GW Instek GRF-1300A - Manual (Page 120)

GRF-1300A User Manual and Teaching Materials
The relationship between dB and dBc
The figures in the table above are based on a 50Ω load. As an
example, as -30dBm is equal to 0.001mW or 10
-6 W, therefore with
a 50Ω load it is 7071.07 uV or 0.007071mV. The formulas and
derivations from the above table are:
10 10
dBm
inmW
P = => R P V × =
=> ) log( 20
uV
V
dBuV × =
further ) log( 10
mW
P
dBm × = ) log( 20
mV
V
dBmV × =
As for dB and dBc, they are relative units. In terms of power, a
difference of 20dB is equal to a difference of 100 times.
Question What is the difference between 0dBm and-50dBm? Is it 50dB or
50dBm?
Answer 50dB
Explanation 0dBm = 1mW, -50dBm = 10 -5 mW, therefore the difference of both
is 10 5 times which equal to 50dB or a difference of 0.99999mW
And 0.99999mW is equal to -0.0000434dBm ≈ 0dBm.
50dBm = 10 5 mW = 100W. Obviously 50dBm is the wrong
answer.
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