P a g e 1 Handout 1 EE442 Spring Semester The Definition of 'db' and 'dbm' A decibel (db) in electrical engineering is defined as 10 times the base-10 logarithm of a ratio between two power levels; e.g., P out/p in (power gain, in other words): N db = 10 * log 10 (P1/P2) All gains greater than 1 are therefore expressed as positive decibels (>0), and gains of less than 1 are expressed as negative decibels (<0). Note that for cases most of us encounter, the linear ratio of P1/P2 must be a positive number (>0) since the logarithm of 0 is undefined and the logarithm of negatives numbers are complex (they contain both a real and an imaginary part for which we have no meaning for power quantities). The db value, though, can theoretically take on any value between and +, including 0, which is a gain of 1 [10 * log (1) = 0 db]. 'dbm' is a decibel-based unit of power referenced to 1 mw. Since 0 db of gain is equal to a gain of 1, 1 mw of power is 0 db greater than 1 mw, or 0 dbm. Similarly, a power unit of dbw is decibels relative to 1 W of power. 1 mw = 0 dbm Accordingly, all dbm values greater than 0 are larger than 1 mw, and all dbm values less than 0 are smaller than 1 mw (see Fig. 1). For instance, +3.01 dbm is 3.01 db greater than 1 mw; i.e., or 0 dbm + 3.01 db = +3.01 dbm (2 mw). 3.01 dbm is 3.01 db less than 1 mw; i.e., or 0 dbm + ( 3.01) db = 3.01 dbm (0.5 mw). The following table gives some numerical examples, so you can see the correlation between mw and dbm. The same set of values plotted on a logarithmic scale would produce a straight line. Because of the logarithmic relationship, the graph bunches the smaller values against the left vertical axis. A magnified version of the 0 to 1 mw region is inset for clarity.
P a g e 2 Fig. 1 - Graph of Power in Units of dbm vs. mw Fig. 2 is a table and graph of db vs. linear gain ratios like the dbm vs. mw in Fig. 1. Note that the numbers and curves are the same; only the axis labels are changed. That is because dbm is a unit of power expressed in db relative to 1 mw (0 dbm). Fig. 2 - Graph of Gain in Units of dbm vs. Linear Ratio
P a g e 3 Linear Gain (output/input ratio) vs. Logarithmic (decibels, db) Gain Fundamentally, gain is a multiplication (or division) factor. As an example, an amplifier might have a gain that increases the signal by a factor of 4 (i.e., 4x) from input to output (see Fig. 3). If a 1 mw (0 dbm) signal is fed into the amplifier, then 1 mw * 4 = 4 mw comes out. In terms of decibels, a factor of 4 is equivalent to 10 * log (4) = 6.02 db, so 0 dbm in plus 6.02 db of gain yields +6.02 dbm at the output. 1 mw * 4 = 4 mw 0 dbm + 6.02 db = 6.02 dbm Fig. 3 - Single amplifier gain. Combining Gains (linear and db) w/positive Values If an amplifier with a gain of 4 is in series with a second amplifier with a gain of 6, then the total gain is 4 * 6 = 24. In terms of decibels, a factor of 6 is equivalent to 10 * log (6) = 7.78 db, and a factor of 24 is equivalent to 10 * log (24) = 13.8 db. Just as 4 x 6 = 24 (linear gain), 6.02 db + 7.78 db = 13.8 db (decibel gain). If a 1 mw signal (0 dbm) is fed into the amplifier, then 4 mw comes out of the first amplifier, and 24 mw comes out of the second amplifier. See Fig. 4. 1 mw * 4 * 6 = 24 mw 0 dbm + 6.02 db + 7.78 db = 13.8 dbm Fig. 4 - Cascaded dual amplifier gain. Combining Gain and Loss (linear and db) This next example shows what happens when a gain < 1 (a loss) is encountered, where an attenuator with a gain of 1/6 is placed after the first amplifier instead of having a second amplifier. See Fig. 5.
P a g e 4 Thus, 4 * 1/6 = 2/3 (linear gain). Similarly, 6.02 db - 7.78 db = 1.76 db (decibel gain). As with the previous example, if a 1 mw signal (0 dbm) is fed into the amplifier with a gain of 4, then 4 mw comes out. That 4 mw then goes into the attenuator with a linear gain of 1/6 and comes out at a power level of 4/6 mw (2/3 mw). The total gain in this case is 4/6 = 2/3, so the output power will be less than the input power. 1 mw * 4 * 1/6 = 2/3 mw = 0.67 mw 0 dbm + 6.02 db + (-7.78 db) = 1.76 dbm Fig. 5 - Cascaded amplifier gain and attenuator. Note that power levels greater than 0 dbm sometimes include the 'plus' sign (+) to emphasize that it is not negative. This is particularly so when power levels are displayed on a block diagram where both positive and negative values are present. Summary When making power measurements in the laboratory or in the field, most people find it easier to add and subtract gains and power levels than to multiply and divide gains and power levels. db and dbm units make that possible. The important thing to remember is to never mix linear gain (ratio) units and wattage power (mw) units with logarithmic gain (db) and power (dbm) units. Quantities must be either in all linear or all decibel units. The following type of calculation is NOT Allowed because it mixes linear values and logarithmic values: 12 mw + 34 mw + 8 mw + 20 db Reference: http://www.rfcafe.com/references/electrical/decibel-tutorial.htm
P a g e 5 How to Calculate Decibels Decibels are calculated using the following formulas: (power uses factor 10 and voltage or current uses a factor of 20) db power 10 log, and reference power = 10 voltage current db = 20 log10 = 20 log10 reference voltage reference current Why is the logarithm of voltage and current ratios multiplied by 20 instead of 10? First, decibels are always about power ratios, so don t think there is a voltage db and a current db that is different from a power db. A db is a db is a db. Using the equations P = V 2 /R and P = I 2 R to substitute for the power values, you ll see that the ratios inside the parentheses of the decibel equation become V 2 /Vref 2 and I 2 / Iref 2. Logarithms treat exponents specially: namely, log (value (Exp) ) = Exp log (value). So, in the case of the voltage and current ratios, the exponent of 2 is brought outside the logarithm calculation such that 10 2 log (ratio) = 20 log (ratio). http://www.arrl.org/files/file/instructor%20resources/a%20tutorial%20on%20th e%20dec-n0ax.pdf Rules for Working with Decibels 1. Always be aware whether you are converting from/to power or amplitude ratios and apply the correct factor (either 10/20). 2. Never mix ratios and decibels algebraically. Work either with one or the other, but never mix them in an equation.
P a g e 6 3. Never multiply decibels they only add or subtract. 4. Keep in mind a few decibel values for rough estimation or quickly checking a calculation. decibels (db) Power Ratio 0 db 1 3 db 2 (approx..) -3 db 0.50 (approx..) 6 db 4 (approx..) -6 db 0.25 (approx..) 10 db 10-10 db 0.10 You can use these for many estimations: Example 1; 7 db = 10 db - 3 db = (10)(0.5) = 5 Example 2; 4 db = 7 db 3 db = (5)(0.5) = 2.5 5. Subtracting reference value decibels (e.g., dbm and dbw) yields an answer in decibels. This is equivalent to dividing power by power (a unitless quantity). https://tableroalparque.weebly.com/uploads/5/1/6/9/51696511/01_-_gain_and_decibels.pdf
P a g e 7 Table of Decibels for Amplitude and Power ratios db Amplitude ratio Power ratio -100 db 10-5 10-10 -50 db 0.00316 0.00001-40 db 0.010 0.0001-30 db 0.032 0.001-20 db 0.1 0.01-10 db 0.316 0.1-6 db 0.501 0.251-3 db 0.708 0.501-2 db 0.794 0.631-1 db 0.891 0.794 0 db 1 1 1 db 1.122 1.259 2 db 1.259 1.585 3 db 1.413 2 1.995 6 db 2 1.995 3.981 10 db 3.162 10 20 db 10 100 30 db 31.623 1000 40 db 100 10000 50 db 316.228 100000 100 db 10 5 10 10 https://www.rapidtables.com/electric/decibel.html