In communication technology often dBc is used, dBc means power level relative to a carrier signal, for example the level of a signal you're trying to receive in a receiver. On spectrum analyzers often dBm is used and that means 0 dB = 1 milliWatt of power in 50 ohms (most spectrum analyzers have a 50 ohms input). When your FFT plot in dBVrms I do not need to ask as your reference ( 0 dB) should then be 1 V rms. But if you plot a spectrum in dB the first thing I will ask is: "dB compared to what?" Often it will be 1 Volt peak but it is better to explicitly state that. ![]() You can plot an FFT spectrum in either dB or dBVrms. (Sometimes the threshold of pain is given as 120 decibels, or 1 watt per. That is correct but you didn't specify what Vs is, is it peak or rms? You should specify what Vs represents to make things clear. When the defining level of 0 decibel (10 -12 watt per square metre) is taken to be at the threshold of hearing for a sound wave with a frequency of 1,000 hertz, then 130 decibels (10 watts per square metre) corresponds to the threshold of feeling, or the threshold of pain. I prefer to keeps things separate and in the "dB" formula stick to same way to express a voltage, so peak or rms.īut if we would take the nominator as amplitude as Vs (i.e not rms value but the amplitude) then: dB = 20*log10(Vs) Indeed you're mixing things and that's OK but mind that that introduces more complexity and it is then easier to make a mistake. \$x\space dB = 20\space log \space \frac \$ and you used that above. For ratios of voltages, currents etc (not power) the formula is:
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