THE DECIBEL SYSTEM

2-3 You do not need to use the formula in most applications. The following shows conversions of dBm to mW: +20dBm = 100mW +10dBm = 10mW +7dBm = 5mW +6dBm = 4mW +4dBm = 2.5mW +3dBm = 2mW 0dBm = 1mW -3dBm = .5mW -10dBm = .1mW For a +10 dBm level, start with the 1 milliwatt reference and move the decimal point one place to the right (+10 dBm = 10 mW). Another 10 dB increment brings the power level to +20 dBm, thereby moving the decimal point another place to the right (+20 dBm = 100 mW). For a -10 dBm level, again start with 1 milliwatt, but this time move the decimal point one place to the left (-10 dBm = .1 mW). An additional 10 dB decrease results in another decimal point shift to the left (-20 dBm = .01 mW). For a 3 dB increase, you double the power. For a 3 dB decrease, you reduce the power by one-half (+3 dBm = 2 mW and -3 dBm = .5 mW). A +6 dBm level is an additional 3 dB change from +3 dBm. In this case, you just double the power level of the +3 dBm (+6 dBm = 4 mW). Q-3. What milliwatt value is equal to +6 dBm? The dB change can be made in either direction. For example, +7 dBm is a decrease from +10 dBm. Reducing the +10 dBm power by one-half, we have +7 dBm, or 5 mW. A +4 dBm power level is a 3 dB decrease from +7 dBm (+4 dBm - 2.5 mW). By using this simple method, you can quickly find any power level that corresponds to a given dBm. Some test instruments you will be using are calibrated in decibels and have a 1 milliwatt zero reference level. Figure 2-1 illustrates such an instrument. Notice that this is an ac voltmeter in which the upper scale of the meter indicates ac voltage and the lower scale indicates decibels. The zero power-level indicator on the decibel scale is located at, or near, center scale. If the power in the line being measured is more than the reference value, the meter will indicate a value to the right of the zero mark (+dB). If the power is less than the reference value, the meter will indicate a value to the left of the zero mark (-dB). Such meters are useful when recording measurements where a direct indication in decibels is desired. However, you must remember that this meter is still a voltmeter and that power measurements are not meaningful unless the circuit impedance is known. If you feel the need to review how to calculate power in ac circuits, refer to NEETS, Module 2.