College of Electronic Technology, Bani-walid
Communication Department
5th Semester
Communication systems Lab
Experiment No.7
FM Modulator
Name of the student: ______________________________
Enrollment No. :_______________
Department: _______________________________________
Communication systems Lab Experiment 7 5th Semester
FM Modulator
4.1 Educational Objectives:
Understanding the operation of voltage controlled oscillator.
Implementing a frequency modulator with voltage-controlled oscillator
4.2 Background Information:
Frequency modulation (FM) is a process in which the carrier frequency is varied by the
amplitude of the modulating signal (i.e., intelligence signal). The FM signal can be
expressed by the following equation:
7-1
If x(λ)=Amcos(2πfm λ), then
7-2
Where
θ (t) = instantaneous modulated frequency
fc = carrier frequency
fm = modulating frequency
β = modulation index = Am(fΔ /fm)
The frequency of FM signal xFM(t) may be expressed as
7-3
From Eq. (7-3) we can find that the frequency of frequency modulated signal
occurs frequency deviation from the center frequency of the carrier when the
intelligence amplitude is variation.
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Communication systems Lab Experiment 7 5th Semester
Frequency Modulator Based on LM566 VCO:
The circuit of Fig. 7-5 is a frequency modulator based on voltage-controlled
oscillator (VCO) IC, LM566. If the SW1 is open, this circuit is a typical VCO
whose output frequency is determined by the values of C3 and VR1, and the
audio input voltage. If the values of C3 and VR1 are fixed, the output frequency is
directly proportional to the voltage difference between pins 8 and 5, (V8-V5). In other
words, an increase in audio input voltage (V5) causes a decrease in the value of (V8-
V5) and a decrease in the output frequency. Conversely, decreasing the audio input
voltage (V5) will cause the output frequency to increase. As discussed above, the values
of C3 and VR1 can also determine the output frequency, which is inversely proportional
to the product of VR1 and C3. That is, the greater the VR1×C3 value the lower the
output frequency.
Fig.7-5 LM566 frequency modulator circuit
If the SW1 is closed, the voltage divider constructed by R1 and R2 provides a dc level
to the audio input (pin 5). By adjusting the VR1, we can easily tune the
VCO center frequency fo. When an audio signal is applied to the audio input, the output
frequency will generate frequency deviations around fo in the variations of audio
amplitude. Thus, a frequency-modulated signal is obtained.
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Communication systems Lab Experiment 7 5th Semester
4.3 EQUIPMENT REQUIRED
1. Module KL-92001
2. Module KL-93004
3. Oscilloscope
4. RF Generator
4.4 Tasks to Study
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Communication systems Lab Experiment 7 5th Semester
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Communication systems Lab Experiment 7 5th Semester
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Communication systems Lab Experiment 7 5th Semester
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Communication systems Lab Experiment 7 5th Semester
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Communication systems Lab Experiment 7 5th Semester
7.5 Questions:
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