Fiber Optic Solutions for 125 MBd Data
Communication Applications at Copper
Wire Prices
Application Note 1066
Introduction
Fiber optic cables have historically been used when the
distance is too long, or the data rate is too high, for the
limited bandwidth of wire. Optical communication links
are also favored when the environment through which the
data will pass is electrically noisy, or when electromagnetic
radiation from wire cables is a concern. Optical fibers have
numerous technical advantages over conventional wire
alternatives, but the cost of fiber optic solutions has always
been higher until now.
Fundamental Advantages of Optical Communication
Non-conductive optical cables have none of the traditional
problems associated with wire. When using a fiber optic
solution, system designers do not need to be concerned
about environmental noise coupling into cables, or worry
about whether there is a termination resistor at the end of
the cable. Conflicts between systems with different refer-
ence potentials do not happen when using insulating fiber
optic media because optical cables do not have conductors
or shields that can be improperly grounded when the
cables are installed or maintained. The fiber optic receiver
is the only portion of the optical link which is sensitive to
noise, and it can easily be protected because it is contained
within the host system which is receiving the data. A simple
power supply filter is usually sufficient to protect the fiber
optic receiver from the host system’s electrical noise. Elec-
trostatic shielding can be applied to the receiver if the host
system is particularly noisy, but electrostatic shields are
not needed in most applications if the circuit techniques
recommended in this application note are used.
The Inherent Disadvantages of Wire
Systems which must communicate are often connected
to different reference potentials which are not necessarily
zero volts, or in other situations ground references that are
thought to be 0 V are electrically noisy. Metallic connections
between systems with different ground potentials can be
implemented by using the proper isolation and grounding
techniques, but if these techniques are not strictly adhered
to conductive cables will introduce conflicts between
systems operating at different ground potentials. Data
communication system designers must exercise caution to
ensure that conductive cables do not exceed radiated noise
limits established by the FCC, and cable installers need to
route wire cables away from other power conductors that
might couple electrical noise into the data by magnetic
induction. Conventional wire transmission lines must also
be terminated using a load resistor equal to the charac-
teristic impedance of the metallic cable. This termination
resistor must always be connected to the receiving end
of every wire cable to ensure that pulses are not reflected
back toward the data source causing interference with the
transmitted data.
A Fiber Optic Solution at Wire Prices
The traditional argument for using copper wire has always
been that fiber optic solutions cost more, but Avago Tech-
nologies’ Versatile Link components now enable system
designers to overcome cost barriers that have historically
prevented the use of fiber optic cables in short distance
applications. The HFBR-15X7Z LED transmitter and the
HFBR-25X6Z receiver can be used with large diameter
1 mm plastic or 200
µm
Hard Clad Silica (HCS
TM
) step index
fibers to build unusually low cost data communication
equipment. The fiber optic solution described in this
application note can transmit data at rates up to 125 MBd
for the same price as shielded twisted pair wire, but this
unusually low cost optical data link has none of the disad-
vantages that are inherent to wire cables.
HFBR-15X7Z/25X6Z Distance and Data Rate Capabilities
Various distances and data rates are possible when the Figure 1 shows the performance possible with 1 mm diam-
HFBR-15X7Z and HFBR-25X6Z components are used with eter plastic fiber. The HFBR-15X7Z/25X6Z components can
large-core step index fibers. At low data rates, the distances be used with standard 1 mm plastic cables to build 20 m
achievable are determined by the sensitivity of the receiver, links which are capable of transmitting data at a rate of 125
cable attenuation, and the amount of light which the LED MBd. When low loss plastic fiber is used, distances of 25 m
can launch into the fiber core. As data rate increases, fiber are possible at 125 MBd. As data rate decreases, the dis-
TYPICAL PERFORMANCE AT 25 C
bandwidth will begin to
TYPICAL PERFORMANCE AT 25 C
tance achievable with 1 mm fiber increases. Figure 1 shows
influence how long the optical
data link can be and how fast the data can be transmitted. that a distance of 100 m is typically possible at rates as low
RECOMMENDED OPERATING REGION
A plastic fiber with a 1 mm core diameter will couple more as 33 MBd when using low loss 1 mm plastic fiber.
RECOMMENDED OPERATING REGION
light from the LED than a composite fiber with a 200
µm
180
Composite fiber with a silica glass core and plastic cladding
180
diameter silica glass core and plastic cladding, but greater
140
can achieve greater distances than possible with an all
140
distances are achievable with the composite fiber since it
100
plastic fiber. Figure 2 shows what can be accomplished
100
has significantly lower attenuation than an all-plastic fiber.
80
when HFBR-15X7Z and HFBR-25X6Z components are
80
60
The distance data rate curves shown in Figures 1 and 2 are used with 200
µm
diameter hard clad silica (HCS) fiber.
60
provided to allow designers to quickly determine if HFBR-
40
Substantial increases in cable length are possible when
40
15X7Z and HFBR-25X6Z can be used with large-core optical using 200
µm
HCS
TM
fiber since it has a much lower opti-
fibers to meet their system requirements. Figure 1 shows cal attenuation than 1 mm plastic fiber. Figure 2 indicates
the distances and data rates that can be achieved with
20
125 MBd data rates are typically possible with 125 m
that
20
Avago’s 1 mm plastic fibers and Figure 2 shows what can lengths of 200
µm
HCS
TM
fiber when using the transceiver
be accomplished when using Avago’s 200
µm
hard clad recommended in this publication. Distances of 1 km can
silica fibers.
10
designers utilize the circuits recommended
101.0
If
typically be achieved at data rates as low as 20 MBd due to
1,000
100
100
50
nor-
75
much lower optical losses
METERS
HCS
TM
cable.
1.0
note, digital fiber optic links can
25
in this application
the
l
, LENGTH,
of 200
µm
l
, LENGTH,
data rates within
METERS
mally be implemented at distances and
the shaded portions of Figure 1 and Figure 2. The fiber
optic transceiver shown in this publication was optimized
for operation at 125 MBd. Greater distances can be achieved
at data rates less than 125 MBd by optimizing the trans-
mitter and receiver circuits for operation at lower speeds.
HCS is a registered trademark of OFS.
DATA RATE, SYMBOLS/SEC, MBd
DATA RATE, SYMBOLS/SEC, MBd
180
140
100
180
TYPICAL PERFORMANCE
AT 25 C
140
100
TYPICAL PERFORMANCE
AT 25 C
DATA RATE, SYMBOLS/SEC, MBd
DATA RATE, SYMBOLS/SEC, MBd
80
60
40
RECOMMENDED
OPERATING REGION
20
80
60
40
RECOMMENDED
OPERATING REGION
20
10
1.0
25
l, LENGTH, METERS
50
75
100
10
1.0
100
l, LENGTH, METERS
1,000
Figure 1. Distances and Data Rates Possible with 1 mm Plastic Fiber
Figure 2. Distances and Data Rates Possible with 200
µm
HCS Fiber
5439-2
5439-1
2
Advantages of Encoded Run Limited Data
Fiber optic transceivers are commonly used in systems
that use some form of encoding. When data is encoded the
original data bits are replaced with a different group of bits
known as a symbol.
Data is encoded to prevent the digital information from
remaining in one of the two possible logic states for an
indefinite period of time. When data is encoded, a char-
acteristic known as the “run limit” is established. If data is
not changing, the run limit defines how much time may
pass before the encoder inserts a transition from one logic
state to another. The run length, or run limit of the encoder,
is the number of symbol periods that are allowed to pass
before the encoder changes logic state. Encoders also
force the encoded data to have a 50% duty factor, or they
restrict the duty factor to a limited range, such as 40 to
60%. When data is encoded, the fiber optic receiver can
be AC coupled as shown in Figure 3. Without encoding,
the fiber optic receiver would need to detect DC levels to
determine the proper logic state during long periods of
inactivity, as when there is no change in the transmitted
data. AC-coupled fiber optic receivers tend to be lower in
cost, are much easier to design, and contain fewer compo-
nents than their DC-coupled counterparts.
The output of the HFBR-25X6Z should not be direct cou-
pled to the amplifier and comparator shown in Figure 3.
Direct coupling decreases the sensitivity of a digital fiber
optic receiver, since it allows low-frequency flicker noise
from transistor amplifiers to be presented to the receiver’s
comparator input. Any undesired signals coupled to the
comparator will reduce the signal-to-noise ratio at this
critical point in the circuit, and reduce the sensitivity of the
fiber optic receiver.
+5V
NOISY
SYSTEM
POWER
RECEIVER Vcc
Another problem associated with direct-coupled receivers
is the accumulation of DC offset. With direct coupling, the
receiver’s gain stages amplify the effects of undesirable off-
sets and voltage drifts due to temperature changes. These
amplified DC offsets will eventually be applied to the com-
parator and result in reduced sensitivity of the fiber optic
receiver. The DC offset at the comparator can be referred to
the optical input of the receiver by dividing by the receiver
gain. This division refers the DC offset at the comparator
to the receiver input where it appears as a change in
optical power that must be exceeded before the receiver
will switch logic states. Problems with DC drift can be
avoided by constructing the receiver as shown in Figure 3.
Encoding has other advantages. Encoding merges the data
and clock signals in a manner that allows a timing-recovery
circuit to reconstruct the clock at the receiver end of the
digital data link. This is essential because fiber optic links
can send data at such high rates that asynchronous timing-
recovery techniques, such as over-sampling, are not very
practical. Without encoding, the clock signal required to
synchronously detect the data would need to be sent via
a second fiber optic link. Separate transmission channels
for data and clock signals are usually avoided due to cost,
but problems with time skew between the data and clock
can also arise if separate fibers are used to transmit these
signals.
0V
RECEIVER
COMMON
POWER
SUPPLY
FILTER
LOGIC
COMPATIBLE
OUTPUTS
HFBR-25X6Z
LIMITING
AMPLIFIER
LOGIC
COMPARATOR
Figure 3. Fiber Optic Receiver Block Diagram
3
Characteristics of Encoders
A Manchester encoder replaces each bit with two symbols,
for instance, a logic “1” is replaced by a (“1”,“0”) symbol, and
a logic “0” is replaced by a (“0”, “1”) symbol. Manchester
code is not very efficient since it doubles the fundamental
frequency of the data by substituting two symbols for each
bit transmitted. Block substitution codes such as 4B5B
replace 4-bit groups of data with a 5-bit symbol. Another
popular block substitution code is 5B6B, which replaces
each group of five bits with a 6-bit symbol. Substitution
codes encode the data more efficiently. If a Manchester
code is used to transmit data at 100 Mbits/second the fiber
optic channel must be capable of passing 200 M symbols/
second. Baud (Bd) is expressed in units of symbols/second,
thus the Manchester encoder in this example requires a
serial data link that can work at 200 MBd. If the Manchester
encoder is replaced by a 4B5B encoder, the 100 M bit/
second data can be sent at a signaling rate of 125 MBd. In
binary transmission systems the maximum fundamental
frequency of the data is half the symbol rate expressed in
Bd. When a Manchester encoder is used to send 100 M bit/
second data, at a symbol rate of 200 MBd, the maximum
fundamental frequency of the data is 100 MHz. By using
a 4B5B encoder, the same 100 M bit/second data can be
transmitted at 125 MBd, at a maximum fundamental
frequency of 62.5 MHz.
The minimum fundamental frequency that the fiber optic
link must pass is determined by the encoding rule chosen.
The run limit of the encoder determines the maximum
number of symbol periods that the encoder will allow
TAXIchip is a registered trademark of Advanced Micro Devices, Inc.
before it forces a transition, thus the encoder’s run limit
determines the minimum fundamental frequency of
the encoded data. Manchester code will allow only two
symbol periods to pass without a transition. As many as
three symbol times without a transition will be allowed by
the 4B5B encoder used in the AMD TAXIchip
TM
.
Figure 4 illustrates the attributes of various encoding
techniques. Figure 4 shows that as encoder efficiency
improves the bandwidth needed in the fiber optic com-
munication channel is reduced, or conversely, for a fixed
communication channel bandwidth the number of bits/
second that can be transmitted will go up as encoder
efficiency improves.
Total Solution Cost: 125 Mbd Link Costs
The cost of a 125 MBd link consists of the cost of the data
transceiver, and the cost of the media (cable and connec-
tors). For the recommended +ECL transceiver discussed in
this application note, the material costs in low volume are
approximately $28.
The total material cost for a logic-to-light transceiver is
under $25 in moderate volume, which compares favorably
with the cost of a wire transceiver solution capable of 125
MBd performance over 100 m spans, but the big advan-
tage of this low cost fiber optic technology is its ability to
provide better data integrity than comparably priced wire
alternatives.
SERIAL DATA
SOURCE
100 M BITS/SEC.
0% TO 100% DUTY FACTOR (D.F.)
100 MBd
NRZ DATA
fo = 50 MHz
200 MBd
ENCODED DATA
fo = 100 MHz
MANCHESTER
ENCODER
(50% EFFICIENT)
50% D.F.
4B5B
ENCODER
(80% EFFICIENT)
40% TO 60% D.F.
125 MBd
ENCODED DATA
fo = 62.5 MHz
27-1
SCRAMBLER
(100% EFFICIENT)
≈
50% D.F.
100 MBd
ENCODED DATA
fo = 50 MHz
Figure 4. Attributes of Encoding
4
Cable Costs
The price per meter of HCS cable from Avago and OFS is
comparable to the cost of shielded twisted pair wire in
similar volumes. Connectors cost approximately a dollar,
similar to typical twisted pair RJ jack connectors for data
communications. Connector installation requires no epoxy
or polishing, and can be completed in less than a minute
per connector. Therefore the installed cost of HCS cable
is similar to the installed cost of wire links of comparable
performance.
For shorter distance links, pre-connectored plastic fiber
cable assemblies are available from Avago Technologies
Distributors at attractive prices. For example, a 1 m, duplex,
pre-connectored plastic fiber cable (i.e., part number HF-
BR-RMD010Z) has a suggested U.S. resale price of $5.70
in quantities of 500-999; a 10 m cable would cost $16.50.
Again, these costs compare favorably with the cost of data
grade wire cable assemblies at similar volumes.
The costs of the 125 MBd Versatile Link electronics, cable,
and connectors are all competitive with wire solutions.
However, wire solutions frequently incur additional costs
in use due to unanticipated trouble-shooting of electrical
interference due to poor terminations or adjacent sources
of electrical noise. The inherent electrical isolation of
optical fiber results in a more robust solution and lower
cost to the end user.
+ECL Logic-to-Light Transceiver Cost
Non-Avago
parts:
MC10H116FN ECL Line Receiver
3 Transistors
IC Voltage Regulator
Inductor
Assorted Capacitors
Assorted Resistors
Total non-Avago components
HFBR-1527 Transmitter
HFBR-2526 Receiver
Total optoelectronic components
Complete +ECL transceiver
function
Circuits Recommended for use with the HFBR-15X7Z and
HFBR-25X6Z
The HFBR-15X7Z/25X6Z components can be used in a
diverse range of applications. Not all applications can
be addressed with the circuits shown in this publication,
however, the transceiver recommendation which follows is
useful in a wide range of systems which transmit encoded
data at rates up to 125 MBd. If the design suggestions giv-
en in this publication do not meet your needs, you should
contact an application engineer. The Application Engineer-
ing Department of Avago Technologies’ Industrial Fiber
Products Division has numerous circuit recommendations
that will allow the HFBR-15X7Z/25X6Z to address your
specific needs.
Recommended Transmitter
The transmitter shown in Figure 5 is recommended for use
with 1 mm plastic fiber. The transmitter in Figure 5 applies
a forward current of 20 mA to the HFBR-15X7Z LED. If
200
µm
HCS
TM
fiber is to be used the LED forward current
must be increased to 60 mA and the drive circuit shown in
Figure 6 is recommended. The forward current applied to
the HFBR-15X7Z was chosen so that the LED will couple
the maximum amount of light into the core of the fiber
without overdriving the HFBR-25X6Z receiver when short
optical cables are used.
$2.15
$2.50
$0.98
$0.25
$3.89
$0.78
~$10.55
$6.75
$10.70
$17.45
$28.00
(Note 1)
(Note 2)
(Note 2)
Avago Parts:
(Note 3)
Notes:
1. Costs of non-Avago components are based on very low volume price
quotes. High volume prices are below $5.00 for all required non-
Avago components.
2. Avago Suggested Resale Price in USA, 1994, quantity = 500-999.
Contact your local authorized Avago Technologies Distributor or
Sales Office for additional price information.
3. $28 is a low volume (<1,000) cost estimate. High volume cost is
below $20 for the complete +ECL transceiver function.
5
PCB Design
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