HP 54520 Series - Page 185

Math/FFT Menu
Making FFT Measurements
Computation of dBV
Another common unit of amplitude is dBV. A 0 dBV signal is defined as a 1
Volt rms signal. A dBm reading is converted to a dBV reading by subtracting
13.01 GB.
aBV = 20l0g(0.707107 x Vp in volts)
Vp 7
0316028 + 20log(0.707107 x 0.316228)
= dBm value — 13.01 dB
= 20log
dc Value
The FFT computation produces a de value that is incorrect. It does not take
the offset at center screen into account and is 1.41421 times greater than its
actual value. The de value is not corrected in order to accurately
represent frequency components near dc.
Aliasing
When using FFTs, make sure you avoid signal aliasing. Aliasing occurs when
there are insufficient samples on each cycle of the input signal to recognize
the signal. It occurs whenever the frequency of the input signal is greater
than the Nyquist frequency (sample frequency divided by 2).
When a signal is aliased, it shows up in the FFT spectrum asa 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.
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