Computation of dBm
The vertical units of the FFT functions are dBm, and 0 dBm is defined as a
1 mW signal. The formula for converting a signal of power P into dBm is:
dBm
=
10
log
P
1mW
A handier formula, and the one that is used in the oscilloscope, is for
calculating dBm from the peak voltage.
dBm
=
20
log
V
p
0.316228
V
The bottom term, 0.316228 Volts, is the peak voltage of a 1 mW signal into a
50-
Ω
resistor. The calculation is:
V
P
=
V
rms
0.707107
=
√
P
×
R
0.707107
=
√
1
mW
×
50
Ω
0.707107
=
0.316228V
If you are measuring the power of a signal, then terminate the source into
50
Ω
in order to get the correct dBm reading. However, if you are measuring
voltage, then you do not need to terminate source impedance into 50
Ω
. The
equation above for dBm as a function of peak voltage still applies.
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FFT Basics
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