EEE_310_02

EEE 310: Communication Laboratory

Exp. 2: Frequency Modulation and Demodulation - Appendix

Last Updated: Fall 2012

United International University (UIU)

Name of The Experiment: Frequency

Modulation and Demodulation

Communication LAB

Course: EEE-310 Section: C Group: C3

Expt no. 02

Ahsan Mazumder

ID: 021 103 008




Appendix

Table 2-1 The measured results of MC1648 voltage controlled oscillator.

Input forward bias (V) 3 7 11 15

Output signal frequency (MHz) 1.27 1.5 1.62 1.69

Output Wave of VCO:




0 5 10 150

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Input dc bias(V)

Outp

ut

Sig

nal F

requncy(M

Hz)

The characteristic curve of VCO

Discussion:

Voltage control oscillator is an oscillator in which the input terminal voltage controls output

signal frequency. From the graph above according to the readings taken, it can be seen

clearly that as the input voltage increases, the output frequency also increases. This relation

should be linear but due to different kind of circuitry parameters, this is not linear all over.

Table 2-2 The measured results of MC1648 frequency modulator and demodulator (fm = 1 kHz).

Am Audio signal waveform (with

necessary remarks)

FM signal waveform (with

necessary remarks)

Demodulated waveform (with

necessary remarks)




5V

2V

Table 2-3 The measured results of LM566 frequency modulator and demodulator (fm = 1 kHz).

Am Audio signal waveform

(with necessary remarks)

FM signal waveform (with

necessary remarks)

Demodulated waveform (with

necessary remarks)

5V




2V

Report :

2. Q. What characteristics of the modulated signal found in the experiment implies that this is

actually a frequency modulated signal?

Ans: By definition frequency modulation (FM), utilizes the amplitude of audio signal to modulate

the frequency of carrier signal. The transmitted high and low frequency signals follows the

received audio signal, which has different frequency that keeps on changing. In the experiment it

was seen that the frequency of the modulated signal was changed from that of the input message

signal.

3. Q. Show the block diagram of frequency modulation and demodulation using PLL. Compare it

with the setup used in the experiment.

4. Q. Show the block diagram of frequency modulation and demodulation using FM-to-

AM conversion discriminator. Compare it with the setup used in the experiment.




5. Q. Discuss any deviations found in the experiment.

Ans : The deviation occurs when the message signal is in higher amplitude. Then in the output

the demodulated signal is noisy.

6. Q. Discuss the effects and purpose of the adjustment knobs, namely VR1 in MC1648

modulation block, VR1 in LM566 modulation block, and VR1 in PLL demodulation

block.

Ans: VR1 in MC1648 modulation block is used for varies the amplitude of message signal ,

VR1 in LM566 modulation block is used for adjust the free running , and VR1 in PLL

demodulation block also used for adjust the free running frequency .

7. Q. For long distance communication, which modulation (AM or FM) is preferable? Give

reason.

Ans: FM modulation is preferable in long distance communication rather than AM modulation

since in FM modulation noise can be eliminated. As the frequency of the signal is only

modulated ,a low pass filter may be used to recover the original message signal. But in AM

modulation noise affects the amplitude of the signal and is difficult or impossible to remove.

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