LAB MANUAL/EMIT

EXPERIMENT NO. 5

TITLE: Study of Spectrum Analyzer

AIM: Study of Spectrum Analyzer

Perform harmonic analysis and Total Harmonic Distortion (THD) measurement for sine

and square waves.

Analyse Spectrum of AM & FM and to measure percent modulation and bandwidth.

APPARATUS:

Sr. No. Instrument Specifications

1. Arbitrary Waveform generator

2. Spectrum analyzer

Technical Specifications:

I/p Freq.

Frequency Range

50Ω

Specifications

9kHz to 1.5GHz

Frequency Reference

Aging

Temperature Stability

±2*10-6 /year

±5*10-6

Frequency Readout Accuracy

(Start,Stop,Center,Marker)

±(frequency readout*frequency reference error3

+span accuracy+20% of RBW)

Frequency Span

Range

Resolution

Accuracy

0Hz (zero span),100 Hz to 1.5GHz

Four digits or 2Hz,whichever is greater

±1% of span

Sweep Time

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LAB MANUAL/EMIT

Range

Accuracy

Sweep Trigger

5ms to 2000s

±1%

Free Run,Single,Line,video,External

Measurement Range

50 Ω

Input Attenuator Range

-120 dBm to +30dBm

0 to 60dB,in 5 dB steps

Frequency Response

50 Ω,9kHz to 1.5GHz

20 to 30 C

0 to 55C

±0.75 dB

±1.0 dB

THEORY:

The most common way of observing a signal is to display it on an oscilloscope, with time as x-

axis. This is a view of the signal in time domain. It is also very useful to display signals in

frequency domain. The instrument providing this frequency domain view is the spectrum

analyzer. On its CRT, the spectrum analyzer provides a calibrated graphical display with

frequency on horizontal axis and voltage on vertical axis.

There are two types of spectrum analyzers:

Scanning types, which can scan the frequency and non-scanning types also called real-time

spectrum analyzer. The scanning types are essentially swept receivers, both superhetrodyne and

tuned rf (Trf) whose tuning is electrically swept over the frequency range of observation by a

scanning signal that also controls the horizontal position of the spot on the CRT ray tube. There

are two kinds of real-time non-scanning spectrum analyzers: The multichannel spectrum

analyzer or Fourier analyzer. These real-time analyzers “lock” over all parts of their frequency

display range simultaneously and so present the spectrum of an electrical event as soon as it

happens. The scanning spectrum analyzer can only “lock” at a single frequency at a given

instant. The spectrum analyzer that we are using in the laboratory is the swept spectrum analyzer.

Fig 1 shows the basic block diagram of a spectrum analyzer covering the range of 500 kHz to 1

GHz which is representative of super heterodyne type. the input signal is fed into the diode mixer

which is driven to saturation by a strong signal from the local oscillator, which is linearly tunable

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LAB MANUAL/EMIT

electrically over the range of 2 to 3ghz.the input mixer multiplies (heterodynes) the input signal

and the local oscillator signal together and so provides two signals at its output that are

proportional in amplitude to the input signal but of frequencies that are the sum and difference

between the frequencies of input signal and the local oscillator signal.

The intermediate frequency amplifier is tuned to a narrowband around 2ghz.as the local

oscillator is tuned over the range from 2 to 3 GHz, only input signals that are expected from the

local oscillator frequency by 2 GHz will be converted into the intermediate frequency band pass

through the intermediate frequency amplifier, be rectified in the detector and produce a vertical

deflection on the CRT.The spectrum analyzer will also be sensitive to signals from 4 to 5 GHz. a

LPF with a cutoff a little above 1 GHz at the input suppresses these spurious signals.

Fig.1: Swept Heterodyne Spectrum Analyzer

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LAB MANUAL/EMIT

Fig.2 Block Diagram of Spectrum Analyzer

Fig 2 is a more detailed block diagram of the spectrum analyzer. The frequency of the first local

oscillator is controlled with the select ably attenuated signal from the scan generated combined

with an adjustable bias level from the center frequency control in a voltage control block. The

attenuator controls the frequency axis calibration of the display by controlling the frequency

range over which the first local oscillator is scanned. The adjustable bias level sets the frequency

about which the local oscillator is scanned and thus the center frequency of the display.

Several conversions are used in the intermediate frequency amplifier chain, which ultimately

gets down to a 3 MHz intermediate frequency amplifier having a B.W.of a few 100 Hz to

provide this high selectivity over the whole range of the instrument. the high first IF is necessary

for wide image separation, the low cost IF is necessary for narrowband filtering unobtainable at

the 1st IF.

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LAB MANUAL/EMIT

Frequency Resolution and Bandwidth:

Frequency resolution is the ability of the spectrum analyzer to separate signals closely spaced in

the frequency. The scanning action in the spectrum analyzer slides the input spectrum past the IF

amplifier filter. Because of this, the magnified display of a single frequency CW signal is a plot

of sensitivity char. of the IF amplifier filter.The other factors determining resolution are the

frequency stability of the spectrum analyzer, local oscillator.

Sweep Desensitization:

Sweep desensitization is an effect caused by scanning a spectrum analyzer too fast which results

in loss of amplitude, sensitivity and resolution.The scan velocity in Hz must not exceed the

square of 3 dB B.W.of the IF filter in Hz.

Sensitivity:

The ability of the swept super heterodyne spectrum analyzer to measure small signals is

determined by its own internally generated be noise. Typical noise fig. varies from 24 dB at low

frequency to 40 dB at 12 GHz. Noise power is proportional to B.W. and so the highest sensitivity

to CW signals is obtained by the narrowest B.W.s

Dynamic Range:

The dynamic range of the spectrum analyzer expresses its ability to display the true spectra of

large and small signals simultaneously with signal levels within the dynamic range of the

instrument.

Harmonic Mixing:

Harmonic mixing is used to extend the frequency range of the swept super heterodyne spectrum

analyzer.

PROCEDURE:

1. Capture 98.3MHz radio Mirchi signal by giving appropriate START and STOP

frequency.

2. Generate AM waveform using arbitrary waveform generator. Observe fc-fm, fc, fc+fm

frequencies on spectrum analyzer and calculate modulation index.

%m = Em/Ec

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LAB MANUAL/EMIT

3. Generate FM waveform using arbitrary waveform generator and calculate modulation

index. %m = ∆f/frequency of modulating signal(fm)

4. Using markers measure bandwidth of FM

For m=0.5, no of side bands = 4

For m= 1,no of side bands = 6

For m= 2,no of side bands = 8

For m= 5,no of side bands = 16

Bandwidth= fm* no. of sidebands

5. For sine and square wave input measure total harmonic distortion.

THD = sqrt[(v22+v32+v42+…..)]/V1, where V1 = amplitude of fundamental frequency

V2 = amplitude of second harmonic

V3 = amplitude of third harmonic and so on.

CONCLUSIO

N:___________________________________________________________________________

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