2-2Power measurements for af circuits are usually indicated in terms of decibels (dB) or decibelsreferenced to 1 milliwatt (dBm). Because the actual calculation of decibel measurements is seldomrequired, the following explanation is somewhat simplified. Most test equipment is designed to measureand indicate decibels directly. This eliminates the need for you to perform complicated calculations.Nevertheless, a basic explanation of the decibel measurement system is necessary for you to understandthe significance of dB readings and amplifier-gain ratings that are expressed in decibels.THE DECIBEL SYSTEMThe basic unit of measurement in the system is not the decibel; it is the bel. The bel is a unit thatexpresses the logarithmic ratio between the input and the output of any given component, circuit, orsystem. It may be expressed in terms of voltage, current, or power. Most often, it is used to show the ratiobetween input and output power to figure gain. You can express the power gain of the amplifier (N) inbels by dividing the output (P_{1}) by the input (P_{2}) and taking the base 10 logarithm of the resultingquotient. The formula for determining this gain is:If an amplifier doubles the input power, the quotient of P_{1} to P_{2} will be 2. If you consult a logarithmtable, you will find that the base 10 logarithm of 2 is 0.3, making the power gain of the amplifier 0.3 bel.Q-1.What is the logarithmic ratio between the input and output of a given circuit called?Experience has shown that because the bel is a rather large unit, it is difficult to apply. A morepractical unit, and one that can be used more easily, is the decibel (1/10 bel). You can convert any figureexpressed in bels to decibels by multiplying that figure by 10 or simply by moving the decimal point oneplace to the right. Applying this rule, we find that the above ratio of 0.3 bel is equal to 3 decibels.The decibel (dB) cannot be used to represent actual power; only the ratio of one power compared toanother. To say that an amplifier has a 3 dB gain means that the output power is twice the input power.This gives no indication of the actual power represented. You must be able to state the input power for itto be meaningful. In many applications, a mathematical expression represents the actual power, not apower ratio. One standard reference is the dBm.The dBm is an abbreviation used to represent power levels above or below 1 milliwatt. NegativedBm (-dBm) represents power levels below 1 milliwatt, and positive dBm (+dBm) represents powerlevels above 1 milliwatt. In other words, a dBm value is a specific amount of power; 0 dBm is equal to 1milliwatt. Briefly stated, the amount of power in a given value of dBm is the power which results if 1milliwatt is amplified or attenuated by that dB value. For example, 40 dBm represents an actual powerlevel (watts or milliwatts) that is 40 dB above 1 milliwatt, whereas -10 dBm represents a power level thatis 10 dB below 1 milliwatt. The formula for finding dBm is a variation of the dB power formula:Q-2.What term is used to represent power levels above or below a 1-milliwatt reference?