e. The dB by itself indicates an amount of power loss or gain but it *does not tell exactly how much power *is

involved.

f. A reference level of a milliwatt of power is used with the decibel so that a definite amount of power can

be expressed.

g. To accomplish this, 1 mW is designated as being equal to 0 dBm.

h. This 0 dBm is then used as a reference level and losses and gains are compared to 0 dBm. This means

that a level of -3 dBm is 3 dB below the reference level. And, since a 3 dB loss represents a 50-percent power

loss, a level of -3 dBm is equal to .5 mW. Similarly, a level of +3 dBm is equal to 2 mW since a gain of 3 dB

doubles the power.

i. The frequency used with the reference level of 0 dBm is 1,000 Hertz (1 kHz).

j. This, then, is the standard frequency and testing power used in telephone work-1 kHz at 0 dBm.

k. In transmission measurement, 1 kHz at 0 dBm is supplied by an oscillator. A decibel meter is used to

measure the amount of dB loss or gain at many points along the circuit.