HP 54750A, User Manual, Page: 285 / 478

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 l og
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 l og
V
p
0: 316228 V
The bottom term, 0.316228 volts , is the peak voltage of a 1 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 .
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