Communications Specialties Application Note

UNDERSTANDING THE CONCEPT

OF “dB” AS A MEASUREMENT

OF SYSTEM LOSS

MARKET: General Fiber Optics

DOC. NUMBER:T-22

Overview:

One of the specifications used when describing

a fiber optic transmission system is the

allowable “system loss”, measurable in a unit called

“dB” (decibels). In simple terms, this specification

describes the maximum percentage of signal

strength that can be lost from the point where a

signal is launched by a fiber optic transmitter until

the

signal

reaches

and

enters

the

fiber optic receiver. However, the concept of what

loss in “dB” represents and how it is calculated can

be a bit confusing. The following TECHnique is de-

signed to help demystify how a “dB” value is calcu-

lated and to clearly explain the relationship between

the dB number and the percentage of signal lost

during the transmission process.

sured at a strength of 100 IUs. When it reaches and

exits the receiver, it has a strength of 50 IUs. The

first step in calculating signal loss in dBs is to deter-

mine the ratio between the second number (50) and

the first one (100). In this case, that ratio is 50:100,

which equals 1:2, �½ or .5. In other words, the signal

that reaches the receiver is half as strong as when it

was launched along its transmission path.

Here’s where it may get confusing. Rather than

describe the loss in terms of this ratio, we now apply

a function to the ratio, and the resulting number is

what we use to describe the loss. The function we

apply to the ratio is:

Gain (dB) = 10 Log P out

P in

What does this really mean? First of all, the “Gain”

at the front of the equation indicates that this func-

tion is used not only to measure system loss, but

also to measure system gain – or an increase in

power. In some instances, such as when amplifiers

are used, a system may have a net gain instead of

loss. However, when the number we end up with is

negative, as it will be in our example, it means we

have a negative “gain” – or, in other words, a “loss.”

Now, let’s look at the function to the right of the

equal sign. As it applies to our example, “P out”

means the power or signal strength that exits

or comes “out” of the fiber. “P in” means the strength

of the signal that is put “in” to the fiber. So,

“P out/P in” really just refers to the ratio that we al-

ready calculated. In our example, it’s �½ or .5.

Substituting “.5” in place of “P out/P in”, we now

we have an equation that says:

Details:

As just described, system loss, measured in dB,

describes the ratio between the strength of signal

launched by a transmitter and the strength of that

signal as it exits the fiber at the receiver. Along the

path from transmitter to receiver there are various

factors that contribute to the diminishing of signal

strength. The type of fiber used, the type of connec-

tors used, the wavelength of the transmitted signal,

and several other factors all contribute to how much

or little of the original signal ultimately reaches and

can be processed by the system’s receiver. This

article does not address the level to which each of

these factors contributes to signal loss. For this dis-

cussion, it is sufficient to simply understand that

these types of factors contribute to a weakening of

signal strength as it passes through the system.

Now let’s get into the specifics of how loss, in dB, is

calculated.

Hypothetically, let’s pretend that we have a de-

vice that measures the strength or power of a sig-

nal as it is launched by a fiber optic transmitter in

“imaginary units” — “IUs” for short. We’ll use this

same device to measure signal strength as it

reaches the receiver.

In our first example, the launched signal is mea-

Gain (dB) = 10 Log .5

Finally, we have to apply the log function to the num-

ber .5, and then multiply it by 10. How do you calculate

the Log of .5? Get your hands on a cheap,

scientific calculator that has a “Log” function key on

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55 Cabot Court, Hauppauge, NY 11788

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Communications Specialties Application Note

Page 2: UNDERSTANDING THE CONCEPT OF “dB” AS A MEASUREMENT

OF SYSTEM LOSS

it. Just type in “.5”, hit the Log key, and you get -.301. (It would be virtually

impossible to figure this out without the help of a calculator.) Multiply -.310 by

10 and you end up with -3.1. Our ratio of “1/2” translates to -3.1 dB. Our

system loss is therefore –3.1 dB.

Equation From Previous Page:

Gain (dB) = 10 Log .5

Using the same math process, the following table shows what the loss would be in dB as a function of

the ratio between original launched signal strength and signal strength reaching the receiver.

A:. Stregth of Signal

B: Strength of Signal

at Transmission (in IUs) Reaching Receiver (in IUs)

100

What’s important to understand from looking at

this table is the relationship between the numbers

in the third and fourth columns. As the ratio in the

third column decreases – meaning less and less

signal is reaching the receiver – the dB number also

decreases. The dB value is another way of express-

ing this ratio.

When looking at the specifications for fiber optic

transmission systems, a range or loss budget, in

dBs, is usually given for the maximum allowable

transmission loss. For some systems, the allow-

able loss may be as low as 0 to -3 dB. For other

systems, the number may be as great as -30 dB. It

all depends on the specific system, the components

being used, the type and number of signals being

transmitted, the type of fiber, and many other con-

siderations. However, after reading this article, you

should now understand the basics of what that num-

ber means. For example, a maximum system loss

of -6.99 dB means that the signal reaching the sys-

tem receiver must be at least 1/5th (20/100s) as

strong as the original, launched signal. (See above

chart.) If it becomes any weaker than that, the sys-

tem will not be able to function.

IU = Imaginary Units

Ratio of A:B

.05

.03

.02

.01

Loss in dB

-3.01

-3.98

-5.23

-6.99

-10.00

-13.01

-15.23

-16.99

-20.00

As explained at the beginning of this TECHnique,

there are many factors that contribute toward sys-

tem loss. Refer to

TECHnique 21, Calculating Maxi-

mum Transmission Distance for Fiber Optic Sys-

tems,

for an introduction to these issues. Also, it

should be noted that the term dB is not limited to

use in measuring system loss for fiber optic sys-

tems. It is used throughout the worlds of engineer-

ing and electronics to measure the loss (or gain) of

almost any type of signal.

CSI Products Used in this TECHnique:

All Fiberlink and Pure Digital

Fiberlink®

transmission modules

Related TECHniques

•

TECHnique 21, Calculating Maximum Transmis-

sion Distance for Fiber Optic Systems

•

Education Guide:

Introduction to Fiber Optics

•

Educational Guide:

Introduction to Fiber Optic

Cables and Connectors

•