Inexpensive dc to 32 MBd Fiberoptic Solutions for
Industrial, Medical,Telecom, and Proprietary data
communication Applications
Application Note 1121
Introduction
Low-cost fiberoptic data-communication links have
been used to replace copper wire in numerous industrial,
medical, and proprietary applications. The fiberoptic
transmitter and receiver circuits in this publication ad-
dress a wide range of applications. These recommended
circuits are compatible with unencoded or burst-mode
communication protocols originally developed for use
with copper wire. Complete TTL compatible digital
transceiver solutions, including the schematic, printed
circuit artwork, and material lists, are presented in this
application note, so that users of this low-cost fiberoptic
technology do not need to do any analog design.
Designers are encouraged to embed these complete
fiberoptic solutions into their products and various
methods for electronically downloading the reference
designs are described.
The second reason is that when using plastic optical
fiber (POF), or hard clad silica (HCS) fiber, the total cost
of the data communication link is roughly the same as
when using copper wires.
Wire Communication Protocols and Optical Data Links
Many existing serial wire communication protocols
were developed for differential line receivers or opto-
couplers that can sense the DC component of the data
communication signal. This type of serial data is often
called arbitrary duty factor data because it can remain
in the logic “1” or logic “0” state for indefinite periods of
time. Arbitrary duty factor data has an average value,
which can instantaneously be anywhere between 0 per-
cent and 100 percent of the binary signal’s amplitude,
or in other words, arbitrary duty factor data contains
DC components. Communication protocols that were
developed specifically for use with copper wire often
require an optical receiver that is DC coupled or capable
of detecting if the data is changing from a high-to-low
or low-to-high logic state. That is, the receiver needs
to be an edge detector. At relatively modest data rates
between zero and 10-Mbits/sec it is possible to con-
struct DC coupled TTL-compatible fiberoptic receivers.
The Avago Technologies HFBR-2521Z is a TTL-compat-
ible, DC to 5-Mbit/sec receiver, and the HFBR-2528Z is
a DC to 10-Mbit/sec CMOS or TTL-compatible receiver.
Additional information about DC to 5-Mbit/ sec applica-
tions can be found in Avago Technologies AN-1035, and
applications support for DC to 10-Mbit/sec applications
can be obtained by reading AN-1080. This application
note will focus on higher speed or higher performance
arbitrary duty factor optical data communication links
that work at higher data rates or greater distances than
achievable with the HFBR-2521Z or HFBR- 2528Z com-
ponents. The optical transceivers shown in this applica-
tion note can also be used in burst-mode applications
where the data is transmitted in packets and there are
no transitions between bursts of date.
Why Use Optical Fibers?
Copper wire is an established technology that has been
successfully used to transmit data in a wide range of
industrial, medical and proprietary applications, but
copper can be difficult or impossible to be used in nu-
merous situations. By using differential line receivers,
optocouplers, or transformers conventional copper
wire cables can be used to transmit data in applica-
tions where the reference or ground potentials of two
systems are different, but during and after the initial
installation great care must still be taken not to corrupt
the data with noise induced into the cable’s metallic
shields by adjacent power lines or differences in ground
potential. Unlike copper wires, optical fibers do not
require rigorous grounding rules to avoid ground loop
interference, and fiberoptic cables do not need termi-
nation resistors to avoid reflections. Optical transceivers
and cables can be designed into systems so that they
survive lightning strikes that would normally damage
metallic conductors or wire input/output (I/O) cards;
in essence, fiberoptic data links are used in electrically
noisy environments where copper wire fails. In addition
to all of these inherent advantages there are two other
reasons why optical fibers are beginning to replace
copper wires. The first reason is that training and simple
tools are now available.
The Pros and Cons of Arbitrary Duty Factor or Burst Mode Data
The most important advantage of any existing data
communication protocol is that it already exists, and
typically works reasonably well with copper wires in
many applications. On the other hand, existing pro-
tocols for copper wire are usually not the best choice
for optimizing the performance of a fiberoptic link. For
example, a receiver designed for use with arbitrary duty
factor data, or burst mode data, will typically be 4 dB to
7 dB less sensitive than when the same components are
used in receiver circuits optimized for use with encoded
data. Encoded data normally has a 50 percent duty fac-
tor, or restricted duty factor variation, which allows the
construction of higher-sensitivity fiberoptic receivers.
The best arbitrary duty factor or burst-mode receivers
described in this application note are considerably less
sensitive than the encoded data receivers described in
AN-1122.
When sending arbitrary duty factor data, a separate op-
tical link must be used to send the clock if synchronous
serial communication is desired, or an asynchronous
data communication system can be implemented if the
data is oversampled by a local clock oscillator located
at the receiving end of the fiberoptic data link. To avoid
excessive pulsewidth distortion (PWD), the local oscilla-
tor used to oversample the received data must operate
at frequency that is greater than the serial data rate. For
instance, if the data rate is 32-Mbits/sec, a clock frequen-
cy of 100 MHz will assure three times oversampling of
the received serial data. As the sampling rate decreases,
the PWD of the reclocked data increases. Conversely,
when the sampling rate is increased, the PWD of the
asynchronous data link decreases. At modest data rates
such as 32-Mbits/sec the frequency of the local clock
oscillator will rise sharply if higher oversampling rates
are attempted; for instance, to guarantee five times
oversampling the clock oscillator at the receiver would
need to operate at a frequency slightly greater than 160
MHz. Refer to Figure 1 for a graphical representation of
the relationship between the sampling rate and PWD of
an asynchronous serial data communication link.
The 10Base-T copper standard sends no transitions
between packets of Ethernet data, but the 10Base-FL
standard for optical fiber media inserts a 1 MHz square
wave between each packet of Ethernet traffic.
Figure 1. Relationship Between PWd and Sampling Rate
2
SERIAL DATA
SOURCE
32 M BITS/SEC
0% TO 100% DUTY FACTOR (D.F.)
32 MBd
NRZ DATA
fo = 16 MHz
MANCHESTER
ENCODER
(50% EFFICIENT)
50% D.F.
64 MBd
ENCODED DATA
fo= 32 MHz
4B5B
ENCODER
(80% EFFICIENT)
40% TO 60% D.F.
40 MBd
ENCODED DATA
fo = 20 MHz
40 MBd
ENCODED DATA
fo= 20 MHz
32 MBd
ENCODED DATA
fo = 16 MHz
8B10B
ENCODER
(80% EFFICIENT)
50% D.F.
(27)-1
SCRAMBLER
(100% EFFICIENT)
APPROXIMATELY 50% D.F.
NOTE THAT Fo IS THE MAXIMUM FUNDAMENTAL FREQUENCY OF THE ENCODED DATA.
THE MINMUM FUNDAMENTAL FREQUENCY OF THE ENCODED DATA IS DETERMINED BY THE
ENCODER'S RUN LIMIT
Figure 2. Attributes of Encoding
Burst-mode serial communication systems also have
some interesting characteristics. They usually require
more communication channel bandwidth, since the
most common burst-mode protocols normally use a
Manchester encoder, which transmits more than one
symbol for each bit. Figure 2 shows how the commu-
nication channel’s bandwidth must increase when the
Manchester code normally used in Ethernet data com-
munication systems is applied to unencoded serial data.
The big advantage of encoding is that it merges the
clock and data so that only one communication channel
is needed for both signals. In most high-performance fi-
beroptic communication systems, the data and clock are
merged onto a single serial channel using a method that
has better efficiency than Manchester encoding. Figure
2 shows several common encoding methods with better
efficiency than Manchester code. Other important re-
lationships between bits/second, and symbols/second,
expressed in Baud (Bd) are explained by Figure 2. Note
that arbitrary duty factor unencoded data is one of the
few instances when data rate in bits/second, and the
symbol rate in Bd are equal. Relationships between the
signaling rate expressed in Baud and the fundamental
frequency of digital data communication signals are
also shown in Figure 2.
Burst-mode communication protocols are used in
popular serial communication systems such as Ethernet,
or Arcnet. Burst-mode protocols allow many network
users to share a common pair of copper conductors
with a tapped connection for each user network inter-
face. The key disadvantages of this simple tapped line
3
architecture is that only one user can send data at any
time, and a preamble must be sent to wake up or initial-
ize the receiving node’s timing recovery circuit at the
beginning of each packet of burstmode data. Burst-
mode, shared-wire communication links are not par-
ticularly fast, because no data can be transmitted during
the preamble and each node must wait until the tapped
line is quiet before data can be transmitted. Burst-mode
protocols are not necessarily the best choice for optical
communication links, because optical fibers are not
easily and inexpensively tapped. When Ethernet traffic is
sent via optical fibers, the wiring architecture is changed
from a tapped serial transmission line to hubs that
contain active fiberoptic transmitters and receivers. The
active hubs are then connected to one another in a “star”
configuration, because this star architecture is compat-
ible with existing low-cost fiberoptic transceiver and ca-
bling technologies. Fiberoptic receivers can be designed
to accommodate burst-mode data, but it is much easier
to build highsensitivity fiberoptic receivers when data
is sent continuously. Continuous transmission also has
other advantages. Continuous transmission increases
the throughput of the LAN since there is no dead-time
between packets of data. Throughput is substantially
improved when data is continuously transmitted, be-
cause no time is wasted sending preambles of sufficient
length to allow the receiver’s timing-recovery circuit to
acquire the phase lock required to synchronously detect
each serial data packet. It is interesting to note that the
IEEE 802.3 10Base-FL standard for fiberoptic media uses
a different transmission.
The 1 MHz idle signal described in the IEEE 802.3
10Base-FL standard assures that the burst-mode proto-
col used for copper wire Ethernet is converted to a pro-
tocol that will optimize the performance of a fiberoptic
receiver. More details about inexpensive fiberoptic
solutions suitable for use with higher-efficiency block
substitution codes, such as 4B5B, and 8B10B, can be
found in Avago Technologies Application Notes 1122
and 1123. This publication will stay focused on solutions
compatible with unencoded data, because many system
designers need a fiberoptic solution that can use proto-
cols originally developed for use with copper wires.
Distances and Data Rates Achievable
The simple transceivers recommended in this applica-
tion note can be used to address a very wide range
of distances, data rates, and system cost targets. The
maximum distances allowed with various types of
optical fiber when using Avago Technologies’ wide
range of fiberoptic transceiver components are shown
Table 1. One simple calculation is needed to optimize
the receiver for use at the desired maximum symbol rate
of your system application. No transmitter or receiver
adjustments are needed when using fiber cable length
that vary from virtually zero length up to the maximum
distances specified in Table 1.
Table 1
Transmitter
Component Part #
and Wavelength
HFBR-15X7Z
650 nm LED
HFBR-15X7Z
650 nm LED
HFBR-15X7Z
650 nm LED
HFBR-15X7Z
650 nm LED
HFBR-14X2Z
820 nm LED
HFBR-14X2Z
820 nm LED
HFBR-14X4Z
820 nm LED
HFBR-14X4Z
820 nm LED
HFBR-13X2TZ
1300 nm LED
HFBR-13X2TZ
1300 nm LED
Receiver
Component Part #
and Wavelength
HFBR-25X6Z
650 nm
HFBR-25X6Z
650 nm
HFBR-25X6Z
650 nm
HFBR-25X6Z
650 nm
HFBR-24X6Z
820 nm
HFBR-24X6Z
820 nm
HFBR-24X6Z
820 nm
HFBR-24X6Z
820 nm
HFBR-23X6TZ
1300 nm
HFBR-23X6TZ
1300 nm
Fiber Diameter
Type
1 mm plastic
step index
1 mm plastic
step index
200
mm
HCS step index
200
mm
HCS step index
200
mm
HCS step index
200
mm
HCS step index
62.5/125
mm
multimode glass
62.5/125
mm
multimode glass
62.5/125
mm
multimode glass
62.5/125
mm
multimode glass
Maximum Distance at 32 MBd
with the transceiver circuits
recommended in this publication
27 meters with transmitter in Fig. 3
and receiver in Fig. 4
42 meters with transmitter in Fig. 3
and receiver in Fig. 5
690 meters with transmitter in Fig. 3
and receiver in Fig. 4
1.0 kilometer with transmitter
in Fig. 3 and receiver in Fig. 5
690 meters with transmitter in Fig. 3
and receiver in Fig. 4
1.0 kilometer with transmitter
in Fig. 3 and receiver in Fig. 5
800 meters with transmitter in Fig. 3
and receiver in Fig. 4
1.6 kilometers with transmitter
in Fig. 3 and receiver in Fig. 5
1.3 kilometers with transmitter
in Fig 3. and receiver in Fig. 4
3.3 kilometers with transmitter
in Fig. 3 and receiver in Fig. 5
4
+5 V
HOST
SYSTEM POWER
L1
TDK
#HF30ACB453215
C1
0.1
mF
U1C
Vcc 14 74ACTQ00
9
8
U1D
74ACTQ00
12
11
TTL IN
13
7 GND
4
5
U1A
74ACTQ00
3
10
U1B
74ACTQ00
6
R3
R1
C3
R2
C2
10
mF
1
2
8
U2A
3
HFBR-15X7Z
4
5
1
2
2
6
7
3
1
8
U2B
HFBR-14X4Z
4
5
Figure 3. TTL-compatible LEd Transmitter
Table 2
Transmitter
Fiber Type
R1
R2
R3
C3
HFBR-15X7Z
650 nm LED
1 mm Plastic
120
W
120
W
390
W
82 pF
200
mm
HCS
33
W
33
W
270
W
470 pF
HFBR-14X4Z
820 nm LED
62.5/125
mm
33
W
33
W
270
W
75 pF
HFBR-13X2TZ
1300 nm LED
62.5/125
mm
22
W
27
W
∞
150 pF
Simple TTL Compatible LED Transmitter
A high-performance, low-cost TTL-compatible trans-
mitter is shown in Figure 3. This transmitter recom-
mendation is deceptively simple, but has been highly
developed to deliver the best performance achievable
with Avago Technologies’ LED transmitters. The recom-
mended transmitter is also very inexpensive, because
the 74ACTQ00 gate used to current modulate the LED
can typically be obtained for under $0.40. No calcula-
tions are required to determine the passive component
needed when using various Avago Technologies’ LEDs
with a wide range of optical fibers. Simply use the
5
recommended component values shown in Table 2,
and the transmitter shown in Figure 3 can be used to
address a broad range of applications.
Simple TTL Compatible Receiver
A very simple TTL-compatible receiver that has ad-
equate sensitivity for a wide range of applications is
shown in Figure 4. Equation 1 allows the designer to
quickly determine the values of C6 and C7 so that the
receiver is optimized for operation at any data rate up to
a maximum of 32 MBd.
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