Agilent Technologies 54657A Podręcznik Użytkownika Pobierz pdf (Strona 25)

Aliasing When using FFT’s, it is important to be aware of aliasing. This

requires that the operator have some knowledge as to what the

frequency domain should contain, and also consider the effective

sampling rate, frequency span, and oscilloscope vertical bandwidth when

making FFT measurements. Effective sample rate is briefly displayed

when the

± key is pressed.

Aliasing happens when there are insufficient samples acquired on each cycle

of the input signal to recognize the signal. This occurs whenever the

frequency of the input signal is greater than the Nyquist frequency (sample

frequency divided by 2). When a signal is aliased, the higher frequency

components show up in the FFT spectrum at a lower frequency.

The following figure illustrates aliasing. In waveform A, the sample rate is set

to 200 kSa/s, and the oscilloscope displays the correct spectrum. In

waveform B, the sample rate is reduced by one-half (100 kSa/s), causing the

components of the input signal above the Nyquist frequency to be mirrored

(aliased) on the display.

Aliasing

Since 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.

Figure 2– 2

Operating the Measurement/Storage Module

FFT Measurement

2–9

1 2 ... 20 21 22 23 24 25 26 27 28 29 30 ... 88 89

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Agilent Technologies 54657A Podręcznik Użytkownika Strona 25