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HP 54540 Series - Page 184

HP 54540 Series
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Math/FFT
Menu
Making
FFT
Measurements
Computation
of
dBm
The
vertical
units
of
the
FFT
functions
are
dBm.
0
dBm
is
defined
asa
1
milliwatt
signal.
The
formula
for
converting
a
signal
of
power
P
into
dBm
is:
Pinmw
lmW
A
handier
formula,
and
the
one
that
is
used
in
the
instrument
is
for
calculating
dBm
from
the
peak
voltage.
Vp
in
volis
0.316228V
The
bottom
term,
0.316228
Volts,
is
the
peak
voltage
of
a
|
milliwatt
signal
into
a
50
Q
resistor.
Vo
=
Vrms.
NPXR_
P=
0707107
0.707107
_VimW
x50
Q
=““o707107
=
0.316228
Volts
If
you
are
measuring
the
power
of
a
signal,
then
terminate
the
source
into
50
Q
in
order
to
get
the
correct
dBm
reading.
However,
if
you
are
measuring
a
voltage,
the
50-Q
source
impedance
is
not
a
requirement.
The
above
equation
for
dBm
as
a
function
of
peak
voltage
still
applies.
dBm
=
10log|
dBm
=
20log
10-17

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