Spectrum Analyzer

           Spectrum Analyzer :

Introduction:-

A spectrum analyzer provides a calibrated graphical display on its CRT, with frequency on the horizontal axis and amplitude (voltage) on the vertical axis.

(The most common way of observing signals is to display them on an oscilloscope with time as the

X-axis (i.e.between amplitude of the signal and time). This is the time domain. It is also useful to display signals in the frequency domain. The providing this frequency domain view is the spectrum analyzer.)

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 a given frequency band.

Spectrum analyzers use either parallel filter bank or a swept frequency technique.

In a parallel filter 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 others, as

shown in fig 4.1.

Typically, an audio analyzer has 32 of these filters, each covering one third of an octave.

For wide band narrow resolution analysis, particularly at RF or microwave signals, the swept

Technique is preferred.

as per reference  to the block diagram of fig. 4.2, the saw tooth generator provides the saw tooth voltage ,which drives the horizontal axis element of the scope and this saw tooth voltage is the frequency controlled element of the voltage tuned oscillator. As the oscillator sweeps from fmin to fmax 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, i.e. The IF corresponding to the component is amplified and detected if

necessary and then applied to the vertical plates of the CRO, producing a display between of amplitude and  frequency.

The spectrum produced if the input wave is a single toned A.M .

One of the principal applications of spectrum analyzers has been in the study of the RF spectrum

produced in microwave instruments. In a microwave instrument, the horizontal axis can display

as a wide a range as 2 - 3 GHz for a broad survey and as narrow as 30 kHz, for a highly

magnified view of any small portion of the spectrum. Signals at microwave frequency separated

by only a few KHz can be seen individually.

The frequency range covered by this instrument is from I MHz to 40 GHz, The basic block

diagram is of a spectrum analyzer covering the range 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. This oscillator is

linearly tunable electrically over the range 2 - 3 GHz. 

The mixer provides two signals at its output that are proportional in amplitude to the input signal but of frequencies which are the sum and difference of the input signal and local oscillator frequency.

The IF amplifier is tuned to a narrow band around 2 GH4 since the local oscillator is tuned over

the range of 2 - 3 GHz, only inputs that are separated from the local oscillator frequency by

2GHz will be converted to IF frequency band, pass through the IF frequency amplifier, get

rectified and produce a vertical deflection on the CRT.

From this, it is observed that as the saw tooth signal sweeps, the local oscillator also sweeps

linearly from 2 - 3 GHz. The tuning of the spectrum analyzer is a swept receiver, which sweeps

linearly from 0 to 1 GHz. The saw tooth scanning signal is also applied to the horizontal plates of

the CRT to form 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 a cutoff

frequency above I GHz at the input suppresses these spurious signals.)

The two types of spectrum analyzers are,

1. Fliter Bank Spectrum analyzer.

2. Super hetero dyne Spectrum analyzer.

1. Filter Bank Spectrum analyzer

2. Super hetero dyne Spectrum analyzer

The modern spectrum analyzers use a narrow band super heterodyne receiver. Super heterodyne is nothing but mixing of frequencies in the super above audio range. 
The functional block diagram of super heterodyne spectrum analyzer or RF spectrum analyzer as shown in the

Figure 7.2

The RF input to be analyzed is applied to the input attenuator. After attenuating, the signal is fed

to low pass filter.

The low pass filter suppresses high frequency components and allows low frequency components

to pass through it. The output of the low pass filter is given to the mixer, where this signal is

fixed with the signal coming from voltage controlled or voltage tuned oscillator. 

This oscillator is tuned over 2 to 3 GHz range. The output of the mixer includes two signals whose amplitudes.are proportional to the input signal but their frequencies are the sum and difference of the input signal and the frequency of the local oscillator. Since the frequency range of the oscillator is

tuned over 2 to 3 GHz, the IF amplifier is tuned to a narrow band of frequencies of about 2 GHz.

Therefore only those signals which are separated from the oscillator frequency by 2 GHz are

converted to Intermediate Frequency (IF) band. This IF signal is amplified by IF amplifier and

then rectified by the detector. After completing amplification and rectification the signal is

applied to vertical plates of CRO to produce a vertical deflection on the CRT screen. Thus, when

the saw tooth signal sweeps, the oscillator also sweeps linearly from minimum to maximum

frequency range i.e., from 2 to 3 GHz. Here the saw tooth signal is applied not only to the

oscillator (to tune the oscillator) but also to the horizontal plates of the CRO to get the frequency

axis or horizontal deflection on the CRT screen. On the CRT screen the vertical axis is calibrated

in amplitude and the horizontal axis is calibrated in frequency.

Application:

These Spectrum analyzers are widely used in the field of,

1. Bio medicals

2. RADARS

3.Oceangraphy