Fm Modulation

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION ♦ 27

Communication Systems Experiment No. 5

Experiment No. 5

1.0 Title :Generation of FM using IC 8038.

2.0 Prior Concepts:Definition of frequency modulation, deviation of frequency in FM, bandwidth of FM.

3.0 New Concepts: Proposition 1: IC 8038 is VCO ( voltage controlled oscillator) used to generate FM. In this

oscillator frequency ( carrier frequency) is changed in accordance with the control voltage

(modulating signal).

Concept Structure :

Proposition 2 : Define FM and draw its concept structure.

4.0 Learning Objective :

4.1 Intellectual Skills :

a. Use of IC 8038 as VCO.

b. Interpretation of IC 8038 as FM generator.

4.2 Motor Skills:

a. Generate carrier frequency as output of IC 8038 without applying control voltage (free

running frequency).

b. Apply control voltage as modulating signal and generate FM.

5.0 Apparatus:IC 8038 experimental kit, signal generator, power supply and CRO .

28 ♦ MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

Experiment No. 5 Communication Systems

6.0 Circuit diagram:

Circuit description :

IC 8038 is voltage controlled oscillator. In this circuit, modulating input signal is applied at pin no.8.

The FM output is obtained at pin no.2. When modulating signal is not applied , only carrier signal is

available at the output . The instantaneous change in the voltage applied at pin no. 8 changes the

output frequency at pin no. 2 accordingly. Thus FM output is available when modulating signal input

is applied at pin no.8.

7.0 Procedure :1) Trace the circuit to identify input, process and output points.

2) Make the connections as shown in diagram.

3) Measure the frequency and amplitude of IC 8038 at pin no. 2 without application of control

voltage (i.e. center frequency fc).

4) Apply modulating signal with particular frequency and amplitude to pin no.8 as control voltage

using signal generator.

5) Observe the FM output waveform on CRO.

6) Vary the modulating signal amplitude to observe effect on FM output without changing the

modulating signal frequency.

7) Draw the waveforms of modulating signal, carrier signal and FM output on graph paper.

8.0 Observation Table :Modulating signal frequency = Hz

Carrier signal frequency = Hz



8.1 Specimen calculation for reading no.

mf = δ /fm

Note: To follow different procedure the space is provided for observation table.

9.0 Conclusion:

10.0 Questions :Attempt three questions as directed by teacher.

1) What is the function of IC8038?

2) How is carrier generated in given experimental circuit? Also mention the carrier frequency.

3) What is the function of RA and R

B in given circuit?

4) If capacitor is not connected at pin no. 10, what will be the output?

5) Compare AM and FM with respect to

i) Modulation Index.

ii) Number of sidebands.

iii) Bandwidth requirement.

iv) Carrier power.

6) Why FM is more resistive to noise than AM?



(Space for answer)



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Fm Modulation

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