Units Of Measurement; Watts; Decibels - Keysight V3500A User Manual

4 RF Measurement Basics

Units of Measurement

Watts

Decibels

84
RF power measurements can be expressed in either Watts or
dBm. The V3500A can display results in either Watts
or dBm.
Watts are the standard SI unit for power measurements. In
electrical terms, one volt applied across a one- ohm resistor
dissipates one watt of power.
Decibels (abbreviated dB), are a logarithmic scale used to
express power. Decibels are inherently a ratio measurement
providing the relative magnitude of two different numbers.
For power measurements, the difference between two signals
in decibels is:
where
P and P are the two power levels to be compared.
1 0
Absolute measurement can be made on the decibel scale by
using a standard power level in place of P
to be 1 milliwatt, the resulting units are called "dBm." Power
expressed in dBm is calculated with the formula:
P
P
⎛ ⎞
1
× ----- -
10 log
⎝ ⎠
P
10
0
P
⎛
------------ -
×
= 10 log
⎝
dBm
1mW
10
. If P is defined
0 0
⎞
1
⎠
V3500A User's Guide