I understand that when we measure noise floor, we use dBm/Hz, that is,
the noise power per 1Hz bandwidth. The ideal noise floor is -174
dBm/Hz.
However, when I measure a signal power at spectrum analyzer, I would
get, for example, -30dBm. What is the bandwidth of my measurement? Is
the bandwidth that we use in measuring the signal is less than 1Hz?
Thanks,
feilip
dB measurements are made relative to something else.
Wot you do is divide what you have measured by what is the reference and
then take the log of the answer and you get your answer in dB.
Then you have to say what your answer was reffered to and make sure that any
other bits you used in the referenced are included in the thing you measured
and what it was referenced to.
So.... if your reference is the weight of an Elephant and you were looking
to measure a pig then you would take the weight of your pig and divide it by
the weight of your elephant and take the log of it and express your answer
as dB/Welph.
Now, things get complicated when you are looking at how much your pig weighs
and the frequency it farts at when compared to your reference Elephant and
the frequency it farts at so then you end up with units like dB/WElphHz.
You will only realise the worth of these units when you have to work with
fly farts per weight of a fly referenced to elephant farts per weight of an
elephant or maybe not but most times you can write 2 dBWF/WElphHz rather
than 0.000000000000001299876 or
399691589691256120.
Most times when you are comparing stuff in their original units then you
have to multiply the dB answer by 10. However, if the units are different
then you have to use 20 instead because E=mC^2 and the squared term is a
multiply by two in log terms.
See...
DNA