Q. Explain the working of spectrum analyzer and its applications.

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Draw block diagram of spectrum analyzer and explain its operation write down its importance applications.

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Explain the block diagram of frequency selective wave analyzer.

Sol. Spectrum Analyzer: The most common way of observing signals is to display them on an oscilloscope, with time as the Y-axis amplitude of the signal versus time. This is the time domain. It is also useful to display signals in the frequency domain. The instrument providing this frequency domain view is the spectrums analyzer. A spectrum analyzer provides a calibrated graphical display on its CRT, with frequency on the horizontal axis.

Displayed as vertical lines against these coordinates are sinusoidal components of which the input signal is composed. The height represents the absolute magnitude and the horizontal location represents the frequency.

   These instruments provide a display of the frequency spectrum over a given frequency band spectrum analyzers use either a parallel filter bank or a swept frequency technique. In a parallel filter bank analyzer, the frequency range is covered by a series of filters whose central frequencies and bandwidth are so selected that they overlap each other, as shown.

Referring to the block diagram of the sawtooth generator provides the sawtooth voltage which drives the horizontal axis element of the scope and this sawtooth voltage is frequency controlled element of the voltage tuned oscillator. As the oscillator sweeps from of its frequency band at a linear recurring rate, it beats with the frequency component of the input signal and produce an If, whenever a frequency component is met during its sweep. The frequency component and voltage tuned oscillator frequency beats together to produce a difference frequency, IF. The IF corresponding to the component is amplified and detected if necessary and ten applied to the vertical plates of the CRO, producing a display of amplitude versus frequency.

The spectrum produced if the input wave is a single tuned A.M. is given. One of the principal applications or spectrum analyzer has been in the instrument, the horizontal axis can display a range as wide a range as 2-3 GHz small portion of the spectrum's Signals at amplitude microwave frequency separated by only a few kHz can be seen individually.

   The frequency range covered by this instrument is from 1 MHz to 40 GHz. The basic block diagram range from 500 kHz TO 1 GHz, which is representative of a super heterodyne type.

    The input signal is fed into a mixer which is driven by a local oscillator; the oscillator is linearly tunable electrically over the range 2-3 GHz. The mixer input signal but of frequency which are the sum and difference of the input signal and lock oscillator frequency.

   The if amplifier is tuned to a narrow band around since the lock oscillator is tuned over the range of only inputs that are separated pass through the if frequency amplifier get rectified and produce a vertical deflection on the CRT.

 From this is observed that as the sawtooth signal sweeps the local oscillator also sweeps linearly from GHz. The tuning of the spectrum analyzer is a swept receiver, which sweeps linearly from 0 to 1 GHz. The sawtooth scanning signal is also applied to the horizontal plates of the CRT to from the frequency axis. (The spectrum analyzer is also sensitive to signals from 4-5 GHz referred to as the image frequency of the super heterodyne. A low pass filter with and a cutoff frequency above 1 GHz at the input suppresses these spurious signals.) Spectrum analyzers are widely used in radars, oceanography and biomedical fields.