c. Seeing how formulas for db loss and

a. Example 1. Suppose you have a circuit

gain are solved has given you an idea of how db's

like that shown in figure 120 input power is .001

express the power ratio in a circuit. There is no need

watt and the output power is .000001 watt. The

to go into any further computations. Instead the

input power is greater than the output power so you

formula have been worked out for several common

have a power loss. To find the power loss, you

db losses and gains.

The results of these

divide .001 by .000001 which equals 1,000. In

computations are given in Table IV.

Table IV this corresponds to 30 db. So your answer

is: -30 db.

b. Example 2. Suppose you know the db

gain is 40 db. The ratio of the output power to the

input power is found by referring to Table IV where

40 db corresponds to a ratio of 10,000. If you know

the input power is .001 watt, -you can find the

output power by multiplication: .001 watt x 10,000 =

10 watts.

4-24.

THE DBM REPRESENTS A REFERENCE

LEVEL

a. Before explaining the dbm, let's first

find out what is meant by a reference level. The

simplest way to explain it is to use an everyday

example.

b. Suppose you had 0 in a savings

account at the beginning of a year. This 0 could

act as a reference level. You could actually plot

4-23.

USING TABLE IV

your savings account for the whole year on a graph

as shown in figure 121. You'd put 0 in the center

You can find out how much power is lost or

of the vertical axis. And then place plus signs above

gained if you know the number of db's lost or

0 and minus signs below 0. Then as the year

gained. Or you can find out the number of db's lost

went on, if you deposited in February, you'd plot

or gained when you know the amount of power lost

+ (above 0). Then, if in March you had to

or gained. To show you how to use Table IV, here

withdraw you'd plot this to show that you had -

are a few examples.

(below 0). And as the year progressed you'd

plot the other months as shown.

Figure 120. Circuit showing power loss.

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