Chapter - Amplitude Modulation and Demodulation, PPT, ADC, Semester, Engineering - Computer Science Engineering (CSE) PDF Download

Amplitude Modulation and Demodulation

I. Objectives

1. To investigate amplitude modulated signal in the time domain and the frequency domain.
2. To be familiar with AM modulator and demodulator.
3. To calculate the modulation index of an AM wave.

II. Equipments

1. GW Audio Generator (GAG-808B)
2. Tektronix PG501A 2MHz Function Generator
3. Tektronix TDS220 Digital oscilloscope
4. Advantest R3132 Spectrum Analyzer
5. Bread board kit

III. Pre Lab

Before you start this lab, you should have knowledge about AM and the spectrum of amplitude modulated wave in the time domain and the frequency domain.

Amplitude Modulation (AM)

Amplitude modulation is the process in which the amplitude of the carrier signal is varied according to the instantaneous value of the message signal (modulating signal/baseband signal).

1 This laboratory session is prepared by Sarbagya Buddhacharya.

Time-Domain Representation of AM Signal

Let us consider a message signal and a carrier signal represented as

According to the definition of AM, the amplitude of the carrier signal is varied according to the instantaneous value of the modulating signal. So, the general expression for the amplitude modulated wave can be written as shown below.

Equation (4.2) represents the amplitude modulated waveform. It has three terms.
 Term I of the equation represents the unmodulated carrier signal.
 Term II of the equation represents upper side band (USB). USB represents the portion of the signal that has the frequency , which is higher than that of carrier.
 Term III of the equation represents the lower side band (LSB). LSB represents the portion of the signal that has the frequency , which is lower than that of carrier.
Thus from Equation (4.2), it can be seen that the amplitude modulated wave has three main components: carrier signal, USB, and LSB. The carrier signal does not contain any information; the message is contained in the side bands, i.e., USB and LSB.

Modulation Index (m)

The modulation index is a measure of the degree of modulation. This is usually expressed as a percentage called the percentage modulation.

Frequency-Domain Representation of Amplitude Modulated Wave 

From the expression of in Equation (4.2), the frequency-domain representation of the AM wave can be obtained from the Fourier transform of this equation.

Equation (4.4) is the frequency domain representation of the amplitude modulated wave. If Equation (4.4) is illustrated, we obtain Figure 4.4(a), which contains negative frequencies also. Practically negative frequencies do not exist, so the spectrum of amplitude modulated waveform is represented by Figure 4.4 (b). Here, it can be seen that, if or 100%, then the amplitude of the side bands are half of the amplitude of the carrier signal.

AM Modulator:
 

Figure 4.4 shows the electronics circuit of a simple AM modulator. It is essentially a CE amplifier having a voltage gain of A. The carrier signal is the input to the amplifier. The modulating signal is applied in the emitter resistance circuit.

The carrier ec is applied at the input of the amplifier and the modulating signal es is applied in the emitter resistance circuit. The amplifier circuit amplifies the carrier by a factor A, so that the output is Aes. Since the modulating signal is a part of the biasing circuit, it produces low frequency variations in the emitter circuit. This in turn causes variations in “A”.
The result is that amplitude of the carrier varies in accordance with the strength of the signal. Consequently, amplitude modulated output is obtained across RL. It may be noted that carrier should not influence the voltage gain A; only the modulating signal should do this. To achieve this objective, the carrier should have a small magnitude while the modulating signal should have a large magnitude.

AM Demodulation

The process of detection provides a means of recovering the modulating signal from the modulated signal.
Demodulation is the reverse process of modulation. The detector circuit is employed to separate the carrier wave and obtain the original message signal. Since the envelope of an AM wave has the same shape as the message, independent of the carrier frequency and phase, demodulation can be accomplished by extracting the envelope.

Diode as an AM demodulator

Figure 4.5 shows the circuit for diode as an AM demodulator. The diode is either forward–biased (when the AM signal is higher in value than the voltage across the capacitor) or reverse–biased (when the AM signal is lower in value than the voltage across the capacitor). When the diode is forward biased, it acts like a short circuit and the voltage across the capacitor follows the voltage of the source. When the diode is reverse-biased, it is acting like an open circuit and the capacitor simply discharges through the resistor. If the value of the time-constant of the capacitor and resistor, i.e.,  = RC, is suitable (not too large or too small), the charging and discharging of the 5 capacitor results in a signal that follows the message signal with some small ripples. If the value of  = RC is too
large, the discharge may be too slow that some parts of the envelope of the AM signal are not followed. If the value of  = RC is too small, the discharge may be too fast that the output signal contains extremely large ripples and it may be hard for any added low pass filter to remove these large ripples. Figure 4.6 illustrates the output of envelope detection.

Figure 4.6: Demodulation process based on envelope detection

IV. Procedure

AM Modulator

Here, we are going to observe the AM wave in the time domain and in the frequency domain. The time-domain display is seen in an oscilloscope while the frequency spectrum is seen in a spectrum analyzer. Use the carrier signal with frequency 2 MHz and the peak-to-peak value of 1.36 V. Use the modulating signal with frequency 10 kHz and the peak-to-peak value of 5 V. The modulating signal is generated from a GW audio generator while the carrier signal is generated from Tektronix PG501A 2MHz Function Generator. The circuit for an AM modulator is connected in bread board kit according to Figure 4.4.

Note that, in the circuit, the white-color wire is ground while other colors (red, green or blue) are positive voltage inputs.

1. Connect audio generator to oscilloscope and set the output signal to 10 kHz frequency, 5 V peak-to-peak.
There are two options for signal type (sine or pulse), you can use sine wave. Now the Audio generator generates the desired signal of frequency 10 kHz, and peak-to-peak value 5 V.

2. Similarly, set the function generator output at 2 MHz frequency and 1.36 V peak-to-peak value.

3. Connect the output of audio generator to the modulating input of the circuit.

4. Connect the output of function generator to the carrier input.

5. Connect the AM output to the oscilloscope. Now make the oscilloscope display in readable form. You will see the display as in Figure 4.2.

6. Using cursor measurement, measure the value of and . You will obtain = 280 mV and = 128 mV.

7. Now the modulation index can be calculated using equation 4.4; it will be equal to 37.25%.

8. Connect the AM output to spectrum analyzer. Set spectrum analyzer with centre frequency = 2 MHz and span = 100 kHz.

9. You will see three peaks: center peak is higher than the side peaks. Center peak is at 2 MHz and corresponds to the carrier signal. Two side peaks are one box away from the center peak (i.e., 10 kHz away from the carrier) and correspond to the message signal or side bands (USB and LSB).

10. You can use Display of the spectrum analyzer to find the difference between side bands and carrier amplitude. You will obtain the difference to be 23.3 dB.

If you change the frequency or the amplitude of the modulating signal through audio generator, corresponding changes are seen in the AM wave. If frequency is increased, then frequency of the signal displayed in oscilloscope will be increased and in spectrum analyzer gap between side lobes and carrier is increased. If amplitude is increased, then amplitude of the signal displayed in the oscilloscope is increased and in the spectrum analyzer peak value of side lobes will be increased.

AM demodulator

The circuit is connected as shown in figure 4.5.

1. Connect the AM output of modulator to the AM input of the demodulator.
2. Connect the output of the AM demodulator to the oscilloscope. Make the oscilloscope display to readable format.
3. You will see sinusoidal signal. The frequency of the signal will be equal to 10.34 kHz and peak-to-peak value equal to 54 mV.

Thus, the sinusoidal signal is demodulated. Here, amplitude of the demodulated signal is attenuated, as we are using passive devices such as diode for demodulation. For obtaining the exact signal (same as the message signal) at the demodulated output, amplifiers should be implemented.V. Your Task

Set carrier signal with frequency equal to 2 MHz and peak-to-peak value equal to 1.5 V. Set the frequency of the modulating signal to 10 kHz. Now, for the peak-to-peak values of the modulating signal given in the Table 4.1,
perform following and fill up the table.
1. Generate the AM modulated wave using the modulator circuit.
2. Calculate the modulation index from the AM wave displayed on the oscilloscope.
3. Note down the difference in the amplitude value of carrier and side bands from the spectrum of AM wave displayed on the spectrum analyzer.
4. Demodulate the AM wave using demodulator circuit and note down the frequency and amplitude value of the demodulated output using oscilloscope.