General Purpose Oscilloscope Menus
FFT Menu
Computation of dBm
The vertical units of the FFT functions are dBm, and 0 dBm is dened as a
1 mW signal. The formula for converting a signal of power P into dBm is:
dB m
=10
log
P
1
mW
A handier formula, and the one that is used in the instrument, is for
calculating dBm from the peak voltage.
dB m
=20
log
V
p
0
:
316228
V
The
bottom
term,
0.316228
volts
,
is
the
peak
voltage
of
a1
mW signal
into
a
50
resistor
.
The
calculation
is:
V
p
=
V
r
ms
0
:
707107
=
p
P
2
R
0
:
707107
=
p
1
mW
2
50
0
:
707107
=
0
:
316228
V
If
you
are
measuring
the
power
of
a signal,
then
terminate
the
source
into
50
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.
The
equation
above
for
dBm
as
a
function
of
peak
voltage
still
applies
.
10-33
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