Figure 123. Use of oscillator and transmission measuring set.

4-29.

SUMMARY

g. To accomplish this, 1 mw is designated

Here are the most important points covered

as being equal to 0 dbm.

in this information sheet:

h. This 0 dbm is then used as a reference

a. The db is a transmission measuring unit

level and all losses and gains are compared to 0

used to express power loss and gain in a telephone

dbm. This means that a level of -3 dbm is 3 db

system.

below the reference level. And, since a 3 db loss

represents a 50 percent power loss, a level of -3 dbm

b. A minus sign placed before db (-3 db)

is equal to .5 mw. Similarly, a level of +3 dbm is

indicates a power loss.

equal to 2 mw since a gain of 3 db doubles the

power.

c. A plus sign placed before db (+3 db)

indicates a power gain.

i. The frequency used with the reference

level of dbm is 1,000 Hertz (1 kHz).

d. Db losses and gains in a circuit are

added algebraically.

j. This, then, is the standard frequency and

testing power used in telephone work: 1 kHz at 0

e. The db by itself indicates an amount of

dbm.

power loss or gain but it does not tell exactly how

much power is involved.

k. In transmission measurement 1 kHz at 0

dbm is supplied by an oscillator. And a decibel

f. A reference level of 1 mw of power is

meter is used to measure the amount of db loss or

used with the db so that the db can express a definite

gain at many points along the circuit.

amount of power.