HFBR-0536 Datasheet HFBR-0536, HFBR-0537, HFBR-0536Z | P1-P3

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.

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 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.

Inexpensive DC to 32 MBd Fiberoptic Solutions for

Industrial, Medical,Telecom, and Proprietary Data

Communication Applications

Application Note 1121

Figure 1. Relationship Between PWD and Sampling Rate

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 2. Attributes of Encoding

SERIAL DATA

SOURCE

32 M BITS/SEC

MANCHESTER

ENCODER

(50% EFFICIENT)

64 MBd

ENCODED DATA

= 32 MHz

32 MBd

NRZ DATA

= 16 MHz

4B5B

ENCODER

(80% EFFICIENT)

40 MBd

ENCODED DATA

= 20 MHz

8B10B

ENCODER

(80% EFFICIENT)

40 MBd

ENCODED DATA

= 20 MHz

)-1

SCRAMBLER

(100% EFFICIENT)

32 MBd

ENCODED DATA

= 16 MHz

50% D.F.

40% TO 60% D.F.

50% D.F.

APPROXIMATELY 50% D.F.

0% TO 100% DUTY FACTOR (D.F.)

NOTE THAT F

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

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

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.

P1-P3 P4-P6 P7-P9 P10-P12

HFBR-0536

Mfr. #:

Buy HFBR-0536

Manufacturer:

Broadcom / Avago

Description:

Fiber Optic Development Tools 32MBd Data Comm Evaluation Kit

Lifecycle:

New from this manufacturer.

Delivery:

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HFBR-0536

HFBR-0536

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