ple of how to use the bel to compute the ratio of power received to power transmitted, let's assume that received power was one- hundredth of the transmitted power: Because log -loglo obtain B = -log '10 100 = - 2 Understanding Light Note that a negative value indicates a power loss while a positive value would indicate a power gain. Although the bel was used for many years to categorize the quality of transmission on a circuit, industry required a more precise measurement. This resulted in the adoption of the deci- bel (dB) as a preferred power measurement. Decibel The decibel (dB) represents the standard method used today to denote power gains and losses. It is a more precise measurement as it represents one-tenth of a bel. The power measurement in decibels is computed as follows: where dB is the power ratio in decibels, PO is the output or received power, and P1 is the input or transmitted power. Returning to our previ- ous example where the received power was measured to be one-hun- dredth of the transmitted power, the power ratio in decibels becomes dB=10em(1007) Because log 10 1/X===10910 X, we obtain dB = -loglo 100 = 20 DECIBELS ABOVE 1 mW Note that the terms bel (B) and decibel (dB) do not denote power values. Instead, they represent a ratio or comparison between two power values, such as input and output power. Because it is often desirable to express power levels with respect to a fixed reference, it is common to use a one-milliwatt (mW) standard input for comparison purposes. In the wonderful world of communications testing of copper cable in North America, 1-mW signal occurs at a frequency of 800 Hz. To remind ourselves that the resulting power measurement occurred with respect to a 1-mW input signal, we use the term decibel-milliwatt (dBm) Chapter Two output power dBm = 1-mW input and serves to remind us that the output power measurement occurred with respect to a 1-mW test tone. Although the dBm is referred to as decibel-milliwatt in most literature, it actually means one decibel above one milliwat