For example, an antenna with 10 dB of gain connected to a preamp with 20 dB of gain gives a total gain of 10 + 20 = 30 dB. In the case of the voltage and current ratios, the calculation is 10 × 2 log (ratio) = 20 log (ratio).Īnother point to note about decibels is that gains and losses of stages in a radio system can be added together if they are specified in dB. We now use the identity log (value Exp) = Exp × log (value). Substituting for power values using the equations P = (V 2/R) and P = (I 2R) makes the ratios inside the parentheses of the decibel equation become V 2/V ref 2 and I 2/I ref 2. The reason comes from the definition of the dB–it is not because a “voltage dB” or a “current dB” differs from a “power dB.” A dB is a dB. This brings up the question of why the logarithm of voltage and current ratios are multiplied by 20 instead of 10. R i = 20log 10 ( current/ reference current) R v = 20log 10 ( voltage/ reference voltage) For voltages and currents, the ratio R v and R i respectively is The above expression is valid for the ratio of one power level to another, but not for one current level to another, or for one voltage level to another. For example, dBm means decibel milliwatts. But always a reference quantity is required, even if it is simply implied by a letter appended to the end of the expression. However, often decibels appear to be absolute measures. Notice that in the above cases, where decibels are used, one signal is compared to another. A signal that is 1/10 the amplitude of another signal is -20 dB. A signal that is 10 times the amplitude of another signal is 20 dB. Accordingly, a signal that is twice the amplitude of another signal is roughly 6 dB relative to it because log 10 2 = 0.3010. Where P1 and P2 are the respective powers of the two signals. The ratio R of the two power quantities, by definition is: For this reason, modern sampling oscilloscopes offer in addition to the linear option a logarithmic scale. In power and sound ratios, the factor is frequently in the millions, which makes it difficult to graph the two quantities within an oscilloscope display. Decibels, (derived from bels, old units which, like unwieldy farads, are rarely used) are convenient and user-friendly terms for expressing the ratio between two amounts of power on a logarithmic scale or the amount of change, amplification or attenuation, over time of a single amount of power.

Dbi to dbm conversion