Computation of dBV
Another common unit of amplitude is dBV. A 0 dBV signal is defined as
1 Vrms signal. You can convert a dBm reading to a dBV reading by
subtracting 13 dB.
dBV
=
20 log
0.707107
×
V
p
1V
=
20 log
V
p
0.316228V
+
20
log
0.707107
×
0.316228
V
1V
=
dBm value
−
13.01dB
dc Value
For efficiency reasons, the FFT computation produces a dc value that is
incorrect.
Aliasing
When using FFTs, it is important to avoid aliasing. Aliasing occurs when
there are insufficient samples on each cycle of the input signal to recognize
the signal. Aliasing occurs whenever the frequency of the input signal is
greater than or equal to the Nyquist frequency (sample frequency divided by
2).
When a signal is aliased, it shows up in the FFT spectrum as a signal of a
lower frequency. Because the frequency span goes from 0 to the Nyquist
frequency, the best way to prevent aliasing is to make sure that the
frequency span is greater than the frequencies present in the input signal.
Keep in mind that most periodic signals that are not sine waves have
frequency components that are much higher than the frequency of the signal.
Presetting FFT Parameters
The FFT vertical parameters, Magnify, Magnify span, and Center frequency
are set to the default values whenever the operand (or operator, if the FFT
magnitude operator is selected from the Math menu) is changed.
FFT Menu
FFT Basics
21–16
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