Agilent Technologies 24A, 54621D, 42A, 54621A, 22A, 41A manual FFT Measurement

Making Measurements

FFT Measurement

FFT Measurement

FFT is used to compute the fast Fourier transform using analog input channels or math functions 1 + 2, 1 – 2, or 1 * 2. FFT takes the digitized time record of the specified source and transforms it to the frequency domain. When the FFT function is selected, the FFT spectrum is plotted on the oscilloscope display as magnitude in dBV versus frequency. The readout for the horizontal axis changes from time to frequency (Hertz) and the vertical readout changes from volts to dB.

Use the FFT function to find crosstalk problems, to find distortion problems in analog waveforms caused by amplifier non-linearity, or for adjusting analog filters.

FFT Units 0 dBV is the amplitude of a 1 Vrms sinusoid. When the FFT source is channel 1 or channel 2 (or channel 3 or 4 on the 54624A), FFT units will be displayed in dBV when channel units is set to Volts and channel impedance is set to 1 MΩ .

For the 54640-series, FFT units will be displayed in dBm when channel units is set to Volts and channel impedance is set to 50Ω . For the 54620-series, if you want the result to be in dBm, you must connect a 50Ω load (10100C or equivalent) to the analog channel input, and then perform the following conversion:

dBm = dBV + 13.01

FFT units will be displayed as dB for all other FFT sources or when a source channel’s units has been set to Amps.

DC Value The FFT computation produces a DC value that is incorrect. It does not take the offset at center screen into account. The DC value is not corrected in order to accurately represent frequency components near DC.

Aliasing When using FFTs, it is important to be aware of frequency 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.

The FFT sample rate is displayed directly above the softkeys when the FFT menu is displayed.

Aliasing happens when there are frequency components in the signal higher than half the effective sample rate. Since the FFT spectrum is limited by this frequency, any higher components are displayed at a lower (aliased) frequency.

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