

VECTOR NETWORK ANALYZER



ı Vintage, entry level used machine > $20k Rohde & Schwarz ZVL

ı The Amateur Radio Community has created a number of one and two port VNAs with impressive capability. Not serviceable, No NIST – traceable calibration possible Limited dynamic range above 500 MHz, depending on instrument Require external PC and software. A good toy to sneak in the budget and practice with.

42

VECTOR NETWORK ANALYZER



ı Many EMC folks already have them Familiar Frequency domain data lets EMC engineer apply his/her

intuition about frequency response, attenuation, bandwidth, etc

ı Can't do precise port calibrations ESRP, ESR, ESPI and ESCI offer SOL - "open", "short", "load" Need to purchase external directional coupler

ı Factors that limit dynamic range to (only) 40 dB typical Directional coupler directivity (basic figure of merit) Quality of external 50 ohm reference affects dynamic range Quality of external “plumbing” including cables and adapters Somewhere above 1 GHz, we should abandon this idea and instead use a

real VNA. Dominant issues are: lack of precise reference, plane location, and the need for RF adapters

43

Test Receiver/Spectrum Analyzer withTracking Generator

TG Input


Measurements

ı Insertion Lossı Return (reflection) Loss


Insertion Loss, (“Through”, “Transmission”)

ı Easy, most people understandı May not be sensitive enough to detect problems with "low loss" devicesı DUT must have conducted input port and conducted output port

ı Not appropriate for source or load devices, e.g., "one port" such as preamplifier, near-field probe, antenna

ı SMALL measurement result, e.g. 0.1 dB, is GOOD

45

Input Output

50 ohm load Input

CalibratedRF Source

CalibratedRF Receiver

DUT


Insertion Loss, (“Through”, “Transmission”)

ı Plotted versus frequency, in dB, requires two (in & out) RF ports at EUT

ı Scalar result, Doesn’t distinguish between reflected & absorbed power

46

A B

RF Source(TG)

RF Receiver(Spec. An)

0 dBcoupler

Step 2: Insert Device Under Test, then measure

Step 1: Normalize test cable / path to 0 dB

A B

CalibratedRF Source

CalibratedRF Receiver

DUT


Presenter

Presentation Notes

How about attenuator?

Return (Reflection Loss) Testing

ı Not everybody knows how to do

ı Appropriate for one and two port devices

ı Great for one port devices

ı Reveals resonant behavior of DUTs

ı BIG measurement result is GOOD – usually minimum of 20 dB

47

RETURN (REFLECTION) LOSS

TestReceiver

TrackingGenerator Device Under TestDirectional

Coupler

Forward Power

Reflected Power

Freq.

20 dB


Results of Return (Reflection) Loss Testing

ı Good match Incident power is absorbed at DUT (load), very little returns (reflects) to source.

GOOD Result = Big return loss, ≥20 dB

ı Bad match Most of incident power is REFLECTED at DUT, very little is absorbed,

most of incident power returns (reflects) back to source BAD Results = Small return loss,

ı Resonant networks show INCREASE in return loss at their favorite frequenciesı With TG/Spec An, Result is a SCALAR, with VNA, a VECTOR (S11)

TestReceiver


Coupler

Forward Power

Reflected Power



ı Return Loss defined as: -20Log(ρ)ı # of dB the reflected signal is less than ("below“) the incident signal.ı GOOD MATCH = BIG RETURN LOSS!

ı Very sensitive to small +/- changes above/below Zo;ı Return Loss = 40 dB, VSWR = 1.02 (nearly perfect match)ı Return Loss = 20 dB, VSWR = 1.22 (good match)

ΓL = Vreflected ρ φ

ranges from 0 to 1.0

=ZL- Zo

ZL+ Zo

=Vincident



TestReceiver


Coupler

Forward Power into DUT

Reflected Power from DUT


Return Loss Testing: How to Do It

TestReceiver

DirectionalCoupler

TrackingGenerator

ı Step1: Establish maximum measurable RL using 50 ohm reference loadı Step 2: Replace reference load with Device Under Test, and measure RL

1) 50Ω load, 2) DUT


Return Loss Testing: Measurement Demos

TestReceiver

DirectionalCoupler

TrackingGenerator

ı Good 50 ohm loadı 3 dB attenuatorı Different Unmarked 50 ohm “through”

terminations

1) 50Ω load, 2) DUT


Return Loss Testing: Measurement Limitations

ı "Reference plane" poorly defined – Can’t separate DUT data from fixtureı Sensitivity of measurement directly proportional to directivity of

directional coupler – typically 30-40 dB for inexpensive onesı Accuracy of load termination used for load reference calibration

Network

50

50

50TG

Spec An

DUT

Directional Coupler


Return Loss Testing: Recommended Equipment

ı "Test Receiver/Spectrum Analyzer + Tracking Generatorı Rohde & Schwarz – any of these ESL, ESPI, ESCI, ESRP, ESR, ESU

with tracking generator option installedı Directional Coupler ("bridge"); Narda 3020, Mini-Circuits ZFDC-10-2 ı Key parameter is directivity, the higher the better. Match frequency range

to your interestsı An expensive 50 ohm type "N' or "SMA" termination that "bottoms out"

your return loss measurement to the claimed directivity of your coupler

Network

50

50

50TG

Spec An

DUT

Directional Coupler


Faster Hardware Troubleshooting of Intermittent […] Signals UsingReal-Time FFT Spectral Analysis

Hosted by: Rohde & Schwarz and Interference TechnologyPresented by: Lee Hill, SILENT Solutions LLC

Dates & LocationsThursday, March 10 – Austin, TXTuesday, March 15 – Irvine, CAThursday, March 17 – Milpitas, CATuesday, March 22 – Livonia, MIThursday, March 24- Chelmsford, MA

Outline

ı Why I’m excited about“Real Time” spectral analysis.“Persistence Mod”

ı Quick review of “EMI signals”

ı Demonstrations and Debug Tips

ı Please interrupt me with questions

© 2016 ROHDE&SCHWARZ 56

Local Oscillator3.4-6.6 GHz

PreampPreselection

Digital Magic

DetectorsDisplayMarkers

Limit linesTrace evaluation

IF (RBW) Filters

Full RF Spectrum (9 kHz - 3 GHz)

Bandpass Filtered

RF Spectrum 2-8 MHz Mixed RF

+ and -

Attenuator

Old Style “Swept” Spectrum Analyzers or “Stepped” Tune ReceiversThe “old days”ı “Swept tuned” Spectrum Analyzerı “Step tuned” Receiverı Sequentially step or sweep a narrow window across the frequency axisı Good “intercept” with “continuous wave”, periodic signal ONLYı Significant “blind time” when sweeping or stepping


Old Style “Swept” Spectrum Analyzers or “Stepped” Tune ReceiversThe “old days”ı “Swept tuned” Spectrum Analyzerı “Step tuned” Receiverı Sequentially step or sweep a narrow window across the frequency axisı Good “intercept” with “continuous wave”, periodic signal ONLYı Significant “blind time” when sweeping or stepping

S OL W

Freq.narrow


“FFT” , “Fast Fourier Transform”“Time Domain” InstrumentsNOW, with FFT (“time domain”)

ı Much FASTERı Measure lots of signal frequencies simultaneously

“in parallel”, over a WIDE CHUNK of the freq. axisı Calculate RBW instead of waiting for filter responseı Still some “blind time” to do processing (not “100% intercept”)ı And only 1 measurement at each frequency in each chunk

Freq.wide


“Real Time” FFT

“Real – Time” FFT (“time domain”)ı FASTEST – dedicated measurement hardware for FFTı WIDE (up to 40 or 80 MHz) chunk of spectrumı Calculate RBW instead of waiting for filter responseı Zero “blind time” “100% intercept”ı Multiple measurements at each frequency in each chunkı Allows for generation and display of signal statistics, i.e.,

how often and how strong a signal is present at a given frequency

Freq.40 MHz


Review of Signals We’re Going to See

ı Definitions and buzzwords that EMC engineers like to use when discussing different types of noise


To1/To

finite rise / fall time

EMI Signals – Funny Namesı Continuous (“CW”)ı Periodicı “Power Signals”

RBW wider than line spacingF

a) Measurement value increaseswith increased RBW

b) Cannot identify (resolve)individual lines

RBW narrower than line spacingF

a) Measurement valuedoes not change with RBW

b) Can identify (resolve)individual spectral lines

“Broadband” (SMPS, motor control, sparky) “Narrowband” (clocks, steady RF)


With Permission Dr. T. Hubing, Clemson University

Theoretical, perfect Impulse Infinite Energy

Freq.Time

Single, real pulse

Finite Energy is spread out

Time Freq.40 MHz chunk in Real-Time FFT

EMI Signals – Funny NamesA Single Real Pulse “Broadband Discontinuous” (Ex. ESD)ı Measurement value increases with increased RBWı Finite Energy, Average power = 0ı Represent with Fourier Transform → Energy Spectrum


Let’s Watch 100% Signal InterceptThat’s Possible w/ “Real Time” FFT

1) Standard FFT, spectrum view

2) Real-Time FFT, spectrum view“100% (signal) intercept”

DEMONSTRATION OF Spark Noise Similar to ESD


A Totally Different Way to View, Measure, Identify EMI Sourcesı With Real Time, many measurements / frequency, can generate statisticsı The trace color shows how often a signal occurs at a

specific frequency and amplitudeı Color = “Hit Rate” = how often signal occurs at particular frequency

Height = amplitude of signal at particular frequencyı A signal histogram versus frequencyı Descriptive terms: % occupancy, amplitude, frequency

Persistence Mode

F

Amplitude


Next, We Are Going to Watch Multiple Signals at the same time, as they occupy the SAME frequency


Demonstration of Real Time FFTDisplaying “Signal under Signal”ı “Signal under Signal”ı Two signals at same frequency, but

different amplitude different % occupancy (“100% = CW” = “White hot”)

ı PWM motor + motor controller (broadband)ı USB mouse (broadband + narrowband)ı 20 MHz digital clock on PCB (narrowband)


Demonstration of Real Time FFTDisplaying “Signal under Signal”

ı One Bluetooth Pair. Master + Slave

ı Measuring + displaying the radiated signal with antenna


Demonstration of Real Time FFTMeasuring Class D Audio Amp Noise ı Common-mode current on speakerphone USB cable.


A Debug Technique Not Everyone Uses


Demodulation (not an FFT thing )

ı Why do we only “Look At” EMI?ı In the “Old Days”, everyone listened to EMIı Can distinguish between data, clocks, ambient signals, etc.


Summary – Old Technology

ı Previous generation spectrum analyzers and test receivers use “swept tuned” or “stepped tuned” architecture.

ı OK for measuring continuous (i.e., not intermittent) periodic signals in narrow spans. When you ALWAYS know when (time) and where (frequency) the signal is.

ı Cannot visualize pulse spectrumsı Cannot visually distinguish between two or more broadband signals in a single

piece of spectrum ı Cannot visualize narrowband EMI signals “under” a larger broadband signalı “Blind” during large parts of sweep or step & dwell timeı Challenging to use for EMI debug expect for “trivial” signals like narrowband

clocks that have 100% occupancy


Summary: Real-Time FFT

ı “Real Time” FFT with persistence mode gives us anew, powerful way to visualize EMI signals that werepreviously tough or just impossible to display:

Broadband and intermittent (pulse) signals Multiple broadband signals occupying the same spectrum Multiple broadband and narrowband signals occupying the same spectrum

ı The “visualization” that is possible with Real-Time FFT is difficult to describe in words, you have to see it


Summary Take-Aways

ı Real-Time FFT provides:

100% signal intercept over a 40 or 80 MHz frequency span. FFT machines without real-time hardware will “miss” signals

A new intuitive way to VISUALIZE EMI signals Especially non-periodic, “blinky” and broadband signals

(e.g., SMPS, motor control, & sparky signals - different from digital clocks)

The ability to clearly display “signal under signal” situations: Multiple broadband signals occupying the same frequency span Weak narrowband signals “underneath” larger broadband signals Multiple, co-located RF emitters that occupy the same frequency span

such as Bluetooth, WiFi, Near Field Communication (NFC)


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