HP 8591C Manual do Utilizador descarregar pdf (Página 203)

100

101

Measuring

Amplitude

Modulation

with

the

Fast

Fourier

Transform

Function

A Fourier transform, transforms time domain data (zero span) into the

frequency domain. The fast Fourier transform (FFT) function of the spectrum

analyzer allows measurements of amplitude modulation (AM). It is commonly

used to measure AM at rates that cannot be measured in the normal

frequency domain due to spectrum analyzer limitations on narrow resolution

bandwidths. For a given AM rate, the FFT function can generate a trace

faster than using the frequency domain for the equivalent spectrum analyzer

measurement.

The FFT function calculates the magnitude of each frequency component

from a block of time-domain samples of the input signal. It uses a flat top

filter response. This implementation is a post-detection Fourier transform and

it cannot be used to resolve continuous wave or carrier signals.

When

(MEAS~USER),

FFT Menu, and SINGLE FFT are pressed,

sample-detection mode is selected and a sweep is taken to obtain a sample

of the input signal. Then the spectrum analyzer executes a series of

computations on the time data to produce the frequency-domain results.

CDNTINUS

FFT can be used instead of SINGLE FFT and the spectrum

analyzer will be put in continuous sweep mode with an FFT being performed

at the end of each sweep.

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