Introduction To Telecommunication Laboratory

2 AM MODULATORS

2.1 Objectives

• Understanding the principle of amplitude modulation (AM).

• Understanding the waveform and the frewuency spectrum of AM signal
and calculating the percentage of modulation.
• Desingning an amplitude modulator using MC1496.
• Measuring and adjusting an amplitude modulator circuit.

2.2 Discussion

Modulation is the process of impressing a low-frequency intelligence signal onto

a high-frequency carrier signal. AM is a process that a high-frequency carrier
signal is modulated by a low-frequency modulating signal (usually an audio). In
amplitude modulation the carrier amplitude varies with the modulating amplitude,
as shown in Fig. 2-1. If the audio signal is Am cos ( 2π f mt ) and the carrier signal is
Ac cos ( 2π f ct ) , the amplitude-modulated signal can be expressed by,

x AM (t ) = [ ADC + Am cos( 2π f mt )] Ac cos( 2π f c t )

= ADC [1 + m cos( 2π f mt )] Ac cos( 2π f ct ) (2.1)
= ADC Ac [1 + m cos( 2π f mt )] cos( 2π f c t )

Where
ADC = dc level

Am = audio amplitude

Ac = carrier amplitude

f m = audio frequency

f c = carrier frequncy

m = modulation index or depth of modulation = Am ADC

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Fig. 2-1 Amplitude modulation waveforms

Rewriting Eq. (2-1), we obtain

ADC AC m {cos [ 2π ( f c + f m )t ] + cos [ 2π ( f c − f m )t ]} + ADC Ac cos( 2π f c t )

1
xAM ( f ) = (2.2)
2

The first term o the right side of Eq.(2-2) represents double sideband signal
and the second term is the carrier signal. According to Eq. (2-2), we can plot the
spectrum of AM modulated signal as shown in Fig. 2-2. In an AM transmission
the carrier frequency and amplitude always remain constant, while the side
bands are constantly varying in frequency and amplitude. Thus, the carrier
contains no message or information since it never changes. This means that the
carrier power is pure dissipation when transmitting an AM signal. Thus, the
transmitting efficiency of amplitude modulation is lower than that of double-
sideband suppressed crrier(DSB-SC) modulation, but the amplitude demodulator
circuit is simpler.

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Fig. 2-2 Spectrum of AM signal

The m in Eq. (2-1) , called modulation index, is an important parameter. When

m is a percentage, it is usually called percentage modulation. It is defined as
Modulating Amplitude A
m= × 100% = m × 100% (2.3)
DC Level ADC

It is difficult to measure the ADC in practical circuit so that the modulation

index is generally calculated by
Emax − Emin
m= × 100% (2.4)
Emax + Emin

Where E max = AC + Am and E min = Ac − Am as indicated in Fig.2-1.

As mentioned above, audio signal is contained in the side bands so that the
greater the sideband signals the better the transmitting efficiency. From Eq.
(2-2), we can also find that the greater the modulation index, the greater the
sidebands and the better the transmitting efficiency. In practice, the
modulation index is usually less or equal to 1, if m > 1 , it is called over modulation.

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Table 2-1 A comparison between various balanced modulator outputs under

various input frequency conditions

In the following experiments we will implement an AM modulator using a

monolithic balanced modulator MC1496. According to different input signal
frequencies, the MC1496 may be used as a frequency multiplier, an AM
modulator, or a double sideband suppressed carrier (DSB-SC) modulator. Table
2-1 shows the summary of different input, output signals and circuit
characteristics.

Fig. 2-3 shows the internal configuration of MC1496. The differential amplifier
Q5 and Q6 is used to drive the differential amplifiers Q1Q2 and Q3Q4. the
constant-current source generator Q7 and Q8 provides the differential amplifier
Q5 and Q6 with a constant current. Overall gain of MC1496 can be controlled by
externally connecting a resistor between pins 2 and 3. For AM modulation, the
modulating signal should be applied to pins 1 and 4, and the carrier to pins 8 and
10. The bias current to pin 5 is commonly provided by connecting a series
resistor from this pin to the power supply.

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Fig. 2-3 MC 1496 internal circuit

Fig. 2-4 shows an AM modulator circuit whose carrier and audio signals are
single-ended inputs, carrier to pin 10 and audio to pin 1 . The gain of entire
circuit is determined by the R8 value. The R9 determines the amount of bias
current. Adjusting the amount of VR1 or the audio amplitude can change the
percentage modulation.

Fig. 2-4 Amplitude modulator using MC1496

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2.3 Experiment Equipments

• Module KL-92001

• Module KL-93002

• Oscilloscope

• RF Generator

2.4 Procedures

Procedure 1 Amplitude Modulation

1. Locate AM modulator circuit on Module KL-93002. Insert connect plugs

in J1 and J3 to set R8=1kΩ and , R9=6.8 kΩ.

2. Connect a 250mVp-p, 1 kHz sine wave to the audio input (I/P2), and a 250

mVp-p, 100kHz sine wave to the carrier input (I/P1). Use RF generator

for the carrier signal.

3. Connect the vertical input of the oscilloscope to the AM output (O/P).

Observe the output waveform and adjust the VR1 fo the modulation index

of 50%. Record the result in Table 2-2.

4. Using the results above and Eq. (2-4), calculate and record the percentage

modulation of output signal in Table 2-2.

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5. Using the oscillospe, observe the output signals for the audio amplitudes

of 150mVp-p and record the results in Table 2-2.

6. Repeat step 4.

7. Connect a 150mVp-p, 1 kHz sine wave to the audio input(I/P2), and a 100

mVp-p, 100kHz sine wave to the carrier input (I/P1).

8. Using the oscilloscope, observe the AM signal at output terminal (O/P) and

record the result in Table 2-3.

9. Using the results above and Eq. (2-4), calculate and record the percentage

modulation of output signal in Table 2-3.

10. Repeat steps 8 and 9 for carrier amplitude of 300mVp-p.

11. Connect a 150mVp-p, 3 kHz sine wave to the audio input(I/P2), and a 250

mVp-p, 100kHz sine wave to the carrier input (I/P1).

12. Using the oscilloscope, observe the modulated signal at output terminal

(O/P) and record the result in Table 2-4.

13. Using the results above and Eq. (2-4), calculate and record the percentage

modulation of output signal in Table 2-4.

14. Repeat steps 12 and 13 for the audio frequency of 1kHz.

15. Connect a 150mVp-p, 2 kHz sine wave to the audio input(I/P2), and a 250

mVp-p, 500kHz sine wave to the carrier input (I/P1).

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16. Using the oscilloscope, observe the modulated signal at output terminal

(O/P) and record the result in Table 2-5.

17. Using the results above and Eq. (2-4), calculate and record the percentage

modulation of output signal in Table 2-5.

18. Repeat steps 16 and 17 for the audio frequency of 300kHz.

2.5 Results
Table 2-2
(Vc=250mVp-p, fc=100kHz, fm=1kHz)
Audio Output Waveform Percentage
Amplitude Modulation

Emax=
Emin=

250 mVp-p

Emax=
Emin=

150mVp-p

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Table 2-3

(Vm=250mVp-p, fc=100kHz, fm=1kHz)

Carrier Output Waveform Percentage
Amplitude Modulation

Emax=
Emin=

100 mVp-p

Emax=
Emin=

300mVp-p

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Table 2-4

(Vc=250mVp-p, Vm=150mVp-p fc=100kHz)

Audio Output Waveform Percentage
Frequency Modulation

Emax=
Emin=

3kHz

Emax=
Emin=

1kHz

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Table 2-5

(Vc=250mVp-p, Vm=150mVp-p fm=2kHz)

Carrier Output Waveform Percentage
Frequency Modulation

Emax=
Emin=

500kHz

Emax=
Emin=

300kHz

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2.6 Questions

1. In Fig. 2-4, if we change the value of R8 from 1kΩ to 2kΩ, what is the
variation of the AM output signal?

2. In Fig. 2-4, if we change the value of R9 from 6.8kΩ to 10kΩ, what is the
variation in the dc bias current of the MC1496?

3. Determine the ratio of Emax to Emin if m=50%.

4. What is the function of the VR1?

Before you leave the lab;

• Turn off the power to all the equipment.

• Disassemble the circuit and place the Small components in the

plastic tray.

• Straighten up your lab station.

• Report any problems or suggest improvements to your TA.

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