IEC 61280-2-12:2014

Fibre optic communication subsystem test procedures - Part 2-12: Digital systems - Measuring eye diagrams and Q-factor using a software triggering technique for transmission signal quality assessment

IEC 61280-2-12:2014 defines the procedure for measuring eye diagrams and Q-factor of optical transmission (RZ and NRZ) signals using software triggering technique as shown in 4.1. Keywords: measuring eye diagrams and Q-factor of optical transmission (RZ and NRZ) signals, software triggering technique

Procédures d'essai des sous-systèmes de télécommunication à fibres optiques - Partie 2-12: Systèmes numériques - Mesure des diagrammes de l'oeil et du facteur de qualité à l'aide d'une technique par déclenchement logiciel pour l'évaluation de la qualité de la transmission de signaux

L'IEC 61280-2-12:2014 définit la procédure de mesure des diagrammes de l'oeil et du facteur de qualité de la transmission de signaux optique (RZ et NRZ) à l'aide d'une technique par déclenchement logiciel, présentée en 4.1. Mots clés: mesure des diagrammes de l'oeil et du facteur de qualité de la transmission de signaux optique (RZ et NRZ), technique par déclenchement logiciel

General Information

• Status: Published
• Publication Date: 05-May-2014
• ICS: 33.180.10 - Fibres and cables
• Technical Committee: SC 86C - Fibre optic systems, sensing and active devices
• Drafting Committee: WG 1 - TC 86/SC 86C/WG 1
• Current Stage: PPUB - Publication issued
• Start Date: 06-May-2014
• Completion Date: 31-Jul-2014

Standards Content (Sample)

IEC 61280-2-12 ®
Edition 1.0 2014-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic communication subsystem test procedures –
Part 2-12: Digital systems – Measuring eye diagrams and Q-factor using a
software triggering technique for transmission signal quality assessment

Procédures d'essai des sous-systèmes de télécommunication à fibres
optiques –
Partie 2-12: Systèmes numériques – Mesure des diagrammes de l'œil et du
facteur de qualité à l'aide d'une technique par déclenchement logiciel pour
l'évaluation de la qualité de la transmission de signaux

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IEC 61280-2-12 ®
Edition 1.0 2014-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic communication subsystem test procedures –

Part 2-12: Digital systems – Measuring eye diagrams and Q-factor using a

software triggering technique for transmission signal quality assessment

Procédures d'essai des sous-systèmes de télécommunication à fibres

optiques –
Partie 2-12: Systèmes numériques – Mesure des diagrammes de l'œil et du

facteur de qualité à l'aide d'une technique par déclenchement logiciel pour

l'évaluation de la qualité de la transmission de signaux

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX R
ICS 33.180.10 ISBN 978-2-8322-1545-6

– 2 – IEC 61280-2-12:2014 © IEC 2014
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Abbreviated terms . 6
4 Software synchronization method and Q-factor . 6
4.1 Example of asynchronous waveform and eye diagram reconstructed by
software triggering technique . 6
4.2 Q-factor formula . 7
5 Apparatus . 9
5.1 General . 9
5.2 Optical bandpass filter . 10
5.3 High frequency receiver . 10
5.4 Clock oscillator . 11
5.5 Electric pulse generator . 11
5.6 Sampling module . 11
5.7 Electric signal processing circuit . 12
5.8 Optical clock pulse generator . 12
5.9 Optical sampling module . 12
5.10 Optical signal processing circuit . 12
5.11 Synchronization bandwidth . 12
5.12 Monitoring system parameters . 13
6 Procedure . 13
6.1 General . 13
6.2 Measuring eye diagrams and Q calculations . 13
Annex A (informative) Example of the signal processing required to reconstruct the
synchronous eye diagram . 15
Annex B (informative) Adequate sampling time width (gate width) . 17
Bibliography . 18

Figure 1 – Asynchronous waveform and synchronous eye diagram of 40 Gbps RZ-
signal reconstructed by software triggering technique . 7
Figure 2 – RZ synchronous eye diagram reconstructed by software triggering
technique, time window, and histogram . 8
Figure 3 – Example of relationship between Q-factor and window width . 8
Figure 4 – Test system 1 for measuring eye diagrams and Q-factor using the software
triggering technique . 9
Figure 5 – Test system 2 for measuring eye diagrams and Q-factor using the software
triggering technique . 10
Figure A.1 – Block diagram of the software triggering module . 15
Figure A.2 – Example of interpolating a discrete spectrum and determining beat
frequency . 16
Figure B.1 – The typical calculated relationship between the adequate sampling time
width (gate width) and the bit rate of the optical signal . 17

Table 1 – Monitoring system parameters . 13

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –

Part 2-12: Digital systems –
Measuring eye diagrams and Q-factor using a software triggering
technique for transmission signal quality assessment

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The International Electrotechnical Commission (IEC) draws attention to the fact that it is
claimed that compliance with this document may involve the use of patents concerning
software synchronization given in Clause 4 and procedure for calculating eye-diagrams and
Q-factor given in Clause 6.
IEC takes no position concerning the evidence, validity and scope of these patent rights.
The holders of these patent rights have assured the IEC that they are willing to negotiate
licences either free of charge or under reasonable and non-discriminatory terms and
conditions with applicants throughout the world. In this respect, the statements of these
holders of these patent rights are registered with IEC.

– 4 – IEC 61280-2-12:2014 © IEC 2014
For US patent 6,744,496, information may be obtained from:
Alcatel-Lucent
Intellectual Property Business Group
16 Brookside Dr.
Sutton, MA 01590 USA
For Japanese patent 3987001 and US patent 7190752, information may be obtained from:
Nippon Telegraph and Telephone Corporation
9-11, Midori-cho, 3-Chrome Musashino-Shi
Tokyo 180-8585 Japan
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those identified above. IEC shall not be held responsible for
identifying any or all such patent rights.
ISO (www.iso.org/patents) and IEC (http://patents.iec.ch) maintain on-line data bases of
patents relevant to their standards. Users are encouraged to consult the data bases for the
most up to date information concerning patents.
International Standard IEC 61280-2-12 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics.
The text of this standard is based on the following documents:
CDV Report on voting
86C/1150/CDV 86C/1220/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61280 series, published under the general title Fibre optic
communication subsystem test procedures, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.

INTRODUCTION
Signal quality monitoring is important for operation and maintenance of optical transport
networks (OTN). From the network operator’s point of view, monitoring techniques are
required to establish connections, protection, restoration, and/or service level agreements. In
order to establish these functions, the monitoring techniques used should satisfy some
general requirements:
• in-service (non-intrusive) measurement
• signal deterioration detection (both SNR degradation and waveform distortion)
• fault isolation (localize impaired sections or nodes)
• transparency and scalability (irrespective of the signal bit rate and signal formats)
• simplicity (small size and low cost).
There are several approaches, both analogue and digital techniques, which make it possible
to detect various impairments:
• bit error rate (BER) estimation [1,2]
• error block detection
• optical power measurement
• optical SNR evaluation with spectrum measurement [3,4]
• pilot tone detection [5,6]
• Q-factor monitoring [7]
• pseudo BER estimation using two decision circuits [8,9]
• histogram evaluation with synchronous eye diagram measurement [10].
A fundamental performance monitoring parameter of any digital transmission system is its
end-to-end BER. However, the BER can be correctly evaluated only with out of service BER
measurements, using a known test bit pattern in place of the real signal. On the other hand,
in-service measurement can only provide rough estimates through the measurement of digital
parameters (e.g., BER estimation, error block detection, and error count in forward error
correction) or analogue parameters (e.g., optical SNR and Q-factor).
An in-service optical Q-factor monitoring can be used for accurate quality assessment of
transmitted signals on wavelength division multiplexed (WDM) networks. Chromatic dispersion
(CD) compensation is required for Q monitoring at measurement point in CD uncompensated
optical link. However, conventional Q monitoring method is not suitable for signal evaluation
of transmission signals, because it requires timing extraction by complex equipment that is
specific to each BER and each format.
The software triggering technique [11-14] reconstructs synchronous eye-diagram waveforms
without an external clock signal synchronized to optical transmission signal from digital data
obtained through asynchronous sampling. It does not rely on an optical signal’s transmission
rate and data formats (RZ or NRZ). Measuring method of eye diagrams and Q-factor using the
software triggering technique is a cost-effective alternative to BER estimations. With eye
diagrams and Q-factor using software triggering test method, signal quality degradations due
to optical signal-to-noise ratio (OSNR) degradation, to jitter fluctuations and to waveform
distortion can be monitored.
This is one of the promising performance-monitoring approaches for intensity modulated
direct detection (IM-DD) optical transmission systems.

Numbers in square brackets refer to the Bibliography.

– 6 – IEC 61280-2-12:2014 © IEC 2014
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –

Part 2-12: Digital systems – Measuring eye diagrams and Q-factor using a
software triggering technique for transmission signal quality assessment

1 Scope
This part of IEC 61280 defines the procedure for measuring eye diagrams and Q-factor of
optical transmission (RZ and NRZ) signals using software triggering technique as shown in
4.1 [14].
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61280-2-2, Fibre optic communication subsystem basic test procedures – Part 2-2: Test
procedure for digital systems – Optical eye pattern, waveform, and extinction ratio
measurement
ITU-T Recommendation G.959.1: 2012, Optical transport network physical layer interfaces
3 Abbreviated terms
ASE amplified spontaneous emission
BER bit error rate
CD chromatic dispersion
EDFA Er-doped fibre amplifier
IM-DD intensity modulated direct detection
RZ return-to-zero
NRZ non-return-to-zero
OBPF optical bandpass filter
OSNR optical signal-to-noise ratio
OTN optical transport networks
PMD polarization mode dispersion
SNR signal-to-noise ratio
WDM wavelength division multiplexing
4 Software synchronization method and Q-factor
4.1 Example of asynchronous waveform and eye diagram reconstructed by software
triggering technique
Figure 1 shows an example of a 40 Gb/s RZ-synchronous eye diagram constructed from
asynchronous sampled data using the software triggering technique. The inset in Figure 1
shows an asynchronous waveform obtained from the same asynchronous sampled data.

Asynchronous
waveform
Sampling frequency: 40,379 MHz (asynchronous)
Eye diagram reconstructed
by the software triggering
technique
Sampled data
−1
0 5 10 15 20 25
Time  (ps)
IEC  1198/14
Figure 1 – Asynchronous waveform and synchronous eye diagram of
40 Gbps RZ-signal reconstructed by software triggering technique
4.2 Q-factor formula
As shown in Figure 2, the Q-factor can be calculated from a histogram of “mark” (“1”) and
“space” (“0”) levels in the time window, in which an appropriate time window is established in
a large part of the eye opening. The time window is separated into “mark” (“1”) and “space”
(“0”) levels, the average µ and standard deviation σ of the “space” (“0”) level data and the
0 0
average µ and standard deviation σ of the “mark” (“1”) level data are calculated, and the Q-
1 1
factor is calculated by substituting the obtained µ , σ , µ , and σ into Formula (1).
0 0 1 1
The Q-factor depends on the position of the centre of the time window. For optical
transmission signal quality evaluation, the maximum value obtained by calculating Formula (1)
while changing the position of centre of the time window is defined as the Q-factor.
µ −µ
1 0
(1)
Q=
σ +σ
1 0
The Q-factor also depends on width of the time window. Assuming that the signal waveform is
sinusoidal RZ with duty ratio of 50 % (Figure 3(a)) or sinusoidal NRZ (Figure 3(b)) and σ = σ ,
0 1
calculated relationships between Q-factor and window width are shown in Figure 3(c). A
suitable window width is 0,1 UI or less for an RZ signal and 0,2 UI or less for an NRZ signal.

Amplitude  (arb. unit)
– 8 – IEC 61280-2-12:2014 © IEC 2014
Mark
Histogram
Time window
σ
µ
Space
σ
µ
−1
Time
IEC  1199/14
Figure 2 – RZ synchronous eye diagram reconstructed by
software triggering technique, time window, and histogram
NRZ
RZ
0,5
0,5
0 0,2 0,5 0,7 1
0 0,2 0,5 0,7 1
Time  (UI)
Time  (UI)
IEC  1201/14
IEC  1200/14
Figure 3a – Sinusoidal RZ
Figure 3b – Sinusoidal NRZ
with duty 50 %
RZ
NRZ
0 0,1 0,2 0,3 0,4 0,5
Window width
IEC  1202/14
Figure 3c – Calculated relationships between Q-factor and window width
Figure 3 – Example of relationship between Q-factor and window width
Amplitude  (a.u.)
Amplitude  (a.u.)
Q factor  (dB)
Amplitude  (a.u.)
5 Apparatus
5.1 General
Test systems are mainly composed of an optical bandpass filter, a high frequency receiver, a
clock oscillator, an electric pulse generator, a sampling module, an electric signal processing
circuit with an AD converter and a software triggering module (Figure 4); or, an optical
bandpass filter, an optical clock pulse generator, an optical sampling module, an optical signal
processing circuit with an AD converter, a low frequency receiver and software triggering
module (Figure 5).
In the typical case, eye diagram and Q-factor measurements are performed after the optical
amplifier of the repeaters, optical-cross connects, and other nodes, because sufficient signal
power level and CD compensation are required for the Q-factor monitoring.
Repeater or optical switching node
High frequency
receiver
Software triggering
Measurement
module
result
Sampling module
AD converter
Electric signal
Optical
processing circuit
band-pass filter
Electric pulse
generator
Clock oscillator
Eye pattern waveform and Q-factor measuring circuit using
the software triggering technique
Transmission line
IEC  1203/14
Figure 4 – Test system 1 for measuring eye diagrams and
Q-factor using the software triggering technique

– 10 – IEC 61280-2-12:2014 © IEC 2014
Repeater or optical switching node
Optical sampling
Software triggering
module
module
Measurement
result
AD converter
Optical band- Optical clock pulse
pass filter generator
Low frequency receiver
Optical signal
processing circuit
Eye pattern waveform and Q-factor measuring circuit
using the software triggering technique
Transmission line
IEC  1204/14
Figure 5 – Test system 2 for measuring eye diagrams and
Q-factor using the software triggering technique
5.2 Optical bandpass filter
The optical bandpass filter (OBPF) should be used to remove unnecessary ASE noise from
the optical amplifier or/and to extract the necessary channel from the WDM signals. The
bandwidth of the optical filter B should be broader than the bit rate of the optical signal. The
opt
shape of the OBPF is shown in ITU-T Recommendation G.959.1: 2012, Figure B.2, where two
parameters, the power suppression ratio of adjacent channel and the central frequency
deviation, are defined.
5.3 High frequency receiver
The high frequency receiver is typically a high-speed photodiode, followed by electrical
amplification. The high frequency receiver is equipped with an appropriate optical connector
to allow connection to the optical interface point, either directly or via an optical jumper cable.
Precise specifications are precluded by the wide variety of possible implementations.
However, the high frequency receiver shall follow the general guideline based on IEC 61280-
2-2 as follows:
a) acceptable input wavelength range, adequate to cover the intended application;
b) responsivity, adequate to produce an eye-pattern;
For example, assume that a non-return-to-zero (NRZ) optical data stream with an average
power of −15 dBm is to be measured. If the sensitivity of the signal processing circuit with
sampling module is 10 mV/div, a responsivity of 790 V/W is required in order to produce
an eye-pattern of 50 mV peak-to-peak.
c) optical noise-equivalent power, low enough to result in accurate measurements;
For example, assume that a non-return-to-zero (NRZ) optical data stream with an average
power of −15 dBm is to be measured. If the effective noise band width of the measurement
system is 470 MHz, and if the displayed root-mean-square noise is to be less than 5 % of
the asynchronous eye-pattern height, the optical noise-equivalent power should be
–1/2
145 pw-Hz or less.
d) Upper cut-off (−3 dB) frequency, B Hz;
mes
In order to ensure repeatability and accuracy, the upper cut-off frequency (bandwidth),
B , of the measurement system should be explicitly stated in the detail specifications.
mes
For NRZ format signals, the high frequency receiver and sampling module that have a
combined impulse response with a −3 dB bandwidth of 0,75/T (where T is the bit interval,
in seconds, of the data signal) are often used. For RZ format signals, the spectral content
may be significantly higher than the NRZ signal at the same signal bit rate. This can lead
to measurement system bandwidth that is in excess of the clock frequency.
e) lower cut-off (−3 dB) frequency, B Hz;
low
In order to avoid significant distortion of the detected eye-pattern due to lack of low
frequency spectral components, the lower cut-off frequency, B , of the measurement
low
system should be sufficiently low compared with 1/T . T is the total sampling time
samp samp,
described in 5.12. DC coupling is not always necessary for Q-factor measurements,
because the DC component of the eye-pattern will be cancelled by µ − µ in Formula (1).
1 0
f) transient response, overshoot, undershoot, and other waveform aberrations should be
minor so as not to interfere with the measurement;
The upper cut-off frequency (bandwidth), B , of the measurement system should
mes
primarily determine the system transient response.
g) the corresponding software clock recovery loop bandwidth should be high enough for
tracking of the signal under tests phase noise. The resulting loop bandwidth is related to
the sampling rate and synchronization algorithm. In practice, the loop bandwidth is at least
100 times less than the sampling rate. For example, in IEC 61280-2-2 loop bandwidths of
4 MHz are recommended for 10 G NRZ data, which would yield a recommended sampling
rate of 400 MSample/s. With better control of the signal VCOs, the recommended loop
bandwidth could be reduced.
h) output electrical return loss, high enough that reflections from the sampling module
following the receiver are adequately suppressed, from 0 Hz to a frequency significantly
greater than the bandwidth of receiver;
A time-domain measurement may be very inaccurate if significant multiple reflections are
present. A minimum value of 15 dB for the return loss is recommended when many
components are employed following the receiver. The effective output return loss of the
receiver may be improved with in-line electrical attenuators, at the expense of reduced
signal levels. Finally, the return loss specification extends to DC, since otherwise, a DC
shift in the waveform will occur, causing Q-factor measurements to be in error.
5.4 Clock oscillator
The clock oscillator generates a clock signal that corresponds to the sampling rate. The
generated clock signal jitter at frequencies above the software clock recovery loop bandwidth
shall be sufficiently smaller than the bit period for clear eye diagrams, and is sent to an
electric pulse generator and a signal electric processing circuit. A high clock frequency is
desirable for wide clock recovery bandwidth.
5.5 Electric pulse generator
The electric pulse generator should be capable of providing an electric short pulse train or
electrical clock signal with proper slew rate to the sampling module. The electric pulse
repetition frequency is identical to the sampling rate.
5.6 Sampling module
The sampling module should sample the electrical signals at a specified repetition rate with a
specified sampling time width (sampling window) by using the electric pulse train generated
by the electrical pulse generator and detect the level of the sampled signals. The sampled
values are sent to the electric signal processing circuit.
The accuracy of Q is dependent on the measurement system bandwidth B .
mes
– 12 – IEC 61280-2-12:2014 © IEC 2014
5.7 Electric signal processing circuit
The electric signal processing circuit should reconstruct the eye-diagram waveform and
calculate the Q-factor (and the amplitude histogram) utilizing the asynchronous sampled
signals from the sampling module and the clock signal from the clock oscillator. Q-factor
formula is shown in 4.2.
Within the electric signal processing circuit, the electric signal sampled by the sampling
module is digitized by the AD converter, and then the temporal axis is calculated from that
digitized value in the software triggering module. An example of a principle of signal
processing in the software triggering module is shown Annex A [14].
5.8 Optical clock pulse generator
The optical clock pulse generator generates an optical pulse train and a clock signal at the
sampling rate. The generated optical pulse train and a clock signal are sent to the optical
sampling module and the optical signal processing circuit respectively. The repetition
frequency of the optical pulse train is synchronous with the clock signal. The generated
optical pulse train jitter at frequencies above the software clock recovery loop bandwidth shall
be sufficiently smaller than the bit period for clear eye diagrams. The higher optical clock
frequency is desirable for wide clock recovery bandwidth.
5.9 Optical sampling module
The optical sampling module should sample the optical signal at a specified repetition rate
with an adequate sampling time width (sampling window or gate width) that depends on the bit
rate of the optical signal. Varying a sampling time width leads to change the upper cut-off
of the measurement system. The sampled optical signal is sent to the
(-3 dB) frequency B
mes
optical signal processing circuit.
The calculated relationship between the adequate sampling time width (gate width) and the bit
rate of the optical signal is shown in Annex B.
5.10 Optical signal processing circuit
The optical signal processing circuit should reconstruct the eye-diagram waveform and
calculate the Q-factor (and the amplitude histogram) utilizing the asynchronous sampled
signals from the sampling module and the clock signal from the optical clock pulse generator.
The Q-factor formula is in 4.2.
Within the optical signal processing circuit, the optical signal sampled by the optical sampling
module is digitized by the low frequency receiver and the AD converter. Then, the temporal
axis is calculated from that digitized value in the software triggering module. The bandwidth of
the low frequency receiver shall be over 2 times the sampling rate. An example of a principle
of signal processing in the software triggering module is shown Annex A [14].
5.11 Synchronization bandwidth
In the guidelines of IEC 61280-2-2, an oscilloscope triggering system using a recovered clock
from the signal under test is discussed. The clock recovery bandwidth for eye pattern
measurements will be similar to that of the communications system receiver to suppress
unimportant jitter which does not degrade system level communications. High sampling
frequency more than 1 GSample/s is required to achieve such a wide clock recovery
bandwidth of the communications system receiver by using software synchronization method.
However, low sampling frequency less than 1 GSample/s is desirable for low-cost Q-factor
monitor using software synchronization method, and the clock recovery bandwidth of the Q-
factor monitor may be lower than that of the communications system receiver. If the jitter
frequency is higher than the clock recovery bandwidth, the jitter will appear in the eye diagram,
and the horizontal eye opening will be decreased by the jitter. Therefore, the low-cost Q-factor

monitor is more sensitive to high frequency jitter than the measuring instruments with high
clock recovery bandwidth.
5.12 Monitoring system parameters
For the measurement of the eye diagram and Q-factor of the optical transmission signals
using the software triggering technique, appropriate parameters for the test system shall be
selected. The optical filter bandwidth, B , determines the bandwidth and optical SNR of the
opt
optical signal to be processed. The measurement system bandwidth, B , is determined by
mes
the high frequency receiver and the sampling module in test system 1 (Figure 4) or the optical
sampling module in test system 2 (Figure 5); it influences the eye diagram and Q-factor. The
sampling number, N , is the number of sampled points for drawing the amplitude
samp
histogram. The sampling number, N , is the total number of sampled points. The sampling
total
rate, R , is repetition rate of the sampling clock. The total sampling time, T , is a
samp samp
parameter that is related to the clock recovery bandwidth. The terms T , N , N and
samp samp total
R are related as
samp
N = T / T × N (2)
total bit window samp
T = N / R (3)
samp total samp
The monitoring system parameters are listed in Table 1.
Table 1 – Monitoring system parameters
B Optical filter bandwidth
opt
B
Measurement system bandwidth
mes
T Time of 1bit
bits
T Time of window width
window
N
Number of samples
samp
R Sampling frequency
samp
T Total sampling time
samp
6 Procedure
6.1 General
By using the software triggering technique, eye diagrams can be reconstructed from
asynchronous sampled data, and Q-factor can be calculated from those waveforms.
6.2 Measuring eye diagrams and Q calculations
The procedure for measuring eye diagrams using the software triggering technique and Q-
factor measurement is shown below.
a) Turn on the measuring instruments and wait a sufficient amount of time until its
temperature and performance are stable.
b) Connect the optical signal on the transmission line to the test system, as shown in Figure
4 or Figure 5An EDFA is required only if the power from the transmission line is
insufficient to provide a sufficiently high signal level to high frequency receiver or low
frequency receiver. When an EDFA is used, an ASE from the EDFA modifies the OSNR.
Therefore, it is necessary to confirm that the required Q-factor measurement can be
realized.
– 14 – IEC 61280-2-12:2014 © IEC 2014
c) Reconstruct the eye diagram through the asynchronous sampled data and calculate the Q-
factor from the amplitude histogram using software triggering.
NOTE Q-factor can be calculated by Formula (1).

Annex A
(informative)
Example of the signal processing required
to reconstruct the synchronous eye diagram
The software triggering technique for measuring the eye diagrams and Q-factor of RZ optical
transmission signals reconstructs synchronous eye diagrams from asynchronous sampling
data through a signal processing technique. Figure A.1 shows a block diagram of the software
triggering module, which is necessary to reconstruct eye diagrams from digital data obtained
through asynchronous sampling.
As shown in Figure A.1, the asynchronous sampling data that was digitized by the AD
converter is divided into two branches, one of which is sent directly to the eye diagram display
as an amplitude signal (a vertical axis signal). The other signal is branched again into two
signals. For one of these branches, discrete Fourier transform is performed to obtain the
discrete spectrum. The obtained discrete spectrum data is interpolated, and a precise peak
frequency is obtained from the spectrum. (This peak frequency is used as the beat frequency
between the clock frequency of the optical transmission signal and a frequency that is a
multiple of the sampling frequency. Figure A.2 shows an example of obtaining a beat
frequency by interpolating the discrete spectrum). For the other branched signal, the phase of
the signal component at the beat signal when the amplitude signal is obtained is detected, the
temporal axis (horizontal axis) is normalized at one unit interval (UI), and the temporal axis
signal is sent to the eye diagram display so that the centre of the temporal axis becomes 0
degree phase.
The principles are explained here using the RZ optical transmission signal, but even if
measuring NRZ optical transmission signals that do not have a clock frequency component,
synchronous eye diagrams can be reconstructed using the software triggering technique by
non-linear calculation of the asynchronous sampling data before the discrete Fourier
transform processing.
On typical software synchronization method, since the beat frequency is assumed to be
constant during the total sampling time, T , averaged clock frequency during T is
samp samp
detected for synchronization. The jitter transfer function is corresponding to transfer function
of rectangular impulse response with width of T , and therefore the clock recovery
samp
bandwidth (equivalent noise bandwidth) becomes 1/(2T ). For example, the sampling
samp
frequency, R , is 40 MSample/s, the total number of sampling points, N , is 10 000, the
samp total
equivalent clock recovery bandwidth becomes 2 kHz which is lower than that of the typical
communications system receiver.
Eye diagram
Vertical
y
axis
Asynchronous
y
i
sampling data
Interpolated Beat
Discrete
x
spectrum frequency
spectrum
Horizontal
Timing
x
i
Fourier Inter- Peak
axis
recon-
transform polation detection
struction
Phase φ
Phase
detection
IEC  1205/14
Figure A.1 – Block diagram of the software triggering module

– 16 – IEC 61280-2-12:2014 © IEC 2014
−5
Enlarged
−10
−15
−20
−25
−30
−35
10,996 10,998 11,000 11,002 11,004
Frequency  (MHz)
IEC  1206/14
Figure A.2 – Example of interpolating a discrete
spectrum and determining beat frequency
Amplitude  (dB)
Annex B
(informative)
Adeq
...