The eye diagram analyzer constructs both conventional eye diagrams
and special eyeline diagrams to perform extinction ratio and mask tests
on digital transmitters. It also makes a number of diagnostic
measurements to determine if such factors as waveform distortion,
intersymbol interference, or noise are limiting the bit error ratio of a
transmission system.
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Hardware
Filter
Data
Data
Laser
Transmitter
Optical-toElectrical
Converter
HP 70842B
Error Detector
HP 70841B
Pattern Generator
Trigger
Clock In
Clock In
Clock Out
Clock
Out
Ch 1
HP 71501A
Eye Diagram Analyzer
HP 70311A Clock Source
10-MHz Ref
Ch
2
10-MHz Ref
Fig. 1. $"$.& .,(-'$.. , *,4
' .,$ . -. - ./* /-$(" ( 2
$",' (&23 ,
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network (SONET), a North American standard, and the synĆ
chronous digital hierarchy (SDH), an international standard.
Both standards are for highĆcapacity fiberĆoptic transmission
and have similar physical layer definitions. These standards
define the features and functionality of a transport system
based on principles of synchronous multiplexing. The more
widely used transmission rates are 155.52 Mbits/s, 622.08
Mbits/s, and 2.48832 Gbits/s.
One of the goals of the standards is to provide midĆspan
meet" so that equipment from multiple vendors can be used
in the same telecommunications link. The standards specify
extinction ratio and eye mask measurements on the transĆ
mitted eye diagram to help ensure that transmitters from
various vendors are compatible.1,2 The eye diagram shown
in Fig. 2a is from a laser transmitter operating at 2.48832
Gbits/s. It shows the characteristic laser turnĆon overshoot
and ringing.
An important specified test parameter for these transmission
systems is the extinction ratio (ER) of the eye diagram. It is
typically defined as:
(a)
ER 10 log
P avg(logic 1)
P avg(logic 0)
where Pavg(logic 1) is the mean or average optical power level
of the logic 1 level and Pavg(logic 0) is the mean or average
optical power level of the logic 0 level. For SONET/SDH
transmission systems, the minimum specified extinction ratio
is 10 dB. In some cases, the extinction ratio is expressed as
the linear ratio of the two power levels. A good extinction
ratio is desired in these systems to maintain an adequate
received signalĆtoĆnoise ratio.
Although the definition of extinction ratio is relatively
straightforward, the measurement methodology to determine
the mean logic levels is not specified in the standards, such
as SDH standard G.957. The histogram and statistical analysis
capability of digitizing oscilloscopes can be used to deterĆ
mine the mean and standard deviation (sigma) of a waveĆ
form. However, there are no standard criteria for setting the
windows and limits for the collection and evaluation of the
data to determine the mean logic levels.
(b)
The Telecommunications Industry Association/Electronics
Industry Association (TIA/EIA) has developed a recommended
methodology for making eye diagram measurements called
the Optical Fiber Standard Test Procedure #4 (OFSTPĆ4).3 It
recommends that voltage histograms be used to determine
the most prevalent logic 1 and 0 levels of the eye pattern
measured across an entire bit period. The OFSTPĆ4 also
points out the importance of removing any residual dc offset
from the extinction ratio measurement because this can draĆ
matically affect the measurement accuracy.
(c)
Fig. 2. (a) Conventional eye diagram. (b) Eyeline diagram.
(c) Eyeline diagram with eye filtering.
August 1994 HewlettĆPackard Journal
Over the years, the designers of digital transmission systems
have learned that the eye diagram should have a particular
shape to achieve a good BER. Often these designers have
constructed areas, or masks, inside and around the eye diaĆ
gram. The eye diagram waveform should not enter into these
masked areas. The polygons in the center of the eye diaĆ
gram shown in Fig. 3 and the lines at the top and bottom
correspond to the mask used to evaluate optical transmitters
Amplitude
1+Y1
1.0
–Y2
0.5
–Y1
0.0
–Y1
0.0
X1
X2
X3
Unit Interval
X4
1.0
STM-1
(155.52
Mbits/s)
STM-4
(622.08
Mbits/s)
STM-16
(2.48832
Gbits/s)
X1 / X4
0.15 / 0.85
0.25 / 0.75
X3 – X2
0.20
X2 / X3
0.35 / 0.65
0.40 / 0.60
Y1 / Y2
0.25 / 0.75
Y1 / Y2
0.20 / 0.80
0.20 / 0.80
Fig. 3. %7)6 86%271-88)6 )=) (-%+6%1 1%7/7 *36 "#" 86%27?
1-77-32 7=78)17
-28)2()( *36 97) -2 "#" 7=78)17 )4)2(-2+ 32 8,)
86%271-77-32 &-8 6%8) 8,) 7->) %2( 7,%4) 3* 8,) 1%7/ ',%2+)7
#,) < %2( = '336(-2%8)7 %6) 74)'-*-)( *36 )%', &-8 6%8) %2(
8,)-6 6)0%8-:) 437-8-327 %6) &%7)( 32 8,) 1)%2 03+-'%0 0):)0
%2( 8,) 1)%2 03+-'%0 0):)0 #,) 1%7/ *36 8,) 03;)6 &-8
6%8)7 -7 % ,)<%+32 ;,)6)%7 8,) 1%7/ *36 ?&-87
86%271-77-32 -7 % 6)'8%2+0) #,) 6)')-:)6 &%2(;-(8, *36 8,)
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'31132 6)*)6)2') &%2(;-(8, *36 86%271-88)6 ):%09%8-32
%6(;%6) 03;?4%77 *-08)67 %6) '311320= )1403=)( 83
%',-):) 8,-7 6)74327)
#3 (%8) )<-78-2+ -278691)28%8-32 ,%7 &))2 -2%()59%8) 83
()7-+2 &9-0( %2( 8)78 348-'%0 86%271-88)67 79**-'-)280= 83
1))8 8,) 6)59-6)1)287 3* 8,)7) 86%271-77-32 78%2(%6(7 -2
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%6) 3*8)2 (-**-'908 83 3&8%-2 4%68-'90%60= )<8-2'8-32 6%8-3 1)%?
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7=78)17 %7=?83?97) 1%7/ '3140-%2') 8)78-2+ ;-8, ()*%908
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)67 -2 (-%+237-2+ 86%271-88)( ! 463&0)17 ;390( &) % 1%?
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(-740%= %2( 1%-2*6%1) %2( 8,) )=) (-%+6%1
4)6732%0-8= #,) 4)6732%0-8= -7 7836)( 32 % ?&=8) !
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1)%796)1)287 '%2 &) 1%() )%7-0= #,-7 -278691)28 '32*-+9?
6%8-32 -7 7,3;2 -2 -+ Fig. 4. ,383+6%4, 3* 8,) )=) (-%+6%1 %2%0=>)6 ;-8, %2
4%88)62 +)2)6%836 7=78)1
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-2 -+ &38, -278691)287 ,%:) 1-'63;%:) 7%140)67 83 7%1?
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03+?83?(-+-8%0 '32:)68)6 (-+-8->-2+ 37'-0037'34) ,%7 %
86-++)6 -2498 8,%8 -7 97)( 83 78%68 % 7%140) 2 %((-8-32 %2
-2'6)1)28%0 ()0%= -7 %(()( 83 8,) 86-++)6 7-+2%0 73 8,) 7%1?
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-7 '327869'8)(
#,) 7%140) 6%8) 3* 8,) )=) (-%+6%1 %2%0=>)6 -7 238 ()8)6?
1-2)( &= %2 )<8)62%0 86-++)6 &98 -8 -7 7)8 %''36(-2+ 83 8,)
*6)59)2'= '328)28 3* 8,) -2'31-2+ 7-+2%0 -87)0* 7 8,) -2?
'31-2+ 7-+2%0 -7 (-+-8->)( -8 -7 %2%0=>)( 83 ()8)61-2) %2
%446346-%8) 7%140) *6)59)2'= 83 (3;2?'32:)68 -8 348-1%00=
-283 8,) -28)61)(-%8) *6)59)2'= 7)'8-32
7 7,3;2 -2 8,) &03'/ (-%+6%1 -+ 8,) ,%7
8;3 -()28-'%0 7-+2%0 463')77-2+ ',%22)07 ;,-', '%2 7%140)
%2( (-+-8->) 7-+2%07 *631 (' 94 83 > 2498 7-+2%07 83
)%', ',%22)0 %6) 7%140)( &= % 1-'63;%:) 7%140)6 %8 % 6%8)
*7 &)8;))2 > %2( > #,) 7%140) 6%8) -7 ()4)2?
()28 9432 8,) 7-+2%0 *6)59)2'= %2( 8,) 8=4) 3* 1)%796)?
1)28 &)-2+ 1%() #,) 3984987 3* 8,) 7%140)67 %6) *)( -283
8,) ('?83??> 7)'8-327 #,) 7)'8-327 '328%-2 7;-8',?
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1-'63;%:) 7%140)6 %2( 7911)( -283 8,) 7-+2%0 7)4%?
6%8)0= #,) 3984987 3* 8,) 7)'8-327 %6) 7%140)( %8 8,)
7%1) 6%8) %7 8,) -2498 7-+2%0 %2( 8,)2 '32:)68)( 83 % (-+-8%0
7-+2%0 &= 8,) 7
2') 8,) 7-+2%07 %6) (-+-8->)( 8,)= %6) *)( -283 8,) &9**)6
1)136-)7 #,)7) &9**)67 ,30( 8,) 7%140)7 928-0 8,) 86-++)6
43-28 -7 ()8)61-2)( #,) &9**)6 1)136-)7 1%/) -8 4377-&0)
83 :-); 7-+2%07 &)*36) 8,) 86-++)6 ):)28 ;-8,398 97-2+ ()0%=
9+978 );0)88? %'/%6( 3962%0
Oscilloscope
10-MHz IF
S
T
Sampler
Drive
ADC
DSP
10-MHz
Trigger
Display
Phase
Trigger
X1/mT
(a)
ADC
S
Microprocessor
Display
Sampler
Drive
Trigger
Fig. 5. &),(&#&"! . %&0" 01.(
+),.&/+* +# 0%" "5" !&$.)
*(56". *! +*2"*0&+*(
/),(&*$ +/ &((+/ +," Incremental
Delay
(b)
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"##& &"* 5 .+((7+## 2"./1/ #."-1"* 5 &)&(.(5 &* 0%"/" )+!"/
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,,(&"! 0 0%&/ ,+&*0 / #."-1"* 57!+)&* )1(0&,(& 0&+* /
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&*2"./" &/ ,".#+.)"!
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!+)&* 1/&*$ 0" %*&-1" (("! %.)+*& .","0&0&2" /)7
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/0", 0%.+1$% 0%" )"/1."! 32"#+.) 3&0% /," &#&"! 0&)"
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%"."#+." #&2" 5 ("/ +# 0%" &*,10 32"#+.) ." /'&,,"!
"03""* /),("/ +3"2". &# 0%" /),(" ,".&+! 3"." /"0 0+
"4 0(5 */ /),(&*$ 3+1(! + 1. 0 0%" /)" ,+&*0 +*
"2".5 #&#0% 5 (" *! *+ *"3 &*#+.)0&+* 3+1(! " $&*"!
+. 0&)" /,* +# */ *! 0%" *1)". +# 0. " ,+&*0/
Switchable
Low-Pass
Filters
CH
1
ADC
S
Synthesizer
(fs)
Memory
DSP
IF Step-Gain
Amplifiers
Trigger
Circuitry
Microprocessor
Switchable
Low-Pass
Filters
CH
2
ADC
S
IF Step-Gain
Amplifiers
Fig. 6. &),(&#&"! (+ ' !&$.) +# 0%" "5" !&$.) *(56".
1$1/0 "3("007 '.! +1.*(
Memory
DSP
Display
10 ns
51 ns
RF . . .
Input
51 ns
...
...
Input: Ideal 100-MHz Square Wave
fsignal = 100 MHz
tsignal = 10 ns
51 ns
51 ns
...
51 ns
51 ns
...
...
...
51 ns
51 ns
...
...
51 ns
...
ADC
Memory:
Instrument State:
Time Span = 10 ns
Number of Trace Points = 10
N=5
510 ns
T s + 1 + (N
fs
T +
T signal) ) T
Time Span
10 ns
+
+ 1 ns
10
Number of Trace Points
Therefore: Ts = 51 ns
#/3* 2- .-',21 2'+# 0#1-*32'-, -0 -$ ,1 '1 0#9
/3'0#" -0 2&'1 2'+# 0#1-*32'-, 2&# !23* 1+.*# .#0'-" '1
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54#$-0+ 2&# 1+.*',% .-',2 +-4#1 $-050" ,1 ," $2#0
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',.32 1'%,* 5-3*" # "'1.*7#"
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0#.#21 '% 1&-51 , #7#*',# "'%0+ -$ 2&# 1+# *1#0
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1+.*',% -$ 54#$-0+
20,1+'22#0 1 '% , 2&'1 +-"# 13!!#11'4# 1+.*#1 !-+#
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1#04# 5&#2&#0 ,-'1# ',2#017+ -* ',2#0$#0#,!# -0 +'1+2!&
0'..*# '1 !31',% #7# !*-130#
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1#2 2- 2&# !*-!) 02# "'4'"#" 7 2&# .22#0, *#,%2& ," 1 '1
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#6+.*# 1&-5, ', '% 2&# !*-!) $0#/3#,!7 '1 8 2&#
.22#0, *#,%2& '1 '21 ," $1'%,* '1 8 -0 2'+#
1., -$ ,1 ," 2&# ,3+ #0 -$ 20!# .-',21 #/3* 2- -$ ,1 '1 0#/3'0#" 1 !, # !-+.32#" 2- # ,1
15 ns
61 ns
Input: 1-Gbit/s, 15-Bit Pattern
fclock = 1 GHz
Pattern Length = 15 bits
fsignal = 66.67 MHz
Tsignal = 15 ns
Instrument State:
Time Span = 15 ns
Trace Length = 15
N=4
61 ns
T s + 1 + (N T signal) ) T
fs
Time Span
15 ns
+
+ 1 ns
T +
15
Trace Length
Therefore: Ts = 61 ns
Fig. 8. 0+-,'! 0#.#2'2'4#
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250 kHz
125 kHz
62.5 kHz
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13 dB
16 dB
19 dB
22 dB
1
16
32
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128
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512
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An applicationĆspecific user interface was designed for the
eye diagram analyzer. The goals of the user interface were to
offer turnkey measurements that meet the requirements of
the standards, to be easy to use, and to make the interface
to the pattern generator as transparent as possible. Many of
the specific measurements will be described in the next secĆ
tions. One easeĆofĆuse feature is the ability to set the time
base in bit unit intervals, instead of having to recall the clock
period to display the eye diagram. Time delay can also be
set in bit intervals, which makes it convenient to scroll
through the bits that make up the pattern. A menu key, READ
PAT GEN, is used to interrogate the pattern generator. It reĆ
turns the clock frequency, the pattern length, and the data,
clock, and trigger levels, which are used to set up the display.
Fig. 10. Eyeline diagram with software filtering applied and user
correction table corresponding to a fourthĆorder BesselĆThomson
response.
appropriate correction is applied at each frequency. Finally,
an inverse FFT is applied to the result to transform it back to
the time domain. These same processing routines are availĆ
able for userĆdefined corrections or filters. For example, this
capability can be used to calibrate the eyeline lightwave
measurements by correcting for any optical converter rollĆ
off. In addition, it can be used to evaluate eyeline diagrams
under different filter response conditions.
As previously mentioned, the transmission standards require
that the eye mask measurements be made in a receiver bandĆ
width that corresponds to a fourthĆorder BesselĆThomson
response with a reference frequency at three fourths of the
bit rate. Hardware filters or special lightwave converters are
often employed to achieve this. By using the user corrections
available in the HP 71501A, the ideal transfer function can
be implemented with a software filter. The filtered display
shown in Fig. 10 was made with a software filter applied
that corresponded to the desired ideal BesselĆThomson
response.
The eye diagram application program is written in HP
Instrument BASIC and is downloaded into the host instruĆ
ment, the HP 70820A microwave transition analyzer module.
The microwave transition analyzer is an extremely flexible,
generalĆpurpose instrument that offers both timeĆdomain and
frequencyĆdomain signal analysis capability.4 This versatile
measurement capability is available simultaneously with the
eye diagram measurement capability, and is simply
accessed through its own menu.
A number of parametric measurements are often performed
on eye diagrams to determine their quality. Some of the
more prevalent measurements include eye opening height,
eye opening width, amplitude of the crossing level, jitter at
the transition point, and the rise and fall times of the bit
transitions. Fig. 11 shows a number of these measurements
made with the HP 71501A eye diagram analyzer on a laser
transmitter operating at 622.08 Mbits/s. The extinction ratio
was determined by taking a vertical histogram of the eye
diagram windowed over one bit interval. An algorithm that
recursively adjusts the limits for subsequent evaluations of
the histogram data converges on the most prevalent logical
Up to 128 magnitude and phase points can be loaded as
user corrections. Shown in the lower half of Fig. 10 is a porĆ
tion of the user correction table that corresponds to the
fourthĆorder BesselĆThomson response. This table of magniĆ
tude and phase corrections was generated directly from trace
math functions that are available in the HP 71501A. The linĆ
ear portion of the phase has been corrected to remove the
delay. This allows the trace to remain in the same place
when the corrections are applied. Software filters for the
major SONET/SDH transmission rates are stored on the ROM
memory card. This capability could also be used to develop
an equalizer or tailor a specific response to improve the reĆ
ceived eye diagram. Thus, the transmitted equalized" eye
diagram could be simulated and observed before the final
hardware equalization filter is constructed.
Fig. 11. Table of eye diagram parametric measurements.
August 1994 HewlettĆPackard Journal
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