ISO/FDIS 15548-1

ISO/DISFDIS 15548-1:2025(en)
ISO/TC 135/SC4/WG 1SC 4
Secretariat: AFNOR
Date: 2025-04-0512-17
Non-destructive testing — Equipment for eddy current examination
— —
Part 1:
Instrument characteristics and verification
Essais non destructifs — Appareillage pour examen par courants de Foucault —
Partie 1: Caractéristiques de l'appareil et vérifications
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ISO #####-#:####(X/FDIS 15548-1:2025(en)
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ISO/DISFDIS 15548-1:2025(en)
Contents
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Eddy current instrument characteristics . 2
5 Verification . 9
6 Measurement of electrical characteristics of instrument . 11
Annex A (informative) Principle of frequency beat method . 38
Annex B (informative) Method of evaluation of linearity range between output and input . 40
Annex C (normative) Summary of characteristics and verification levels . 42

Foreword . v
Introduction . Erreur ! Signet non défini.
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Eddy current instrument characteristics . 1
4.1 General characteristics . 1
4.1.1 Type of instrument . 1
4.1.2 Power supply . 2
4.1.3 Safety . 2
4.1.4 Technology. 2
4.1.5 Physical presentation . 2
4.1.6 Environmental effects . 2
4.2 Electrical characteristics . 2
4.2.1 General . 2
4.2.2 Functional block diagram . 3
4.2.3 Generator unit . 3
4.2.4 Input stage characteristics . 4
4.2.5 Balance . 4
4.2.6 HF signal and demodulation . 5
4.2.7 Demodulated signal processing . 5
4.2.8 Signal output . 6
4.2.9 Digital interface . 7
4.2.10 Digitization and data resolution . 7
5 Verification . 8
5.1 General . 8
5.2 Levels of verification . 8
iii
ISO #####-#:####(X/FDIS 15548-1:2025(en)
5.3 Verification procedure . 9
5.4 Corrective actions . 9
6 Measurement of electrical characteristics of instrument . 10
6.1 Measuring requirements . 10
6.2 Generator unit . 10
6.2.1 Excitation frequency . 10
6.2.2 Harmonic distortion . 11
6.2.3 Harmonic distortion . 12
6.2.4 Maximum output voltage . 12
6.2.5 Maximum output current . 13
6.2.6 Output . 13
6.3 Input stage characteristics . 13
6.3.1 Maximum allowable input voltage related to saturation and non-linearity . 13
6.3.2 Input impedance . 15
6.4 Balance . 16
6.4.1 Excitation frequency . 16
6.4.2 Residual output value at balance . 16
6.5 Demodulation . 16
6.5.1 Orthogonality of signal components . 16
6.6 Demodulated signal processing . 18
6.6.1 Gain accuracy and linearity . 18
6.6.2 Phase-setting accuracy . 19
6.6.3 Bandwidth . 20
6.6.4 Cross-talk . 23
6.6.5 Common-mode rejection . 23
6.6.6 Maximum instruments noise . 24
Annex A (informative) Principle of frequency beat method . 26
Annex B (informative) Method of measurement of linearity range between output and input . 27
Annex C (informative) Table of characteristics . 28
Bibliography . 30
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ISO/DISFDIS 15548-1:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents.www.iso.org/patents. ISO shall not be held responsible for identifying any or all such
patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.htmlwww.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee SC
4, Eddy current testing, in collaboration with the European Committee for Standardization (CEN) Technical
Committee CEN/TC 138, Non-destructive testing, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This secondthird edition cancels and replaces the firstsecond edition (ISO 15548-1:2013), which has been
technically revised.
The main changes are the as follows:
— inclusion of digital instrument, a ;
— revision of the measurement procedures and ;
— introduction of acceptance criteria .
A list of all parts in the ISO 15548 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
Field Code Changed
v
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Introduction
The evaluation of the characteristics of general-purpose eddy current instruments permits a well-defined
description and comparability of eddy current instruments.
By careful choice of the characteristics, a consistent and effective eddy current examination system can be
designed for a specific application.

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DRAFT International Standard ISO/DIS 15548-1:2025(en)

Non-destructive testing — Equipment for eddy current examination
— —
Part 1:
Instrument characteristics and verification
1 Scope
This document specifies the characteristics of general-purpose eddy current instruments and provides
methods for their evaluation and verification.
This document can be completed by an application document specifying acceptance criteria for the
characteristics of the eddy current instrument.
Where accessories are used, these are characterized using the principles of this document (e.g. additional
external amplifiers).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel
ISO 12718, Non-destructive testing — Eddy current testing — Vocabulary
ISO 15549, Non-destructive testing — Eddy current testing — General principles
ISO 18173, Non-destructive testing — General terms and definitions
ISO GUIDE 99, International vocabulary of metrology — Basic and general concepts and associated terms
(VIM)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 12718, ISO 18173 and ISO/IEC
GUIDE 99 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obphttps://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/https://www.electropedia.org/

4 Eddy current instrument characteristics
4.1 General characteristics
4.1.1 Type of instrument
a) a) An instrument has a general-purpose application (e.g. crack detection) when the relationship
between the measured quantity and the output of the instrument is established by the user. A range of
probes can be connected to the instrument. The instrument may have a display that should be
configurable by the user. The instrument manufacturer shall provide a list of adjustable parameters.,, in
order that the user can design the examination system. The examination system shall be in accordance
with ISO 15549. The user shall be able to vary the excitation frequency, gain, balance, phase and filters
(unless an automatically setting is used).
b) b) An instrument is of specific application (such as coating thickness measurement, magnetic
permeability, or electrical conductivity measurement) when the relationship between the measured
quantity and the output is explicitly specified in the range of application. The probe is specific to the
instrument. For this type of instrument, the ISO 15548 series may be partially applied.
4.1.2 Power supply
The instrument can be powered by internal batteries or by an external AC or DC power supply. The nominal
values of voltage, frequency and power consumption shall be stated, together with the tolerance for correct
operation.
4.1.3 Safety
The Applicable safety regulations for the instrument and its accessories shall meet the applicable safety
regulations, for example,can exist, e.g. electrical hazard, surface temperature, explosion, etc.
4.1.4 Technology
The instrument can be completely analogue or mainly digital or partly digital and analogue.
The excitation can be single frequency, multi-frequency, swept frequency or pulsed.
The instrument can be single or multichannel.
The instrument settings can be manual, remote controlled, stored or preset.
The instrument shall provide the eddy current signal at an analogue or digital interface.
The instrument can be with or without a built-in display.
4.1.5 Physical presentation
The instrument can be portable, cased or rack mounted, with the component parts integrated or modular.
The weight and size shall be specified for the instrument .
The plugs and sockets shall be specified regarding type and pin interconnections.
The instrument manufacturer, manufacturer’s address, model number, serial number, year of manufacturing,
relevant technical data (power requirements, IP class), used standards (if any) and markings (e. g. CE) shall
be clearly readable and located in a readily accessible place.
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4.1.6 Environmental effects
The warm-up time necessary for the instrument to reach stable operating conditions within specified limits
shall be stated.
The temperature, humidity and vibration ranges for normal use, storage and transport shall be specified for
the instrument and its accessories.
The instrument shall conform to relevantApplicable electromagnetic compatibility (EMC) regulations can
exist.
4.2 Electrical characteristics
4.2.1 General
The electrical characteristics of an instrument shall be evaluated after the warm-up time has elapsed.
The electrical characteristics are only valid for the stated operating conditions.
The electrical characteristics apply to various items of the functional block diagram of the instrument. Where
applicable, they are provided by the manufacturer. Some of these characteristics can be verified according to
the methodology described in Clause 6.6.
4.2.2 Functional block diagram
The functional block diagram of a typical general-purpose eddy current instrument is shown in Figure 1.0.
Each part of the eddy current instrument may be analogue or digital.
Figure 1 — Functional block diagram of eddy current instrument
4.2.3 Generator unit
The source of excitation is the generator unit.
The characteristics to bespecifiedbe specified are as follows;
— — type of generator: current or voltage;
— — wave shape of the excitation signal;
— — type of excitation: single or multi-frequency;
— — frequency setting: range, step size, deviation from nominal value;
— — differential source resistance;
— — maximum output voltage and current;
— — amplitude setting, if available: range, step size, deviation from nominal value.
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In the case of sinusoidal alternating excitation, the additional characteristic to be specified is:
— — harmonic distortion.
In the case of non-sinusoidal alternating excitation (triangular, rectangular, etc.), additional characteristics to
be specified are:
— — duty cycle;
— — rise and fall time;
— — linearity;
— — overshoot.
In the case of multi-frequency excitation, it shall be stated whether frequencies are injected simultaneously or
multiplexed, independent or related, and the multiplexing sequence shall be specified, when relevant.
4.2.4 Input stage characteristics
The input stage interfaces the probe to the instrument. It provides impedance matching and amplification, as
required.
The characteristic to be specified are as follows:
— — the maximum allowable input voltage related to saturation and non-linearity;
— — input impedance;
— — input configuration (single ended, differential);
— — number of inputs (parallel, multiplexed)).
In the case of multi-channel instruments, additional characteristics to be specified is:
— — cross-talk.
4.2.5 Balance
Balance is the compensation of an offset of the signal to achieve a predetermined operating point. The
compensation may be performed manually or automatically. When availableIf the compensation, is available,
it shall include both the imbalance of the sensor and provide sufficient residual dynamic for the acquisition of
the desired signals.
Conversely, the instrument with a maximum dynamic range should be balanced accordingly through the
following characteristics:
— — residual value at balance (expressed as a percentage of a specified range, e.g. full-scale output).
— — maximum compensable input voltage.
4.2.6 HF signal and demodulation
4.2.6.1 High Frequency (HF) input filter
Filters reduce the signal frequency content which can have an undesirable effect on the test result.
When applicable, the filters used before demodulation are referred to as carrier frequency filters (HF filters).
These are usually band-pass filters which suppress any signal frequencies which do not correspond to the
excitation frequency.
The characteristics to be specified are as follows:
— — Type of filter;
— — Bandwidth at -3 dB;
— — Attenuation rate;.
4.2.6.2 HF amplification
The characteristics to be specified are as follows:
— — gain-setting range;
— — step-size.
4.2.6.3 Demodulation
Demodulation shall be a synchronous demodulation that extracts the low-frequency amplitude and phase
variations from the HF signal.
For positive polarity of demodulation, a delay in the signal will cause the signal vector to rotate clockwise. The
polarity of demodulation shall be positive and shall be confirmed.
The characteristic to be specified is:
— — orthogonality of signal components (X and Y).
4.2.7 Demodulated signal processing
4.2.7.1 Vector amplification
Vector amplification generally consists of two transmission channels of identical design. These channels
amplify the vector components produced by synchronous demodulation. In some instruments, these
components can be amplified with different gains.
The characteristics to be specified are as follows:
— — common gain setting range, step size, deviation from nominal value for both vector components;
— — individual gain setting range, step size, deviation from nominal value for both vector components.
4.2.7.2 Phase setting
Phase setting permits rotation of the demodulated signal vector on the complex plane.
If a phase setting is available for the instrument, the characteristics to be specified are as follows:
— — phase rotation setting range, step size, deviation from nominal value;
— — amplitude variation of the signal vector with phase setting.
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4.2.7.3 Low Frequency (LF) filtering
The filters used after demodulation are referred to as low-frequency filters (LF filters). The bandwidth of the
filter is chosen to suit the application, e.g. wobble, surface speed.
The characteristics to be specified for each filter are as follows:
— — cut-off frequency setting at 3 dB attenuation: range, step size, deviation from nominal value;
— — rate of attenuation;
— — ripple, if present (e.g. Chebyshev filter).
LF filters may have a variable cut-off frequency synchronized with the testing speed by an external encoder.
In this case, the additional characteristics to be specified are as follows:
— — type of the encoder signal;
— — frequency range of encoder signal;
— — relation between cut-off frequency of the filter and frequency of the encoder signal.
NOTE Devices displaying spatial dimension filters can also be stated in spatial frequency.
4.2.7.4 Crosstalk
Crosstalk is related to multi-channel instruments only. It is the variation of the output of a channel in relation
to the variation of the input of another channel.
The characteristics to be specified are as follows:
— — variation of the output of a channel versus input variation of any other channel.
4.2.7.5 Instrument noise
Instrument noise is the stochastic variation of the output at constant input. The maximum noise occurs usually
at maximum amplification and is influenced by the filter settings.
The characteristic to be specified is:
— — maximum peak-to-peak amplitude of the output at constant input.
4.2.8 Signal output
The type of output can be a display, a hard-copy device, analogue outputs or digital interface.
The type of presentation can be, for example, complex plane, strip chart, imaging or threshold signal.
The characteristics of a display shall include at least the following:
— — type of presentation;
— — size and resolution (number of pixels) for digital displays;
— — grid divisions if present;
— — full-scale-display voltage range or time range;
— — linearity;
— — bandwidth for analogue display or sampling rate for digital displays.
If the analogue output is generated by a digital to analogue converter (DAC), additional characteristics shall
include at least the following:
— — sampling rate per output;
— — D/A resolution: number of bits and voltage per digit;.
If a threshold output is available, it should be characterized by:
— — type (x-, y- amplitude, box, circle, etc.);
— — adjustment range;
— — hysteresis (if available);
— — D/A resolution: number of bits and voltage per digit;.
The characteristics of digital interfaces shall include at least the type of the interface (e. g. USB, LAN, RS232,
CAN, IEEE, …) and could also provide following:
— — data protocol and format;
— — serial or parallel;
— — voltage and current levels;
— — data rate and format;
— — sampling rate;
— — analogue/digital (A/D) resolution, range and linearity.
4.2.9 Digital interface
The characteristics of logical inputs and outputs shall include at least the following:
— — functionality (e.g trigger input, encoder input, gate output);
— — voltage and current levels;
— — setting delay;
— — hysteresis;
— — active high or low;
— — galvanic isolation, if present;
— — external power, if required (e.g. if galvanic isolated);).
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4.2.10 Digitization and data resolution
4.2.10.1 General
Whenever a digitization is performed, the following characteristics shall be specified as a minimum:
— — location of the digitization stage in the signal chain (before or after demodulation);
— — A/D resolution;
— — sampling rate (total sampling rate and sampling rate per channel for multichannel instruments);).
The definition of the digitization technique and the triggering are optional.
4.2.10.2 Location of the digitization stage in the signal chain
Digitization may be performed at the input stage, at demodulation or at signal processing of the X and Y signal
components after demodulation.
4.2.10.3 Triggering on digitization
Digitization can be triggered by using an internal clock (fix rate or synchronized to the test frequency) or an
external encoder, depending on the digitization stage
4.2.10.4 Digitization technique
Digitization can be performed by direct conversion, successive approximation or similar techniques.
4.2.10.5 A/D resolution
In this context, A/D resolution is specified as number of digitization bits. The input voltage corresponding to
one bit can be calculated by dividing the input voltage range by 2N-1, where N is the number of digitization
bits.
4.2.10.6 Sampling rate
Number of conversions per second of the A/D converter.
4.2.10.7 Data rate and resolution at the output
The data rate of the signal data at a digital output of the instrument may differ to the sampling rate of the A/D
converter. It shall be specified at which speed (samples per second) and in which resolution (number of
significant bits per sample) this data is provided for each vector component.
If the instrument has parallel or time multiplexed channels, the information shall be provided per multiplexed
input.
5 Verification
5.1 General
For a consistent and effective eddy current examination, it is necessary to verify that the performance of the
eddy current test instrument is maintained within acceptable limits.
The physical condition of reference blocks used for verification shall be within acceptable limits.
The end-user shall be informed on initial results (before any corrective actions).
The list of characteristics is available in Table C.1 of annex C.0.
For a better understanding, the verification procedure is described identically in all three parts of ISO 15548.
5.2 Levels of verification
There are three levels of verification. Each level specifies the time intervals between verification and the
complexity of the verification (see Table C.1 of annex C).0).
It is understood that initial type testing has already been carried out by the manufacturer or under histheir
control.
a) a) Level 1: Global functional check.
A verification is performed at regular intervals of time on the eddy current test system, using reference
blocks to verify that the performance is within specified limits.
The verification is usually performed by the user during standard usage.
The time interval and the reference blocks are specified in the verification procedure.
b) b) Level 2: Detailed functional check.
A verification on an extended time scale is performed to ensure the stability of selected characteristics of
the eddy current instrument, probe, accessories and reference blocks.
c) c) Level 3: Characterization.
A verification is performed on the eddy current instrument, probe accessories and reference blocks to
ensure conformity with the characteristics supplied by the manufacturer.
The organization requiring the verification shall specify the characteristics to be verified, in accordance with
Annex C,Annex C, as a minimum.
In case of hardware repair of the instrument, a detailed functional check (Level 2 detailed function check) shall
be performed.
In case of upgrade (hardware and/or firmware impacting the parameters verified under the current standard)
of the instrument, a characterization (Level 3 verification) shall be performed.
In case of adjustment and calibration, the end-user shall be informed on the detailed results. Then, a valid
detailed functional check shall be performed.
The main features of verification are shown in Table 1.0.
Table 1 — Verification Levels (see Annex CAnnex C for the list of characteristics)
Level Object Typical time period Instruments Performing entity
User
Frequently,
shall be an ET Level
Stability of system
e.g. begin and end of 1 or higher
Reference blocks
Global functional
performance
test, shift change, according toin
check
accordance with ISO
hourly, daily
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Level Object Typical time period Instruments Performing entity
Stability of selected Less frequently but Calibrated
characteristics of the at least annually and measuring Manufacturer,
Detailed functional
instrument, probes when required (e.g. instruments, competent entity
check
and accessories after repair) reference blocks
All characteristics of Once (on releasing a Calibrated
the instrument, new version) and measuring
Manufacturer
probes and when required (e.g. instruments,
Characterisation
accessories upgrade) reference blocks

5.3 Verification procedure
The characteristics to be verified are dependent on the application. The essential characteristics and the level
of verification shall be specified in a verification procedure.
The examination procedure for the application shall refer to the verification procedure. This can restrict the
number of characteristics to be verified for a specified application.
Sufficient data on the characteristics featured in an instrument, probe and reference block shall be provided,
in orderso that verification can be performed within the scope of this part of ISO 15548document.
5.4 Corrective actions
— — Level 1: When the performance is not within the specified limits, a decision shall be made concerning
the components examined since the previous successful verification. Corrective actions shall be made to
bring the performance within the acceptable limits.
— — Level 2: When the deviation of the characteristic is greater than the acceptable limits specified by the
manufacturer, a decision shall be made concerning the instrument, the probe or the accessory being
verified.
— — Level 3: When the characteristic is out of the acceptable range specified by the manufacturer, a
decision shall be made concerning the instrument, the probe or the accessory being verified.
6 Measurement of electrical characteristics of instrument
6.1 Measuring requirements
All measurements described in the following subclauses are made at the inputs and outputs of the instrument.
These measurements do not require opening the instrument (black-box concept).
Keeping the black-box concept, any alternative method, the equivalence of which shall be demonstrated, may
be used.
Shielded, low inductive resistors (e.g. BNC type feed-through terminators) shall be used as loads. The resistors
shall have a value of 50 Ω. Additional measurements may be made with other values of the resistor.
The characteristics of an instrument can be significantly altered if a load is applied that is not in the range
specified by the manufacturer or the application document. If a different load is required for the instrument
or the application, the load used shall be noted in the test report.
The equipment used for measurements shall be in a valid state of calibration.
The measuring instruments shall have a bandwidth compatible with the frequency range of the eddy current
instrument. Typically, the maximum usable frequency of the measuring instrument shall be at least twice the
maximum frequency of the eddy current instrument.
Equipment measuring voltages (e.g. oscilloscope, voltmeter) shall have a high input impedance ≥1 MΩ.
Measured AC voltages and AC currents can be reported as peak, peak-to-peak or RMS values. The type of the
value shall be denoted.
The measurements described hereafter shall be made at the minimum and maximum excitation frequency
available by the instrument and:
— — for detailed functional check (level 2 verification), at least one frequency per decade in the end-user
range and the used frequencies;
— — for characterization (level 3 verification), at least one, preferably two or three frequencies per decade
on a logarithmic scale (e. g. 10 Hz, 100 Hz, 1 kHz, … or 10 Hz, 30 Hz, 100 Hz, … or 10 Hz, 20 Hz, 50 Hz,
100 Hz, …) between the minimum and maximum excitation frequency available by the instrument.
The filter settings used for a specific application will modify the characteristics, for example, bandwidth, gain
setting accuracy and phase-setting accuracy. In this case, the measurement conditions for verification shall be
specified in the application document.
6.2 Generator unit
6.2.1 Excitation frequency
6.2.1.1 Definition and measurement conditions
The frequency shall be measured at the generator output of the instrument loaded in accordance with 6.1.0.
The percentage deviation from the target value is given by Formula 1:0:

𝑓 −𝑓 𝑓 −𝑓
t m t m
{∆𝐹} = {𝛥𝐹} = × 100 (1)
% %
𝑓 𝑓
t t
where
ft is the target frequency value in hertz (Hz);
fm is the measured frequency value in hertz (Hz);
{∆F}% is the deviation from the target value in percentage (%)

ft is the target frequency value in hertz (Hz);
fm is the measured frequency value in hertz (Hz);
{∆F}% is the deviation from the target value in percentage (%)
The maximum absolute percentage of the deviation in the total range of the frequencies measured shall be
reported.
6.2.1.2 Measurement method
The frequency shall be measured using a frequency counter or digital oscilloscope.
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In the case of simultaneous non-multiplexed multi-frequency instruments, spectrum analysis shall be used.
6.2.1.3 Acceptance criteria
The maximum deviation shall not exceed ±3 % for each excitation frequency value.
6.2.2 Harmonic distortion
6.2.2.1 Definition and measurement conditions
For a generator producing a sinusoidal waveform, the harmonic content is used as a measure of the deviation
from a pure sinusoid.
The harmonic distortion is described by the total harmonic distortion, k
THD
k is defined as the ratio of the equivalent root mean square (RMS) voltage of all the harmonic frequencies
THD
(from the 2nd harmonic) over the RMS voltage of the fundamental frequency according to Formula 2:0:

N 2 N 2
√∑ √∑
𝑈 𝑈
n=2 𝑛 n=2 𝑛
𝑘 = 𝑘 = (2)
THD THD
𝑈 𝑈
1 1
where
U is the RMS value of the first harmonic (fundamental);
U is the RMS value of the nth harmonic;
n
k is the total harmonic distortion
THD
U1 is the RMS value of the first harmonic (fundamental);
U is the RMS value of the nth harmonic;
n
k is the total harmonic distortion.
THD
The distortion factor shall be measured at the generator output of the instrument loaded in accordance
with 6.1.6.1.
In the case of multi-frequency instruments, sufficient instrumentation shall be used, e.g. spectrum analyser.
6.2.2.2 Measurement method
The distortion factor may be measured using a distortion-factor bridge, a spectrum analyser or a high-pass
filter.
6.2.2.3 Acceptance criteria
The maximum distortion factor should be less than -40 dB (1 %)
6.2.3 Differential source impedance
6.2.3.1 Definition and measurement conditions
The differential source impedance Z is the internal impedance of the generator unit, measured at different
s
load resistances. The differential source impedance shall be measured for each independent output.
The method proposed is based on the assumption that the capacity and inductivity of the complex source
impedance Z can be neglected and the impedance can be considered as a resistor R . (see Figure 2). (see 0).
S I
Key
Uout output voltage
I output current
out
I source current
U0 source voltage
Zs source impedance
U output voltage
out
I output current
out
I0 source current
U0 source voltage
Zs source impedance
Figure 2 — Internal impedance of generator unit
6.2.3.2 Measurement method
.
The generator output is loaded with a resistor R (normally 50 Ω) and the voltage U is measured with an
1 1
oscilloscope or an adequate voltmeter.
Repeat the measurement with a resistor R2 (normally R2 = 0,5 R1 or R2 = 2 R1) and measure U2.
Z , expressed in ohms, is given by Formula 3:0:
s
∆𝑈 𝑈 −𝑈
1 2
𝑍 = = (3)
s
∆𝐼 𝐼 − 𝐼
2 1
𝛥𝑈 𝑈 −𝑈
1 2
𝑍 = = (3)
s
𝛥𝐼 𝐼 −𝐼
2 1
14 © ISO #### 2025 – All rights reserved
ISO/DISFDIS 15548-1:2025(en)
where
Zs Zs is the source impedance in ohms (Ω)
U1 is the output voltage measured with load R1 in volt (V)
U2  is the output voltage measured with load R2 in volt (V)
I1 is the is the output current measured with load R1 in ampere
(A)
I2 is the is the output current measured with load R2 in ampere
(A)
ΔU is the difference of output voltage measured with load R and
R2
ΔI is the difference of output current measured with load R1 and
R
Zs is the source impedance in ohms (Ω);
U1 is the output voltage measured with load R1 in volt (V);
U2 is the output voltage measured with load R2 in volt (V);
I is the is the output current measured with load R in ampere (A);
1 1
I is the is the output current measured with load R in ampere (A);
2 2
ΔU is the difference of output voltage measured with load R1 and R2;
ΔI is the difference of output current measured with load R1 and R2;
I and I can be measured using a current probe or calculated by I = U /R .
1 2 x x x
NOTE 1 R can be achieved by two resistors with the same value as R in parallel (R = 0,5 R ) or in series (R = 2 R ).
2 1 2 1 2 1
The values of U and U shall be less than the maximum output voltage and the currents I and I shall be less
1 2 1 2
than the maximum output current.
6.2.4 Maximum output voltage
6.2.4.1 Definition and measurement conditions
The maximum output voltage U is the voltage at the generator terminals with no load applied and the
O,max
generator set to give its maximum output.
6.2.4.2 Measurement method
The maximum output voltage is measured using an oscilloscope or an adequate vol
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