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.
3-37
SS0031
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