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.  

Principle

 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.

Description:

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.

Advantages

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

display.