|Introduction To The dB (http://www.satcom.co.uk/article.asp?article=10)|
Signal stages are cascaded, so powers are multiplied by gain or loss. This yields a lot of multiplications. This suggests the need for a logarithmic representation of power.
A logarithmic scale is used to
Log(x) = power to which base must be raised to give x. The base is chosen to be 10.
Some example logarithm values:
|Represent gains or attenuations logarithmically (base 10) (the
But to make numbers more convenient, scale by a factor of 10 (the deciBel or dB)
Since Log(A x B) = Log(A) + Log(B) we can add gains and losses.
PR = PT + 20 - 1 + 30 - 2 - 204 + 30 -1 + 60 = PT - 68 dB
For converting from a power ratio to dB, first work out powers of 10, e.g:
Then note the smaller factors:
Examples of converting from dB to a Ratio (or more generally, ratio = 10dB/10):
|Applying dB to Other Units|
|By default, dB is a power ratio. But it can be other things,
for example, dB banana = dB relative to 1 banana.
dBW = dB relative to 1 watt, so:
Bandwidth in Hz can be expressed in dB-Hz
Similarly, Noise Temperature:
By default, with dBs we are dealing with power.
Thus a change in power (e.g. due to amplification) can be represented by:
TIP: Take care with "Voltage gain in dB" which is usually a power gain, i.e 20Log(V2 / V1)
|How Big Is A dB?|
Examples of BER vs. Eb/No in dB:
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