HFBR-0527P Datasheet HFBR-0527H, HFBR-0527P | P1-P3

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.

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.

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.

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

) 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.

180

140

100

1.0 25

DATA RATE, SYMBOLS/SEC, MBd

100

, LENGTH, METERS

RECOMMENDED OPERATING REGION

TYPICAL PERFORMANCE AT 25 C

180

140

100

1.0 25

DATA RATE, SYMBOLS/SEC, MBd

100

l, LENGTH, METERS

5439-1

RECOMMENDED

OPERATING REGION

TYPICAL PERFORMANCE

AT 25 C

180

140

100

1.0 100

DATA RATE, SYMBOLS/SEC, MBd

1,000

, LENGTH, METERS

RECOMMENDED OPERATING REGION

TYPICAL PERFORMANCE AT 25 C

1.0 100

1,000

l, LENGTH, METERS

5439-2

180

140

100

DATA RATE, SYMBOLS/SEC, MBd

RECOMMENDED

OPERATING REGION

TYPICAL PERFORMANCE

AT 25 C

Figure 1. Distances and Data Rates Possible with 1 mm Plastic Fiber Figure 2. Distances and Data Rates Possible with 200 µm HCS Fiber

HCS is a registered trademark of OFS.

Figure 1 shows the performance possible with 1 mm diam-

eter plastic ﬁber. The HFBR-15X7Z/25X6Z components can

be used with standard 1 mm plastic cables to build 20 m

links which are capable of transmitting data at a rate of 125

MBd. When low loss plastic ﬁber is used, distances of 25 m

are possible at 125 MBd. As data rate decreases, the dis-

tance achievable with 1 mm ﬁber increases. Figure 1 shows

that a distance of 100 m is typically possible at rates as low

as 33 MBd when using low loss 1 mm plastic ﬁber.

Composite ﬁber with a silica glass core and plastic cladding

can achieve greater distances than possible with an all

plastic ﬁber. Figure 2 shows what can be accomplished

when HFBR-15X7Z and HFBR-25X6Z components are

used with 200 µm diameter hard clad silica (HCS) ﬁber.

Substantial increases in cable length are possible when

using 200 µm HCS

ﬁber since it has a much lower opti-

cal attenuation than 1 mm plastic ﬁber. Figure 2 indicates

that 125 MBd data rates are typically possible with 125 m

lengths of 200 µm HCS

ﬁber when using the transceiver

recommended in this publication. Distances of 1 km can

typically be achieved at data rates as low as 20 MBd due to

the much lower optical losses of 200 µm HCS

cable.

Various distances and data rates are possible when the

HFBR-15X7Z and HFBR-25X6Z components are used with

large-core step index ﬁbers. At low data rates, the distances

achievable are determined by the sensitivity of the receiver,

cable attenuation, and the amount of light which the LED

can launch into the ﬁber core. As data rate increases, ﬁber

bandwidth will begin to inﬂuence how long the optical

data link can be and how fast the data can be transmitted.

A plastic ﬁber with a 1 mm core diameter will couple more

light from the LED than a composite ﬁber with a 200 µm

diameter silica glass core and plastic cladding, but greater

distances are achievable with the composite ﬁber since it

has signiﬁcantly lower attenuation than an all-plastic ﬁber.

The distance data rate curves shown in Figures 1 and 2 are

provided to allow designers to quickly determine if HFBR-

15X7Z and HFBR-25X6Z can be used with large-core optical

ﬁbers to meet their system requirements. Figure 1 shows

the distances and data rates that can be achieved with

Avago’s 1 mm plastic ﬁbers and Figure 2 shows what can

be accomplished when using Avago’s 200 µm hard clad

silica ﬁbers. If designers utilize the circuits recommended

in this application note, digital ﬁber optic links can nor-

mally be implemented at distances and data rates within

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.

HFBR-15X7Z/25X6Z Distance and Data Rate Capabilities

Figure 3. Fiber Optic Receiver Block Diagram

NOISY

SYSTEM

POWER

RECEIVER

COMMON

+5V

POWER

SUPPLY

FILTER

HFBR-25X6Z

LIMITING

AMPLIFIER

LOGIC

COMPATIBLE

OUTPUTS

LOGIC

COMPARATOR

RECEIVER V

5439-3

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.

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.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P16

HFBR-0527P

Mfr. #:

Buy HFBR-0527P

Manufacturer:

Broadcom / Avago

Description:

KIT EVAL FIBER OPTIC 125MBD

Lifecycle:

New from this manufacturer.

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HFBR-0527P

HFBR-0527P

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