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Using the FFT

3–38

TDS 340A, TDS 360 & TDS 380 User Manual

H Filter the input to bandwidth limit it to frequencies below that of the Nyquist

frequency.

H Recognize and ignore the aliased frequencies.

If you think you have aliased frequencies in your FFT, select the source channel

and adjust the horizontal scale to increase the sample rate. Since you increase the

Nyquist frequency as you increase the sample rate, the alias signals should

“unfold” and appear at their proper frequency.

Nyquist frequency

point

Frequency

Amplitude

Aliased frequencies Actual frequencies

Figure 3–25: How aliased frequencies appear in an FFT

The oscilloscope multiplies the FFT time domain record by a Hanning window

before it inputs the record to the FFT function. Figure 3–26 shows how the time

domain record is processed.

The FFT windowing acts like a bandpass filter between the FFT time domain

record and the FFT frequency domain record. The shape of the window controls

the ability of the FFT to resolve (separate) the frequencies and to accurately

measure the amplitude of those frequencies. The Hanning window is optimized

for low leakage and good amplitude measurement accuracy on the different

frequency components of the signal.

If possible, adjust the trigger position so that the most interesting parts of the

signal in the time domain record are positioned in the center region of the

window so that the tapering does not cause severe errors.

FFT Windows

Tektronix TDS 340A User Manual

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