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EIE 312 Electronics Engineering Laboratory

Experiment Topic : FM Modulator

7.1: Curriculum Objectives

1. To understand the characteristics of varactor diodes.

2. To understand the operation theory of voltage controlled oscillator (VCO).

3. To design and implement the frequency modulator by using the voltage-

controlled oscillator.

4. To design and implement the frequency modulator by using MC4046.

5. To design and implement the frequency modulator by using LM566.

7.2: Curriculum Theory

7-1 The Operation Theory of FM Modulation

In frequency modulation (FM), we utilize the amplitude of audio signal to

modulate the frequency of carrier signal. The transmitted high and low frequency

signals will follow the received audio signal, which has different frequency that

keeps on changing. The frequency modulation can be expressed as

(7.1)

If

Then

(7.2)

where
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: Instantaneous modulated frequency.

: Carrier frequency.

: Modulating frequency or audio signal frequency.

: Modulation index,

: Frequency deviation.

Frequency deviation of FM is shown as below

(7.3)

From equation (7.3), we know that when the amplitude of modulating

signal changes, the frequency of FM will change too, and it uses the center point

of carrier frequency to achieve frequency deviation. From Carson’s rule, the

bandwidth (BW) of modulated signal can be expressed as

If the FM signal is the largest amplitude and largest frequency (i.e.

and ) then the bandwidth of FM can be simplified as

7-2 Varactor Diode

Varacor diode is also called tuning diode. Varactor diode is a diode, whose

capacitance can be varied by adding a reverse bias voltage to the pn junction,

When reversed bias voltage increases, the depletion region becomes wide, this

will cause the capacitance value decreases; nevertheless when reverse bias
voltage decreases, the depletion region will be reduced. This will cause the
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capacitance value increases. Varactor diode also can be varied from the

amplitude of AC signal. If an AC signal is added to a varactor diode, the variation

of capacitance of varactor diode will follow the amplitude of modulation signal.

Figure 7-1 is the analog diagram of capacitance of varactor diode. When a

varactor diode is not biased, the concentration will be differed from minor

carriers at pn junction. Then these carriers will diffuse and become depletion

region. The p type depletion region carries electron positive ions, and the n type

depletion region carries negative ions, We can use parallel plate capacitor to

represent the depletion region.

Figure7-1 Analog diagram of capacitance of varactor diode.

The transition capacitance pn junction of the plates can be expressed as

(7.4)

Where

(dielectric constant of silicon).

F/m

A: The PN junction area.

D: Depletion width
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When reverse bias voltage increases, the width of depletion region

d will increase but the cross-section area A remains, therefore the capacitance

value would be reduced. On the other hand, the capacitance value will increase

when reverse bias voltage decreases.

Varactor diode can be equivalent to a capacitor in series with a resistor

(Rs) and an inductor (Ls) as shown in figure 7-2. From figure 7-2, Cj is the junction

capacitor of semiconductor, which only exists in pn junction, Rs is the sum of

bulk resistor and contact resistor of semiconductor material, which is related to

the quality of varactor diode (generally below a few ohms). Ls is the equivalent

inductor of bounding wire and semiconductor material.

Tuning ratio, TR is the ratio of capacitance value under two different

biases for varactor diode. The expression is shown as follow

Figure 7-2 Equivalent circuit diagram varactor diode.

(7.5)

where

TR : Tuning ratio.

CV1 : The capacitance value of varactor diode at V1.

CV2 : The capacitance value of varactor diode at V2.

From this experiment, the characteristics of the varactor diode 1SV55 is

shown as below
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C3v = 42 pF (The capacitance of varactor diode at bias 3V)

TR = 2.65 (3V 30V)

Figure 7-3 Block diagram of MC4046.

7-3 Implementation of FM Modulator by Using PLL MC4046

MC4046 is the phase-locked loop (PLL) integrated circuit. Figure 7-3 is the

internal structure diagram of MC 4046. Pin 1, pin 10 and pin 15 are in N.C. mode.

Pin 5 is the input of INH, which is situated in low voltage level. The VCO

oscillation frequency of MC4046 is determined by the input voltage at pin 9, the

capacitances at pin 6 and pin 7, the resistances at pin 11 and pin 12.

Figure 7-4 is the circuit diagram of FM modulator by using MC4046. By

adjusting the variable resistor VR1 (DC level), we can control the output frequency

at pin 4, which is the frequency fo; capacitor C2, resistor R6 and R7 determine the

oscillation frequency of f0; capacitor C2 and resistor R6 determine the maximum

frequency of f0; capacitor C2 and resistor R7 determine the minimum frequency of

f0, i.e. the modulation bandwidth.

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Figure7-4 Circuit diagram of MC4046 FM modulator.

7-4 Implementation of FM Modulator by Using VCO LM566

LM566 is voltage-controlled oscillator integrated circuit. Figure 7-5 is the

internal structure diagram of LM566. Figure 7-6 shows the circuit diagram of FM

modulator by using LM566. We let SW1 be opened circuit, and the circuit is a

voltage-controlled oscillator. The output signal frequency is controlled by C3, VR1

and audio signal input terminal voltage. C2 is used to eliminate parasitic

oscillation. If C3 and VR1 remain a constant, then the output signal frequency and

the voltage difference between pin 8 and pin 5 (V8-V5) is proportional. In other

words, when input signal voltage (V5) increases, the voltage difference (V8-V5)

between pin 8 and pin 5 will decrease, the output signal frequency will decrease

as well. But, when input signal voltage (V5) decreases, the frequency of output

signal will increase. Another factor that affects the output signal frequency is

VR1 C3 value, the output signal frequency and VR1 C3 is inverse proportionally.

When the VR1 C3 value is getting larger, the output signal frequency is getting

lower. But when the VR1 C3 value is getting smaller then the output signal
frequency is getting higher. From figure 7-6, when we short circuit SW1, then R1
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and R2 provide a DC bias voltage as the DC level of input audio signal. The

center frequency (fo) can be adjusted by using VR1. If audio signal input terminal

is inputted with and AC signal, the VCO output signal frequency will follow the

change of the input audio signal voltage, which the FM signal is deviated.

Figure 7.5 Internal structure diagram of LM566.

Figure 7.6 Circuit diagram of LM566 FM modulator.

7-3 Experimental Procedures

Experiment 1: MC 4046 FM modulator

1. Refer to the circuit diagram in figure 7-4 or figure ACS7-1 on ETEK ACS-

3000-04 module.
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2. By using oscilloscope, observe on the output signal waveforms of

modulated FM signal(FM O/P). Adjust variable resistor VR1 so that the

output signal is 20 kHz square wave, Then record the measured results

in table7-1.

3. At the audio signal input port (Audio I/P), input 300 mV amplitude and 1

kHz sine wave frequency. By using oscilloscope, observe on the output

signal waveforms of FM O/P, the record the measured results in table 7-

2.

4. According to the input signals in table 7-2, repeat step 3 and record the

measured results in table 7-2.

Experiment 2: LM566 FM modulator

1. Refer to the circuit diagram in figure 7-6 or figure ACS7-2 on ETEK ACS-

3000-04 module.

2. Let J1 be short circuit, i.e. the circuit is the FM modulator. J 3 be short

circuit and J2 be opened circuit, i.e. the selected capacitor is C4 = 10 nF.

Adjust variable resistor VR1 so that the frequency at the modulated FM

output port (FM O/P) is 20 kHz square wave. Then record the measured

results in table 7-3.

3. At the audio signal input port (Audio I/P), input 300 mV amplitude and 1

kHz sine wave frequency. By using oscilloscope, observe on the output

signal waveforms of FM O/P, then record the measured results in table 7-

4.

4. According to the input signals in table 7-4, repeat step 3 and record the

measured results in table 7-4.

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7-4: Measured Results

Table7-1 Measured results of MC4046.

FM O/P

Table7-2 Measured results of MC4046 FM modulator.

(f = 1 kHz, Vm = 300 mV)

FM O/P
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Table7-2 Measured results of MC4046 FM modulator.(Continue)

(f = 1 kHz, Vm = 300 mV)

FM O/P

Table7-3 Measured results of LM566.

FM O/P
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Table7-4 Measured results of LM566 FM modulator.

(f = 1 kHz, Vm = 300 mV)

FM O/P

Table7-4 Measured results of LM566 FM modulator. (Continue)

(f = 1 kHz, Vm = 300 mV)

FM O/P
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7-5: Discussion Questions

1. Describe the operation theory of FM modulation.

2. Explain the implementation of FM modulator by using MC4046.

3. Explain the implementation of FM modulator by using LM566.