I am just trying to understand the nitty gritty of using decibels and absolute measurements in dBm. I see them referred to routinely in RF, and using reference sheets I can understand enough to make sense of gains and losses, but I would like instead to acquire a much better understanding for myself, being able to convert and calculate freely (without the need for reference sheets) between any power or dB value.
I think I understand that dB is merely a ratio of measurement. But if, for example, a component under test had 20W at its input and 15W at its output and there is an overall loss of 5W, how would that be worked out and expressed in a ratio of dB?
Trying to work it out I did:
20W (in) = 10log10(20W/1W) = 13dB(W) 15W (out) = 10log10(15W/1W) = 11.76dB(W) so, component power loss = 13dB(W) – 11.76dB(W) = 1.24dB(W)
However, I can see that I am merely subtracting absolute measurements still and not determining the dB ratio of power transfer efficiency or loss. If so, then what would be the equivalent dB ratio for this example?
In addition, I am able to convert 13dB(W) back to watts with:
10 to the power of (13/10) or 10^1.3 = 19.95W
and so too for 11.76dB(W):
10^1.176 = 14.997W
but for 1.24dB(W):
10^0.124 = 1.33W
which does not appear correct, as I expected instead a value of ~ 5W, for which 1.24dB(W) represents between 13dB(W) and 11.76dB(W), so I unsure where I am going wrong. If someone could let me now I would sure appreciate the insight.
Finally, what would it mean to say a receiver has a receive level of -45dBm?
Is it right to conclude that such a receiver is capable of extracting information from signals received at -45dBm less power than what they (signals) were originally transmitted at? Or actually it just occurred to me that since 45dBm is an absolute value that it perhaps means that the receiver could receive signals as low as -45dBm or:
10^(-45/10) = 0.0000316mW
and still be able to extract the information transmitted so long as the S/N ratio is adequate? Which if that was the case then, if one transmits a signal with 20W or 43dBm of power, then the signal could be attenuated by up to 88dBm:
43dBm - -45dBm = 88dBm
and still be detected adequately enough by the receiver in question so that the original information sent is received?
I hope I have explained my questions well enough to be easily understood.