Saturday, 17 March 2012

digital Fourier analyzer

  • digital Fourier analyzer:  
A spectrum analyzer, which uses computer algorithm and an analog to digital conversionphenomenon and produces spectrum of a signal applied at its input is known as digital Fourier ordigital FFT or digital spectrum analyzer.  


 When the analog signal to be analyzed is applied, the A/D converter digitizes the analog signal

(i.e., converts the analog signal into digital signal). The digitized signal, which is nothing but the
set of digital numbers indicating the amplitude of the analog signal as a function of time is stored
in the memory of the digital computer. From the stored digitized data, the spectrum of the signal
is computed by means of computer algorithm.
The block arrangement of a digital Fourier analyzer is illustrated in the figure above fig 5.The
analog signal to be ana1ysed is applied to the low pass filter, which passes only low frequency
signals and rejects high pass spurious signals. This filter section is used mainly, to prevent
aliasing. The output of low pass filter is given to the attenuator. The attenuator is a voltage
dividing network whose function is to set the input signal to the level of the A/D converter. The
use of attenuator prevents the converter from overloading. The function of A/D converter is to
convert the samples of analog data into digital i.e. ., to digitize the analog signal. When the
output of A/D converter is applied to the digital computer, the computer analyzes the digitized
data and adjusts the attenuator setting accordingly in order to obtain the maximum output from
the inverter without any overloading. As soon as the entire analog signal is sampled and digitized
by the A/D converter) computer performs calculations on the data according to the programmed
algorithm and the calculated spectral components are stored in the memory of the computer.

If the spectral display is to be viewed on the oscilloscope, the digital values of spectral
components stored in the computer memory are converted into analog by using D/A converters
and then applied to the CRO. Thus the spectral display of the input waveform is obtained on the
CRT screen.

1. The use of computer avoids most of the hardware circuitry such as electronic switches.
Filters and PLLs. The use of less hardware reduces the cost of the analyzer.
2. More mathematical calculations can be carried-out on the spectral display.
3. The rate of sampling analog signal can be modified in order to obtain better spectral

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