Hi,
why in TLC having negative TX values is valid? How can we transmit something if the transmission power is in the negative?
Why for at least in fiber optics the “no signal received” was established to be -40 dbm rather than 0?
In: 0
It is a relative measure. Logarithm of power ratios is the most common for RF. So a specific level is used as a reference point. Call it power level A. This gives a “bel”. The more common decibel = 10*bel
The signal to be measured is B. So if B = A, then B/A = 1 and log(1) = 0. Therefore a power level identical to the reference power level will get a dB measure of 0.
If B>A, then B/A > 1 and log (B/A) > 0.
If B<A, then B/A < 1 and log (B/A) < 0
dB makes sense if you understand the reference level. In RF communications, that reference level is usually 1mW in which case a customary notation of dBm is used (to indicate that the reference power level is 1mW)
In this case any measure of dBm > 0 means the signal is > 1mW and dBm < 0 means less than 1mW. Now for big transmission systems etc the signals can go into the kilowatts but for most hand held RF devices powered by batteries the signals will be below 1mW which gives negative dBm measures.
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