c. By using 0 dBm as the reference level you can easily tell the net loss in the circuit. Even though the
power level changed several times along the circuit, the received power is at a level of -6 dBm. The letter m
after dB tells us that this is 6 dB below the input level of 0 dBm. So we can say that, regardless of how much
loss or gain has taken place, the circuit net loss is only 6 dB. If this seems a little hard to understand, think back
to the money example. Remember how, at the end of the year, you had saved above the reference level of
0. You called this a net gain for the whole year.
11. The standard frequency for dBm.
a. You know now that 0 dBm represents 1 mW of power. And this 1 mW, in turn, represents the voice
power. Now, since voice power is made up of AC voltage and current, you must also be concerned with
frequency. This means that when you state the input level to a circuit is 0 dBm, you must also specify the
frequency.
b. The standard frequency used for testing in telephone work is 1,000 Hertz (1 kHz). When you make dB
loss and gain measurements, you will use a device that feeds a frequency of 1 kHz at a level of 0 dBm to the
line input. Then, at various points along the line, another device will be used to measure the dB loss or gain.
This device will not only tell how much loss or gain there is, but it will also indicate the level in dBm.
c. These devices are used as shown in Figure 3-14. The one that supplies the power is called an oscillator.
And the one that measures the power is called a decibel meter.
Figure 3-14. Use of oscillator and transmission measuring set.
12. Summary. Here are the most important points covered in measuring power losses and gains:
a. The dB is a transmission measuring unit used to express power loss and gain in a telephone system.
b. A minus sign placed before dB (-3 dB) indicates a power loss.
c. A plus sign placed before dB (+3 dB) indicates a power gain.
d. Decibel (dB) losses and gains in a circuit are added algebraically.
SS0031
3-36
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