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4
RF Measurement Basics
Power of various types of signals
Continuous-wave signals
Continuous-wave (CW) signals are the simplest form of RF signal. CW signals
consist of a constant sine wave. Power measurements on this type of wave result
in a single numerical value.
Modulated signals
Modulated signals are sinusoids whose amplitude, phase, or frequency change
with respect to time. Changes in either the phase or frequency do not change the
power of the signal, but changes in amplitude causes change in the power level.
Two approaches can be taken to characterizing the power of modulated signals:
– Peak power meter
– Average power meter (the V3500A takes this approach)
108
A peak power meter measures the power of the signal very rapidly (faster than
the rate at which the modulation changes). The results can then be displayed
in a graph of power versus time similar to an oscilloscope display. This
approach has the advantage of displaying a great deal of information. One
disadvantage is that the results can be difficult to interpret — especially for the
complex modulation schemes in use today.
An averaging power meter measures the power versus time of the signal, but
then averages the result — condensing the measurement to a single number.
The V3500A is an averaging power meter. In effect, the power meter performs
the following calculation:
T
1
- - P t ( ) t
P avg
=
d
T
0
where
P(t) is the power of the modulated signal as a function of time.
In cases where the modulation format is known, the peak instantaneous power
can be calculated from the
average power.
Keysight V3500A User's Guide
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