User`s guide
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.
Note
The fast ADC Option 101 extends FFT operation. The standard spectrum
analyzer has sweep times (in zero span) up to 20 ms and allows FFT stop
frequencies from 20 Hz to 10
kHz.
With Option 101, spans of 20 ps can be used
and FFT stop frequencies up to 10 MHz are available.
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
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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.
CONTINUS 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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FFT START 0 Hz
FFT STOP
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RES BW 188 kHz
UBW 38 kHz
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Figure 4-l. FFT Annotation
Some of the screen annotation is altered when the FFT function is active. The left edge of
the graticule is relabeled FFT START and represents 0 Hz relative to the carrier. The right
edge of the graticule is relabeled FFT STOP and is the maximum FFT frequency used in the
transformation. The annotation LIN in the upper left corner refers to the scale of the incoming
data being transformed. The FFT results, which are being displayed, are always in LOG scale.
The carrier appears at the left edge of the graticule with the modulation sidebands and any
distortion appearing along the horizontal axis. The amplitude relationships of all the signals
are the same as they would be if the components were displayed with normal swept-tuned
operation in log mode, 10
dB
per division.
4-2 Making Measurements