Chapter 4 Teo Ry

8/12/2019 Chapter 4 Teo Ry

1/36

4. Measurements4.1. Introduction

The measurements are made both whilst designing, to avoid EMC problems(e.g. to find the sources of EMI, shielding measurements) and in final phase, for EMC

certify of a product (emission and susceptibility tests).This chapter begins with the highlighting of the specifically problems and

measurements types.ecause a measurement system, it doesn!t matter its comple"ity, is composed

from sensor, transmission line and the instrument itself, a separate analysis is a naturaloption.

#ithout special emphasis, we present the fundamentals for E$field sensors, %$field sensors and sensors for simultaneous E and % field measurements.

The transmission line is mainly considered from the angle of reducing itsinfluence towards the measurement itself.

&ince the understanding of their real capabilities and limits in the same time isessential for a reliable measurement, we ta'e into consideration

the EMI receiver (instrument especially designed for EMC test wor')

the spectrum analyser (measurement instrument with a broad palette of

applications)

the oscilloscope (instrument for time domain measurement).

4.2. Fundamentals of EMI measurements

It is necessary for EMC to determine whether the e*uipment under test (E+T)is compatible with the electromagnetic environment in which it is designed to operate.

The E+T must not emit levels of electromagnetic energy that could affect theoperation of any other appliance potentially presented in the neighbourhood, and itmust not be susceptibleto fail operation due to the levels of electromagnetic energye"pected in its operation environment.

The main tas' of electromagnetic interference (EMI) testing is to measure andthen to compare versus ade*uate limits, the electromagnetic energy which

is undesirably emitted by E+T (EMI emissions testing Fig. 4.1)

affects the normal operation mode, if is undesirably received by E+T, (EMI

susceptibility testing Fig. 4.2).There are two main routs for the transmission of EMI into or out an E+T via

any of its connecting wires or cables (conducted EMI

) or by any other means thanconduction (radiated EMI).


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Thus, aiming the EMC certifying, for any electrical device, the designer mustperform conducted emission measurements and radiated emission measurements, aswell as conducted susceptibility measurements and radiated susceptibilitymeasurements.

-or EMI emission testing (-ig. ./), E+T is connected to power source andother devices (control0signal, source0load) for normal wor'ing conditions.Instrumentation is constituted from sensors, transmission lines and measuringapparatus. If the sensors are specific, the transmission line and measuring apparatusare generally identical for conducted and radiated emission.

-or the EMI susceptibility testing (-ig. .1), also the E+T is connected fornormal wor'ing conditions, and to in2ect from outside the EMI radiated and

conducted till failing. Instrumentation is constituted from calibrated signal source,transmission line and specifically actuators.

3owersource

Control0&ignal

&ource04oadE*uipment

under test

(EUT)

Control0&ignal leads

3ower leads

SISTEM FOR

TESTIN

4I&5&

&ensor of

EMI current

EMI voltage

!onducted

emission

testin"

Radiated

emission

testin"

EMI analy6er

&pectrum analy6er

7scilloscope

8eceiving antenna and

other sensor of

electromagnetic fields

INSTRUMENT#TION

FOR EMI

ME#SUREMENTS

Sensors

Transmission

line

easuring

apparatus

Fi" 4.1EMI emission testing


3/36

4.2.1. Industrial measurements$ad%anced measurements for researc& and

de%elo'ment

Ta'ing into consideration the involved methods and techni*ues, EMCmeasurements could be classified industrial (common) and advanced measurementsfor research and development.

a) Industrial measurement have as main target the E+T characterisation, areperformed complying to some regulations and have a repetitive, standardisedcharacter. -or an E+T we effectuates both EMI emissions testing, and EMI

susceptibility testing. These measurements are accomplished with a test report, with

the conclusion passed or failed.b) d!anced measurements "or researc# and de!elopment involve a subtle

phenomena understanding and high precision. These efforts are advisable in somespecial cases the E+T doesn!t comply with the standards and regulations and needimprovement in finding the source of EMI problems for elaboration of new specificstandards.

-or the first situation, measurements in fre*uency domain represent the mostcommon option (mainly due to tradition, instrumentation and standards). -or thesecond one, measurements in time domain are, this time, compulsory.

4.2.2. Measurements in controlled sites or in situ measurements

3owersource

Control0&ignal

&ource04oadE*uipment

under test

(EUT)

Control0&ignal leads

3ower leads

SISTEM FOR

TESTIN

Transformer,

Current probes,

In2ector of EMI

!onducted

susce'tiilitRadiated

susce'tiilit

Calibrated &ignal &ources,

9mplifiers

Emission antenna and

other radiators of

electromagnetic fields

INSTRUMENT#TION FOR EMI

SUS!E*TI+I,IT- TESTIN

ctuators

Transmission

line

pparatus

$bser!ing

E%T

Fi" 4.2EMI susceptibility testing


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The first difficulty encountered while EMI measurement performing is theisolation of electromagnetic radiated energy of interest in the tests from theelectromagnetic environment. Thus we can perform EMI measurements in acontrolled test site or in situ measurements &e.g. testing systems or installations int#eir places o" use).

The physical controlled environments for performing EMI tests are $pen rea Test Site ' $TS

%nderground (allery

Screened *ooms or S#ielded +#amber

*F nec#oic Screened +#amber or bsorber ined +#amber +

Trans!erse Electromagnetic +ell ' TEM

(iga#ert- Trans!erse Electromagnetic +ell ' (TEM

Stirred Mode +#amber

T#ree oop ntenna System ' TS.

These test facilities have not to decrease the degree of being representative

and, additionally, a good correlation between the two test set$up is obligatory.9n overview upon the e"isting test methods and the errors for susceptibility andradiation is presented in table ./.

Table 4.1

Test facilit 'erformances

Test sites Freuenc

ran"e

Error In%estment

$pen rea TestSite ' $TS

:; $ /;;; M%6(/; '%6 $ /;;;M%6)

(1 $


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-or the first situation, the real$time measurements for predicting, detecting andsurveying of some natural phenomena (lighting, earth$*ua'es and volcanoeseruptions) and en*uiring any dysfunction under normal e"ploitation.

The second one is representative for the surveying the EM environment (andits pollution).

The disadvantage of in situ measurements is that the ambient signals fromother transmitters or unintentional emitters may mas' the signals emitted by E+T.The electromagnetic environment must be identified and tagged in the test report.

4.2./. Time %ersus freuenc domain measurements

EMI could be continuos or transient. Instrumentation is *uite different for onecase and the other. -or the first situation we perform measurements in fre*uencydomain (for the second, only measurements in time domain are possible)9 signal presentation in these two domain is offered in -ig. .:

The table .1 presents a comparative evaluation between the two measuringmodes

Table 4.2

!om'arison et0een time domain measurements and freuenc domain

measurements

Freuenc domain measurements Time domain measurements

Is represented the amplitude andthe phase versus fre*uency

($) The domain of applicability is

reduced only to periodical signals

The signal amplitude versus time isstored

() The applicability field is wider

(any type of signals)() The instrumentation is available ($) The instrumentation is more

f

u(t) +(f)f

t

u(t)

t

-re*uency

domain

+(f)

Timedomain

Fi". 4./8elationship between time and fre*uency domain analysis


6/36

in almost EMC laboratories$ EMI receivers (%6 $ tens of

>%6)$ spectrum analyser (%6 D hundred

of >%6)

comple"$ oscilloscope (real time band width

;,A >%6 or even >%6) with anattached camera, magnetic tape

recording, a dis' recorder

$ digital transient recorders() &tandards and regulations,

instructions for the users are wellestablished

($) The already ac*uired e"perienceis lower

() They are applicable mainly inadvanced research studies and

modelling

($) The phase measurements are notalways possible and passing fromfre*uency domain to time domain

is not available

() &upplies a global informationand allows the passing through

computation to fre*uency regime.

() +sage of very selective filterspermits$ a great signal to noise ratio

$ a large dynamic range (


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4.2.4. Fundamental uantities and units of measurement

a) The basically measuring *uantities for radiated EMI are electric field E,magnetic field and radiated power or power density *.

Electromagnetic field can be characterised by means of E (electric field), +(magnetic induction or magnetic flu" density), (electric induction or electricdisplacement) and (magnetic field).

Coupling between field and ob2ects is established through the electric currentI, and the electric charge *. E, +, , are the distributed electromagnetic *uantities,unli'e of voltage and current, which are integral *uantities.

The fields (locale properties of space in a certain point) might be determinedby means of forces e"ercised to a particle (with 'nown proprieties) placed in the pointof interest .GF, ./:F.

The force e"ercised to a charge * moving with the velocity % through anelectromagnetic field is

+%EFFF &e +=+= 33 (.:)where

E, electric field, is defined as the ratio of the force, Fe, on a positive test

charge *, considering it as approaching to 6ero

33

eFE lim

;=

(.)Eis e"pressed in H0mF

%is velocity of charge * e"pressed in m0sF

+is magnetic induction (flu" density) e"pressed in Hs0m1

F or in TF.

Ma"netic field, , is defined considering the magnetic induction + andmagneti6ation M, with the relation

M+

1 =;

(.A)

where

; /; = is the permeability for free space in Hs09mF, or in

henry0meter

Mis the magneti6ation achieved in material, e"pressed in 90mF.

The unit for magnetic field isampere per meter 90mF.

&ensors@ata lin'

9nalog signal

conditioning#aveform

recorder

@isplay

&ignal

source

Control unit&torage

@ata

processing

system

@ata users

Fi" 4.4&ystem for time domain measurements


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oth the electric field E and the magnetic field , at a point in space arevectors. -or a rectangular co$ordinate system, the Eand can be written as

-y4 EEE ++= 34E (.B)

-y4 555 ++= 341 (.)

where , and 3 are unit vectors, and E", Ey, E6 or %", %y, %6 are the scalar

components of the electric field or, respective, the magnetic field.

Radiated 'o0eris defined as the average power flowing per unit area. It is e"pressedin #0m1F.The relationship between the power density and E and % fields is

1E* = (.


9/36

The basic conversion relations between units are/ m# J ; dm J $ :; d#, or / # J ; d# J :; dm (./:)

/ m# J ; dm J 11A mH, for 8 J A; (./)

Holtage in dH J 3ower in dm /; d, for 8 J A; (./A)

Holtage in dH J 3ower in dm //,< d, for 8 J B;; (./B)

4./. Instrumentation for EMI measurement

4./.1. Electroma"netic sensors

4./.1.1. +asics of electric and ma"netic field sensors

The antenna, interface between emitter and medium or between medium andreceiver, may be used for EMI measurement.

9 special 'ind of antennas (electrically small antennas), design and developedmainly in the last ; years, have been named electromagnetic field sensors.7ptimisation of these sensors for being used for electromagnetic field measurementsis done faced to accuracy, sensitivity, bandwidth and broadband0transient

performance, topology and symmetry consideration for the installation of suchsensors and associated instrumentation ta'ing into account the least field influence,and the capability of functioning in different mediums (air, water, tissues, nuclearsource region, inside the lightning arcs and corona regions).

The electromagnetic sensor has the following properties .1F, ./


10/36

T&e ma"netic field sensor(magnetic dipole sensor $ -ig .A) is constituted bya bro'en loop connected to the terminal pair.If this sensor is illuminated by an incident magnetic field i, then to its terminalsappears an open circuit voltage of magnetic field sensor, %oc #or, for short circuitterminals, a short circuit current of magnetic field sensor, Isc #.

#e3# (e*uivalent area), #e3l (e*uivalent length), and , (inductance) are the basicparameters of magnetic field sensors.

The relationship between #e3# and #e3l is the following

#e3#e3

,l# =

(./


11/36

If

+7

"or

+7

s7

+s ccc

>>

>>

%oc e

Magnetic dipoleElectric dipole

&ensors Isc #

5orton

e*uivalent

circuits

Thevenine*uivalent

circuits

Kc4

&ensor 4oad

#e3#e3 #l ,

ee3ee3 #l ,

1iEi

Kc

4

&ensor 4oad

C

Kc

+h

+e

+e +h

Ih

Ih

Ie

Ie

&ensor 4oad

iee3 El

C Kc

&ensor 4oad

iee3

t2#

i#e3

t+#

i#e3 1l

Fi". 4.6. &ensors (electric dipole and magnetic dipole) with e*uivalent circuits.

L( ) ,

L( )% s % se #

c#e3#

ee3e

7ss%

ss%

)(L

)(L

)(L

)(L

1l

Elee3i

e +sss% = lE )(L

)(L

,sss% ee3i

# = l1 )(L

)(L

fcr

scr

1

/

f

s

: d

Fi". 4.7 -re*uency response of electric and magnetic field sensors


12/36

If one consider for magnetic "ield sensorthe 5orton e*uivalent circuitfrom -ig. .A, results the 4aplace transform of output voltage

,s7

ss7s%

c

#e3i

c# +

=

#+ )(L

)(L

(.11)

-or8,

7"or

,

7s7,s ccc


13/36

where

( )

+= )sin(cossin)cos(cos

1

/1/cossin

1

/)(

/;;ii

aa

2:b

2u

and += )cos(/1//),( /

/;;/ aa2

!

=+ )cos(

/1

//),( /

/;;/

aa2

!

If we count the currents through the loads (/ and /+ ), we have got),()(),()()(1)( ///////;/ 1/

++= !7I!7IubEI ,,i

),()(),()()(1)( ///////;/ 1/ +++++=+ !7I!7IubEI ,,i

where

/

/

//;;/

//;;/

/;////

/;////

)(

)(

),(),(

),(),(

";";u

";";u

;;!!

;;!!

=+

+=

=+=+

+=++=

with ;and /, the admittance for the magnetic loop mode current, respectively theadmittance for the electric dipole mode current .AF,

If ,,, 777 == 1/ , the sum and the difference of the terminal voltages have thevalues

i

"

E(b,)

7b

8sin

8

I()

Ei(R)

Ei

(8)sincos Ei(8)sin

Ei(8)cos

Ei(8)sinsin

/

6

K4/

K41

y

"

)K4

(;)H()

K4

%6

Ey

a)

-igure . a) @ouble $ loaded loop antenna b) &ingle loop for measuring %6and Ey


14/36

( ) ,,,

i

7II7;7

;"bE


15/36

comple system "or EM+ measurements &e.g. t#ree'loop antenna system ' TS)

=4.1>, =4.21>, =4.2>.

4./.2. Si"nal transmission

The connections from sensor to measuring apparatus generally transferanalogue signals.Thus, the signal transmission lin' must be insensible to measuring field (eliminationof parasitic antenna effect), and it must not disturb the measurand (e.g. theelectromagnetic field).It is also of greatest importance, matching the signal transmission lin' to sensor andmeasuring apparatus, the impedance mismatch being a source of errors.

Common transmission methods of signal from sensor to measuring apparatusare

conducting lin' (shielded cables)

radio telemetry lin' .GF

microwave telemetry lin'

fibre optic lin'.

The conducting cables (shielded coa"ial, twin a"ial cables) are used at leastfor very small distances.

There are two important concepts in the instrumentation cabling topology andsymmetry .1F.a) T#e sensor must measure an electromagnetic 3uantity on a good conductor Fig.

4.?.

In this situation the original conductor becomes the local ground for the sensor, and itinfluences the fre*uency response, accuracy, field configuration. The topology of

instrumentation cabling must fit with that of the conductors presented in thee"perimental configuration, (-ig. .


16/36

If it is possible, is advisable to use the original conductor as a shield tominimise the influence of the conducting cable for a minimal noise pic'up in thecables.

In -ig. .G, is shown a possibility of measuring the electromagnetic fields withonly the sensitive element (an electrically small half$loop or semi$sphere) being

introduced in the region containing the field of interest. This method is used tomeasure the electric and0or magnetic field in a shielded electronic system (plug$in) orgenerally, in measurements close to a good conductor .


17/36

4././.1 EMI recei%ers

The EMI receivers are fre*uency tuneable (audio, radio$fre*uency andmicrowave), variable bandwidth selective voltmeters, presenting ade*uate parameters(high sensitivity, wide dynamic range, good out of band signal re2ection) for

measuring and characterising the electromagnetic interference (a comple" un'nowninput signal).

The 'ernel of the EMI analyser is a super$heterodyne receiver (with one tothree stages of fre*uency conversion), which permits the selection of

measurement intermediate fre*uency (I-) bandwidths (/ '%6 D /; M%6)

post detector bandwidths (/ %6 D /;; '%6)

detector functions

out ports (the mode of present of results)

4././.1.1. Functional descri'tion

-igure ./; is a simplified bloc' diagram of a typical instrument, and -ig. .// is amore detailed diagram.

The sensors for connection to the EMI receiver input are antennas or otherdevices for radiated measurements and current probes or voltage probes for conductedmeasurements.

9s a source of reference input signal, is built$in an internal standardinterference generator that can provide a sine wave, an impulse or random >aussiannoise. The impulse generator produces a series of very shot subnanosecond duration

pulses with a variable pulse repetition of A; %6 /; '%6. The fre*uency coverage of

impulse generator is a few hundred %6 />%6 with a spectral flatness of / d.

The impulse generator can be used

to calibrate (amplitude accuracy) and chec' (dynamic range compression) EMI

receivers to obtain a very accurate measurement by substitution method.

&ensor

Input

atenuator

Calibration

source

8-

stages

Mi"er @etectorsI-

amplifier

Hideo

out ut

9udio

out ut

Holtmeter

*F

band

9and6idt# @etector

selection

4ocal

oscillator

Fre3uency

+

N

-ig. ./; &implified bloc' diagram of EMI receiver


18/36

In the substitution method by switching N from M to C point (-ig../;), themeasurand (un'nown interference signal) is replaced by a suitable *uantity which isad2ustable in value to bring the indicator bac' the value initially indicated by themeasurand.

The input signal is passed through a variable input attenuator, broadband and

accurate. It is a low noise 8- device increasing the dynamic range of EMI meter. It,also, could be used for chec'ing for compression.

T#e radio "re3uency &*F) stagesare low$pass or band$pass filters, e"pecting toeliminate out of band signals, which might induce spurious response within the meter,

and preselectors or 8- amplifiers to eliminate signals outside the band of immediateinterest and thus further reducing the vulnerability to intermodulation and spuriousresponses.T#e preselector (a couple of band pass filters manually or automatically switched,

placed prior to the first preamplifier or mi"er circuit) improves dynamic range andreduces spurious responses, but increases the receptor noise figure owing to additionalfront$end components, and increases signal attenuation.

T#e mierand t#e local oscillatorform t#e "irst stage o" "re3uency con!ersion.The first conversion stage is generally one of down conversion.

,$I FFF = (.:/)where,

-7is the original fre*uency-4is the fre*uency generated of local oscillator

EMI

sensors

4inear I-

output

4og I-

output

8-attenuator

4ow

pass

orband

pass

filters

3reselector

8- amplifier

Mi"er

7verload

detector

I- amplifier5

filters andattenuator

Impulse

generator

for

calibration

4ocal

oscillator

Manual or

electronic

drive

O a"is

output

scaler

9utomatic

gaincontrol

(9>C)

networ'

&ignalweighting

circuits

&electable

detectors*esults

presentation1

scope

spea'er

head setpan display

meter

recorder

O$ plotter

7ut buffer

amplifier

(video, audio

amplifiers)

I- log

amplifier

eat

fre*uency

oscillator

>anged attenuator

-re*uency

a"is on

display

Fi". 4.11. loc' diagram of typical EMI receiver


19/36

-Iis the intermediate fre*uency.

9t low fre*uency because ,$ FF < , the up conversion is usually used.T#e local oscillator &$)is tuned manually by a variable capacitor, or using

an analogue voltage developed either by a front panel potentiometer or generatedinternally or e"ternally as a voltage ramp. The voltage ramp can be scaled and used

simultaneously to drive the O$a"is of plotter or oscilloscope (fre*uency domaindisplay). The digitally synthesised 47, improves several specifications of theinstrument (fre*uency stability and its determining accuracy, finer resolution ondisplay, low noise). -or EMI testing, a very important of the digitally synthesiseddesign is that allowing direct control of the fre*uency from a driving computer.

The mi"ed output drives theIF stages(amplifier, attenuator, bandpass filter).The I-, unli'e the 8- attenuator, reduces the level of the noise indication (advantage),

but also reduces the useful dynamic range of any preceding stages (disadvantage).The rectangular filter, specific to EMI receivers, has a great selectivity. The >aussianshape filter, specific to spectrum analyser, enables a fast sweep rate, but has a lowerselectivity .1F. -or e"ample comparing the : d down bandwidths of the EMIreceiver (8) and spectrum analyser (&9) we have

1

/

B;

: =

*9

9 and

/

/

B;

: =

S9

9

Thus, assuming 5-9 /;;: = results 5-9 1;;B; = for the receiver, and

5-9 /.;;B; = for the spectrum analyser.

The I- signal is applied to aselectable detector, and using the !ideoand audioampli"iersthe results may be presented on an oscilloscope, spea:er, meter, recorder,etc.

T#e beat "re3uency oscillator &9F$) is used for detection of the noise

embedded continuous wave (C#) signals, by obtaining an audible tone produced as aresult of the -7 beating with the I- signal.Most EMI receivers have a sort of circuitry whose purpose is to compress the

amplitude of e"cursions at their outputs without compromising overall dynamic range(an automatic gain control D9>C D voltage, a logarithmic characteristic onto I-stages, appropriate weighting circuits).

4././.1.2 Selecti%it and sensiti%it

T#e selecti!ity c#aracteristicinto the detector is determined almost entirely inthe final I- stage. ecause the large spectrum and the comple"ity of signals presented

to the input port, without narrow bandpass filters at the input to the receiver, the first8- amplifier or mi"er stage will be overloaded.Thus, the resulting signal into the detector, +I-is

d""("S%IF )()(

;

=

(.:1)where,

&(f) is the fre*uency domain characterisation (-ourier or 4aplace transform) ofthe 8- signal>(f) is the fre*uency domain characterisation of the EMI receiver gain fromthe 8- input to its detector input.

The bandwidths .F commonly used in EMI testing are represented in -ig. ./1a,where


20/36

:is the : d down bandwidth of EMI receiver.

Bis the B d down bandwidth of EMI receiver.

n, the random noise bandwidth, is the width, in hert6, of a rectangle which has an

area e*ual to that of the predetector amplitude s*uared versus fre*uency responsecurve and a height e*ual to the ma"imum value of that curve D -ig. ./1b.

iis the impulse bandwidth of the receiver (essentially for the I- stages).

ecause i, is fre*uency variable, it must be determined by using a calibrated impulsegenerator (I>), or an impulse generator and a continuous wave (C#) generator.If the impulse bandwidth of the EMI receiver is 'nown, then

$ the spectrum amplitude of an un'nown broadband signal, +b, in dH0M%6,

can be determined

isb 9%% log1;= (.::)where,

+sis its pea' detected amplitude, in dH, read on the EMI analyser

iis the impulse bandwidth, in M%6.

$ the amplitude of an un'nown narrowband signal, +n in dH, can be

determined by the substitution method

iin 9%% log1;+= (.:)where +iis the calibrated output of an impulse generator which produces an EMIreceiver reading e*ual to that for the un'nown narrowband signal, +n.

7n the other hand, from the relation (.::) or (.:), results that ican readilybe determined by substitution testing, if both a calibrated impulse generator and acalibrated continuous wave generator are available.

T#e recei!er sensiti!ity depends on the types of EMI (narrowband andcoherent or incoherent broadband signals), that are handled by EMC measuringe*uipment

a) For narro6band and inco#erent broadband signals, receiver sensitivity, &,

is defined in terms of its internal noise power, 5, as referred to the receiver input port.

f;

:

n

i

B

d down

;

:

B

>(f)0>(f;)

/

;.

f

a)

fn

:;.A

E*uivalent

area rectangle>(f)0>(f;)F

/

9mplitudes*uared

response

(9rea of rectangle J 9rea

under curve)

b)

-ig. ./1 a) andwidths of typical bandpass amplifier b) 8ectangular area method fordetermining random noise bandwidth


21/36

1=+0

0S, or & J 5 (.:A)

The noise power, 5, is defined as

nF:T90= (.:B)where,

5 J noise power in #- J noise factor of the receiver and -dJ /; log -, is the receiver noise figure

5-A

B:

= 1:/;:


22/36

id9iM5-


23/36

The re*uirements of various EMC standards and test specification re*uires forEMI receivers, a number of special detection "unctions &pea: detection, 3uasi'pea:detection, a!erage detection, root mean s3uare ' *MS detection), and a multitude of

statistical detection processes (amplitude probability distribution 8@, noiseamplitude distribution 0@, pulse duration distribution 8@@, a!erage crossing

rate +*).

a) 8ea: detector

This detector measures the pea' value of an envelope s(t) of I- stagesresulting signal,

+pJ ma"Ps(t)Q, (.A)

and displays the 8M& value of the e*uivalent sine wave

1

p%.

The pea' detector is characterised by rapid charge time (the charge time isshorter than the rise time of the faster I- pulse produced by the foregoing stages in

response to an impulsive signal), and a discharge time ade*uate to the response of thedisplay devices.

@ue to its fast settling time, it is used in spectrum analysers and, generally, inautomated EMI testing.

b) Cuasipea: detector

The function of this detector is to give a reading proportional to thesub2ectively annoyance effect of a discontinuous impulsive noise (a succession of

brief pulses with repetition fre*uency more or less stable, e.g., EMI produced bycorona discharges on %H transmission lines) on listeners to broadcast radio receiver.This is obtained by appropriate weighting of output voltage of pea' detector, acting

on electrical charge time constant, electrical discharge time constant and mechanicaltime constant of the critically damped meter.

-ig. ./A shows the *uasi$pea' detector and the postdetection circuits (bufferamplifier, simulator of mechanical time constant), .:1F.The charge time constant is

+*cc = (.B)The discharge time constant, ad2ustable by the switch N is

+*dd = (.)If the switch N is in position , we have a pea' detector.

9fter the buffer amplifier 9/, there is a two$order low pass active filter, which

simulates the movement of the mechanical indicator. Mechanical time constant is//+*m = (.


24/36

The time constant of the *uasi$pea' detector for different fre*uency range are given inTable .:, .F, ./GF.

Table .:. CI&38 Ruasi$pea' detector characteristics

-re*uency range ;.;/ 1;

'%6

G /A; '%6 ;./A :;

M%6

:; /;;;

M%6B d bandwidth G '%6 1;; %6 G '%6 /1; '%6

Electrical charge timeconstant

/ ms A ms / ms / ms

Electrical discharge timeconstant

/B; ms A;; ms /B; ms AA; ms

Mechanical time constantof critically damped meter

/B; ms /B; ms /B; ms /;; ms

3redetection overloadfactor

:; d 1 d :; d :.A d

3osdetection overloadfactor /1 d /1 d /1 d B d

ecause the rise time of a *uasi$pea' detector is much greater than the risetime of I- stages, they are not recommended for using with spectrum analysers and,generally, with automated EMI testing.

The output of a given *uasi$pea' detector will always decrease along with adecrease in the repetition rate of the pulsed input (38-), whereas the pea' detectoroutput is repetition rate independent. The output of the *uasi$pea' asymptoticallyapproaches to that of pea' detector while the 38- increases, as illustrated in -ig ./Band -ig ./, where are given the relative output of all four of the detector types for G

'%6, respectively /1; '%6 predetection bandwidth, .F, ./GF.

/; %6/ %6 /;; %6 / '%6 /; '%6

$A; d

$; d

$:; d

$1; d

$/; d

; d

3ulse repetition fre*uency (38-) of EMI signal in %6

@etector output relative

to a pea' detector

3ea' detector

9verage

True 8M&

Ruasi$pea'

-ig. ./B 8elative output of CI&38 detectors as function of 38- for BJ G '%6


25/36

4././.2. T&e s'ectrum analser

The spectrum analyser allows measuring the harmonic content of an electricsignal, otherwise the power of every spectral components. It displays the amplitude ofsignals versus fre*uency (fre*uency domain measurements), complementary to theoscilloscope, displaying the signals amplitude versus time (time domainmeasurements).

There are three principal types of spectrum analyser fre*uency scanning superheterodyne receivers

contiguous bandpass filter ban's (spectral analysis in real time for single pulsed

waveform)

digital signal capture and software -ourier transform systems or computation of

signal autocorrelation function (generally, for fre*uency less than a few tens ofM%6).

9 basically bloc' diagram of a fre*uency scanning heterodyne spectrumanalyser is shown in -ig ./


26/36

ecause the spectrum analyser is an alternative for the EMI receiver,especially in diagnostic and pre$compliance EMC testing, and these instruments aresimilar in their basic functions, a comparative description will be performed.

The input of the spectrum analyser is switchable between EMI sensors and theinternal source used for fre*uency and amplitude calibration. +sually EMI sensors areseparate delivered, having their individual calibration curves.

+nli'e the EMI receiver, no preselector or tunable 8- amplifier normally isincorporated as a standard component in the front ends of a spectrum analyser. Thisinvolves the principal disadvantages of the spectrum analyser (lac' of front endselectivity, and thus overloading the mi"er with low signal amplitude levels highnoise figure, and thus low sensitivity).

The wide bandwidth front ends ma'es the spectrum analyser more e"posed tocompression and overload. The variable input attenuator increases the dynamic rangeof the spectrum analyser and may be used to chec' the compression.

The hori6ontal scale of the instrument (the O a"is of the C8T) is driven by asweep generator, which also commands the voltage tuned local oscillator (47)

producing linear e"cursion of fre*uency versus sweep on the O$a"is.

The sweep generator allows the scan speed and widths (domains) choice.The sensitivity of the spectrum analyser is defined as that of EMI receiver, but

the noise figure for spectrum analyser (typically 1; :; d) is higher than for an

EMI receiver (typically B /1 d).

The front panel of the spectrum analyser contains controls for the Fre3uencyspan S6eep time *esolution band6idt#


27/36

If the bandwidth of the instrument (EMI analyser, spectrum analyser), bb, is less thanthat of the emissions being considered in -ig. ./G, the emission will appear as

broadband.If the bandwidth nbis wider than that of the emissions being considered in -ig. ./G,the emission will appear as narrowband.

Thus, whether impulsive noise is displayed as narrowband or broadband, it isup to the resolution bandwidth.

There are four basic methods for differentiating between the broadband andnarrowband emissions

band6idt# test (using the two bandwidths by changing the resolution bandwidth) tuning test(by tuning the instrument for ma"imum response and then deliberate

detuning higher and lower in fre*uency by an amount e*ual to two of its impulsebandwidths)

detection test(by comparing the readings for pea' detector and average detector)

8*F test (by comparing the repetition rate of the EMI pulse to the impulse

bandwidth of the EMI instrument). 8eferring to -ig. .1;, if the pulse repetitionfre*uency $ 38- of EMI e"ceeds the impulse bandwidth of the instrument theEMI is narrowband. #hen 38- is less than the impulse bandwidth, the EMI is

broadband.

T#e "re3uency spandetermines the spacing of the spectral emission lines. Thespectral emissions lines may be either narrowband signals (sine wave sources) orimpulsive noise, which is displayed as narrowband signals (resolution bandwidth ofinstrument is less than pulse repetition fre*uency D 38- of EMI) as is illustrated in-ig. .1; .1F.

If the repetition rate of EMI pulse is much lower and resolution bandwidth isincreased to include more spectral lines, the signal will be displayed as a broadbandsignal (the amplitude of the display is proportional to the spectrum envelopeamplitude at the fre*uency to which the instrument is tuned). In this case the spacing

between the pulses is changed by t#es6eep timeand is not modified by changing thefre*uency span. Thus the 38- is found by ta'ing the reciprocal of the sweep time

between the individual pulses .1F.

9mplitude spectrumdistribution of pulse

8adio fre*uency

$:0 $10 $/0 /010 :0

nbbb

9

Time domain

response of pulse

Fi". 4.1: Comparison of bandwidths of the instrument to pulsed 8- spectrum


28/36

-igure .1/ is a functional bloc' diagram for a modern 8- spectrum analyser

.1AF. The 8- fre*uency range is G '%6 ; >%6, and may be e"tended up to

hundred of >%6 by using e"ternal mi"er.To allow scanning over multiple octave ranges of fre*uency, three stages for

fre*uency conversion are used.The *uality parameters of the local oscillators 47 (fre*uency stability and

accuracy, low$noise, fine resolution on display) are improved by using a synthesisedunit referenced to a very stable *uart6 oscillator.

-or using in EMC wor' tests, the spectrum analyser also contains bandpass ortrac'ing preselectors a range of I- bandwidth the specific detector functions.

The microprocessor controlling of the analyser is very important for operationand data recording simplicity. It is also necessary for including the analyser in anautomatic measurement system (automated EMI testing).

9mplitude spectrum distribution

of repetitive pulse for 8 S 38-

8adio fre*uency

9

Time domain response

of repetitive pulse

8esolutionbandwidth (8)

T J /038-

&pectrallines

38-

Time

5arrow band characteristics

(38- ? 8)

38- J spectral line spacing 4ine spacing changes with

fre*uency span, but

independent of sweep time

Fi". 4. 2;5arrowband display of repetitive pulse


29/36

The strong points of spectrum analysers lie with

their e"tremely wide input fre*uency range (typically /; '%6 to /.: >%6

but as wide as /;; %6 to hundred of >%6), with lower cost per coveredoctave comparing to the EMI receiver

functional display (C8T display is often superior to that used in EMI

receiver)

capability of displaying a large part of the spectrum in a short duration

sweep time, almost real time (the short duration changes in amplitude aremore easily discernible)

adaptability to a broad palette of applications

compact nature.

4./././. T&e Oscillosco'e

There are plenty of situations demanding time$domain measurements forelectromagnetic fields

Measurements of transients in studies of natural and man$made phenomena as

lightning, electrostatic discharges (E&@), nuclear electromagnetic pulse (5EM3).

EMC emission and immunity tests. The detection of pea' amplitude, wave shape,

repetition fre*uency of transient or very low repetition rate fast burst type signals.

:

3reselector1B.< ; >%6

G '%6

1B.< >%6

G '%6

; >%6

3reselector

:.G/;

>%6:/;.

M%6 :/;.

M%6/;.

M%6

:1

/ st 47

synthesi6er

B

/; M%68ef.

B;; M%6

7sc.

1

/ st 47

:.B

>%6

:.G/;

>%6

I-

4og.

4in.

@etectors Hideo 9@C C

@isplay&%

:/;.

M%6

Fi". 4.21 loc' diagram of modern spectrum analyser


30/36

+onducted emission tests call measurements of the e"ported spi'es from ane*uipment under test (E+T) to be measured when the whole device or only someusers dealing with the power management system are on0off switched. EM+immunity testscall to monitoring the amplitude and wave shape of transients orspi'es being in2ected on power lines connected to an E+T, or illuminating an E+T

(5EM3 waveform, E&@ pulse).The main e*uipment used in this scope is the oscilloscopes and the digital transientrecorders.The oscilloscope displays voltage waveforms, being one of the most versatile andfastest measuring instruments.9lthough very comple", the oscilloscope contains the following basic circuit bloc's

a channel for processing and vertical defle"ion (amplitude a"e)

a channel for synchronising and hori6ontal defle"ion (time a"e)

a display.

9t the first channel input, one applies the signal of interest. In order to achieve

high fidelity for displaying the high$speed signals, the data ac*uisition system musthave a wide bandwidth and a constant group delay.The other channel, consisting of trigger and time base, ma'e input signal to be

displayed as a function of time. The sweep constants are ad2ustable from /;$G

second0division to seconds0division.The display is a direct$view vector cathode$ray tube (@HHC8T C8T), or is

based on computer display technology (digital scope display).

T&e #nalo"ue Oscillosco'e

-rom the beginning, the oscilloscope was analogue, so it has a simple and awell$'nown operation mode.

The display for the analogue oscilloscope is a C8T, having the advantagesreal time operation, short dead time, high hori6ontal resolution and many shades ofgrey$scale information.

The real time fre*uency range is ;,A / >%6, seldom reaching a few >%6.

9lso, the sweep rate is very good (/; div0ns). 9ll these recommend the analogueoscilloscopes for processing very fast and fast time variable signals. If the signals arerepetitive, the real time display represents an advantage. 5ot being repetitive, theac*uired signals storage could by accomplished by using a camera or an analoguememory.The disadvantages of the analogue oscilloscopes are

their dependence on a C8T display, constantly being refreshed, so they do not

have any waveform storage facility

difficulties in triggering on0and visualising a specific interference signal having

the amplitude and position variable with respect to the mains fre*uency

lac' of post and pre triggering possibility (inability to display information before

its trigger)

the impossibility of including the analogue oscilloscope in an automatic

measurement system controlled by computer.

T&e i"ital Oscillosco'e

The main difference versus the analogue oscilloscope is the analogue to digital

conversion of the input signal and in conse*uence, it could be controlled by an


31/36

embedded microprocessor and0or included in a numerical measurement system, withall the advantages deriving from this.a) 9loc: diagram and "unctional modes

The digital oscilloscope diagram is shown in -ig..11

The (vertical) deflection system contains a data ac*uisition bloc'

(amplification, analogue$digital conversion), a memory, and an interface with display.This three operation may could be done in different times. The O defle"ion system (the hori6ontal system) is constituted of the triggercircuit and the time base. The time base is controlled by a *uart6 oscillator, which has,also, the role of generating the impulses driving the sampling and the conversion

process.In -ig. .11 the display is a direct$view vector cathode$ray tube (@HHC8T or

C8T), but it may be based on computer display technology (digital scope display).The ability of the digital oscilloscope to capture signals is based on operating modes,timing resolution, t#escope band6idt#.

a)There are two basically ac*uisition modes e3ui!alent time sampling, the signal capture is done in many synchronising cycles,

only applicable for repetitive (periodic) signals

real time sampling,the signal capture being performed in a single synchronising

cycle.a1)In e3ui!alent time sampling mode, the oscilloscope may display the events shorterthan the sample spacing, the signal being repetitive.

Implementing the sampling method, the oscilloscope could reach a A; >%6bandwidth (only for long se*uence of identical pulses or periodic waveform).&ometimes the periodicity is obtained due to the repetition of the phenomenon, (e.g.for an electrostatic discharge simulator which has the possibility of generating a series

of almost identical discharges).

@igital

memory

Control

logic

Trigger

circuit

Input

attenuator0

amplifier

@9C

@9C

u(t)

9

9

E"t.

9@C

#nalo" to di"italcon%ersion

9nalog shift

register

&amples and

hold

IEEE


32/36

a2) In case of singular signals (single pulsed waveform), having high fre*uencycomponents, the sampling must be done in real time. The sampling fre*uency is high,namely, the oscilloscope has a very large bandwidth.Thus, increasing the analogue to digital conversion speed is the basic difficulty of thedigital oscilloscopes.

The direct analogue$to$digital conversion, usually, permits a /;; M%6 realtime bandwidth.a21)9 method for increasing the analogue bandwidth and thus for displaying singleshot fast transients is the one based on Uendless loopV mode recording or circularaddressing mode.-or this type of recording, the sampling and the data storage in memory continuouslyhappens from the beginning to the end and bac' in a circular fashion until a triggerevent occurs causing the end of the capture phase or the going on till a user$specified

post trigger time.#riting is done using an interleaving memory

the memory is partitioned, for e"ample, into :1 sections with a view to capture a

data word every 1 nanoseconds, if the write cycle time of memory is AAnanoseconds

the first data word is routed to the first memory location in memory section one,

after two nanoseconds the second data word is routed to the first memory location

in memory section two, and in this mode after 1 ns :1 JB ns is reached again

the first memory section, which has completed its write cycle (AA nanoseconds).1F.

The recorded data is then permanent stored at a slower rate, the captured waveform isdisplayed, and then the instrument is ready for ac*uisition of the ne"t signals.This method permits an analogue bandwidth of A;; M%6, and thus recording single

shot fast transitions, having the fre*uency components in this range (lightningimpulses, electrostatic discharge).a22)9nother method for increasing the bandwidth and thus for capturing fast singleevents is based on a short time analogue storage of the input waveform, (e.g. using ananalogue shift register ./1F), later performing the analogue$to$digital conversion andthe digital storage, at a much lower speed.)T#e timing resolutioninforms the operator how closely are spaced the samples inthe scopeWs data record.

+sually the manufacturers of digital oscilloscopes specify t#e maimumsampling speed, memory dept#and number o" bits =4.2>.

EsecF

FEsecF0E

basetimescale"ull

samplesdept#memorysamplesspeedsampling =

(.G)-rom this relation, we remar' that unli'e t#ememory dept# and number o" bitsthatare constants, the maimum sampling speedchanges with the sweep speed (full scaletime base). Thus, the scopeWs ma"imum sampling speed applies only to its fastersweep speed. The increase of memory dept# permits the increase of "ull'scale timebase, without diminishing t#e sampling speed.

The time record (time span) may be from nanoseconds till hundreds ofmilliseconds. The longer the time span to be captured and the finer the resolutionre*uired, the more memory is re*uired.

9 method for e"tending the scopeWs sampling speed to longer time records isby using a pea: detection mode. 9lso this mode can reduce the possibility of analiased display, allowing to display high$fre*uency noise that might not be within the

bandwidth (display short impulse of duration smaller then sample spacing).


33/36

9nother method for overcoming this recording limitation is by employingmultiple oscilloscopes or recorders set to different sweep speeds (different time$basesetting).This method is used, generally, for determining the un'nown parameters of signalswith 'nown waveform.

-or e"ample, a transient electromagnetic field, -ig..1:a may be recovered bymatching the segments obtained from three oscilloscopes (recorders), wor'ing in

parallel (-ig..1:b, c, d), suitable triggered, and having set the sweep speeds inconcordance with the region displayed. The recording is done se*uentially and

processing system lin's the three spots by time tied points 9/$ 91and /$ 1./


34/36

-or this wor'ing regime, the scope operates in a Dendless loop mode, while therecorder continuously samples and stores data (constantly overwrite a bloc' ofmemory from start to finish and bac' to the start) until a trigger occurs, causing the

record phase to finish, respectively to continue a user$specified post trigger time, ,

-ig. .1.

The pre trigger and post trigger view permitsa) to display what happened before a failure is occurred, or time variation of

electromagnetic field before a lighting, for e"ampleb) to visualise , in the middle of the display, a transient interference (in this

mode is observed what happened before and after interference), or generally, thehistory of any phenomena.2) T#e "acility o" digital oscilloscope triggering on the wanted interference signal, theamplitude and position with respect to the mains fre*uency being variable, -ig..1A./GF.

) 9etter accuracy "or bot# de"lection systems. The calibration errors of the scopeWsvertical system could be reduced because microprocessor or computer can applycorrection factors to data. 9lso, the accuracy of time base is few orders of magnitudes

better than the correspondent one from an analogue oscilloscope.

4) T#e possibility o" storing t#e 6a!e"orm for visual analysing, comparing to other testresults, or analysing performed by computer. It is possible to perform mathematicaloperations giving additional insight into waveforms (addition, subtraction,multiplication, integration, differentiation and -ast -ourier Transform). The selectedsignal must not be continuously present to input, because it ma'es not the refresh ofdisplay. Thus, it is possible to display a single shot event.

5evertheless, the digital oscilloscope presents a more comple" operation modethen analogue scope one, and some disadvantages typical for analogue to digitalconversion process (aliasing error) and computer mode display (low hori6ontalresolution and large dead times D the period when the oscilloscope is not capturing theinput signal).

+ilio"ra'&

./F 9ntoniu M., altag 7., @avid H.MGsurGri electrice, IaXi, &atya,/GGG.

Mains power waveform

EMI signal (transient

noise burst)Trigger

level

4ine voltage

t

u(t)

1; ms

Fi". 4.26. Transient capture of noise burst on mains power line


35/36

.1F aum Carl E. Electromagnetic Sensor and Measurement Tec#ni3ues, -astElectrical and 7ptical Measurements vol./, @ordrecht, Martinus 5i2hoff 3ublishers,/G


36/36

.1BF #ehr M., Monich >.@etection o" *adiation ea:s by Sp#erically ScannedField @ata, Kurich (EMC) /GG:..1F #eston @. Electromagnetic Compatibility 3rinciples and 9pplications, 5ewor', Marcel @e''er, Inc, /GG/..1

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