DECIBEL MEASUREMENT TV

Widely used in TV and antenna amplifier specs. Used to measure microphone sensitivity.ĭBμV or dBuV dB(μV RMS) – voltage relative to 1 μV. Here is a partial list of those widely used in electronics:ĭBV, dB(V RMS) – voltage relative to 1 V, regardless of impedance.

You can find online calculators that convert between dBm and watts, such as the one at or There are numerous dB abbreviations followed by letters. This is 7 dB above 50 dBm, so the power has gone up by more than 5x. Finally, we gain 10 dB at the antenna for a net result of +57 dBm. If we lose 3 dB in the coax, we are down to +47 dBm (+50 – 3 dB loss). We convert the power to dBm, then do the additions and subtractions and convert back: 100 W = 100,000 mW = 10 5 mW, so the power level is +50 dBm. And -6 dBm would be 0.25 mW.Ĭonsider an example where a transmitter puts out 100 W, a feed line has a 3 dB loss and an antenna has a gain of 10 dB. For example, when power levels are given in dBm, 0 dBm corresponds to the reference power of 1 mW 10 dBm would be 10 times that or 10 mW. The letter indicates the value is decibels with respect to a specific reference value. This means the value was calculated using a specific reference value. The abbreviation dB is often followed by a letter. It may be useful to consider that a change in signal strength of one S unit is a change in signal power of approximately four. Each S unit represents a change in strength of 5 to 6 dB. There received signal strengths on the HF bands are usually reported in S units. Ears respond logarithmically to changes in sound level, explaining why the decibel is a useful tool of comparison.ĭecibels also arise in amateur radio. Each 10 dB represents a factor of ten difference, so 120 dB represents a pressure 1,012 times greater than the reference threshold level. The threshold of discomfort from sound intensity is 120 dB. A vacuum cleaner one meter away puts out about 70 dB. SPL in the average home is about 50 dB above the 0 dB threshold. The usual practice is to have the reference level of 0 dB correspond to a pressure of 0.0002 microbars, the standard threshold of hearing. One area where dB measurement units prevail is that of sound intensity or sound pressure level (SPL). Similarly, a power amp with 20 dB of gain connected to a feed line with 3 dB of loss and an antenna with 12 dB of gain gives a total gain of 20-3+12 = 29 dB. 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.

Decibel measurement